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Plant distributions and competitive interactions along a gradient of tidal freshwater and brackish marshes

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Title:
Plant distributions and competitive interactions along a gradient of tidal freshwater and brackish marshes
Creator:
Latham, Pamela J., 1955-
Publication Date:
Language:
English
Physical Description:
x, 142 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Dissertations, Academic -- Environmental Engineering Sciences -- UF
Environmental Engineering Sciences thesis Ph. D
Salt marsh ecology ( lcsh )
Salt marsh plants ( lcsh )
Greater Orlando ( local )
Fresh water ( jstor )
Salinity ( jstor )
Vegetation ( jstor )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1990.
Bibliography:
Includes bibliographical references (leaves 131-141).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Pamela J. Latham.

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Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
026492046 ( ALEPH )
25222086 ( OCLC )
AJA5446 ( NOTIS )
AA00004763_00001 ( sobekcm )

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Full Text










PLANT DISTRIBUTIONS AND COMPETITIVE INTERACTIONS ALONG A
GRADIENT OF TIDAL FRESHWATER AND BRACKISH MARSHES














By

PAMELA J. LATHAM


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
TO THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


UNIVERSITY OF FLORIDA LIBRA IS


1990














ACKNOWLEDGEMENTS

I would like to extend my thanks to all the volunteers

that braved the Savannah River to help me with my work.

They faced alligators, feral hogs, water moccasins, an

extraordinary insect diversity, sucking muds, intense heat

and cold, rain storms and sleet, failed boats, and small

(and medium-sized) mammals in the sleeping accommodations -

- all for someone else's project. I sincerely appreciate

the help of Rick Bartleson, Lance Peterson, Kelly Latham,

Clay Phillips, Tim Harris, Brad Mace, and Monica Maynard for

their help in transplanting 390 vegetation plots. Kathy

Hallman provided invaluable lab assistance and she and Brad

Mace spent innumerable hours counting, drying, and weighing

plants.

My committee members all provided advice during various

phases of my field work and writing. Wiley Kitchens was

always there to provide anything I needed to carry out my

work and encouraged me to pursue interests beyond those of

the project. Ronnie Best and Jack Stout are responsible for

my work in wetlands ecology. Jack had enough faith in me

after my Master's work to send me off to the University of

Florida to take Ronnie's Wetlands Ecology class. Ronnie

helped establish my interest in wetlands ecology. Clay








Montague gave me the perspective I needed to be comfortable

with systems modelling and computers. Don Graetz somehow

made wetland soils biochemistry both interesting and

applicable to ecological systems. All my committee members

have shown an interest in my work that has kept me

motivated.

Rick Bartleson and Lance Peterson provided more help

and support than I could have ever asked for. Rick worked

harder in the field than anyone I've ever known, has an

unbelievable sense of direction, edited page after page of

manuscript, provided insightful discussion during the entire

project, and was willing to swim across the Savannah River

with me when airboats broke down. Lance began the project

with me, and together we tried and dismissed various forms

of transportation: helicopters, airboats, the Whaler, and

even a canoe. We settled on the johnboat, in which Lance

was able to attain speeds up to Warp 4. Through it all,

Rick and Lance provided support, enthusiasm, encouragement,

stability, and friendships I'll always cherish.

Jack Putz provided me with the scientific tools to

carry out this work; most of the hypotheses and design were

all generated from insightful discussions with him. Jack

did his best to keep me on the straight and narrow path of

science and I greatly appreciate the friendship that came

along with the science. Leonard Pearlstine was unfortunate

enough to be the closest to my work and was patient enough


iii








to listen to and read nearly every idea I've had in the past

5 years; he was always willing to help me work out problems

by listening to me. Susan Vince's experience with tidal

salt marshes and transplant experiments was invaluable in

setting up my own experiments. Ken Portier, the only

statistician I know who can successfully integrate

statistics and biology and make it look easy, provided early

statistical designs for my transplant experiments.

In addition to volunteers, there were some folks who

just made things easier for me during the 4 years of field

work. Monica Maynard of the Georgia Coop Unit always

welcomed us at the Refuge trailer during her striped bass

surveys and made what were otherwise barren sleeping

quarters a temporary home. Daryl Hendricks, a Refuge

employee, was always there to help get things ready, warn us

about potential problems or get us out of a jam, repair

anything mechanical, and somehow manage to like us anyway.

Ray Porter was always around for us "just in case." The

Savannah National Wildlife Refuge personnel at the refuge

and the Savannah Office were all helpful in finishing this

project. John Davis has been a staunch supporter of this

project.

My Chesapeake Bay Retriever, Mac, was also instrumental

in my work. Mac was a valuable field assistant, carrying

equipment for me in the field and keeping a watchful eye for








anything unusual. He diligently watched over me when I was

alone in the field or working late at the office.

Throughout the entire project, Clay Phillips provided a

source of support and encouragement on which I could depend.

I could not have accomplished the field work, the project

goals, the qualifiers, the writing, or the defense without

him.
















TABLE OF CONTENTS


ACKNOWLEDGEMENTS . .

ABSTRACT . .


1. GENERAL INTRODUCTION .
Overview .. .
The System: Freshwater Tidal Marshes .
Environmental Influences .
Soil Characteristics .
Species Interactions .
Tidal Marshes of the Lower Savannah River .
The Savannah River Tide .
Objectives . .


. 1
. 1





* 10
. 11


2. SPECIES ASSOCIATION CHANGES ACROSS A GRADIENT OF
FRESH, OLIGOHALINE, AND MESOHALINE TIDAL MARSHES
ALONG THE LOWER SAVANNAH RIVER .. 12
Introduction . .. 12
Methods .. .. .. 14
Study Site . 14
Vegetation and Environmental Gradients. .17
Analysis . 19
Results .. . .. 21
Discussion .. 32

3. SPATIAL DISTRIBUTIONS OF THE SOFTSTEM BULRUSH,
SCIRPUS VALIDUS, ACROSS A SALINITY GRADIENT 41
Introduction . 41
Methods . 42
Results . 44
Species Associations .. 44
Spatial Pattern .. 44
Relationship of IV with Environmental Variables 49
Discussion .. .... 51
Species Diversity . 51
Distribution Patterns .. .51

4. MORPHOLOGICAL PLASTICITY IN SCIRPUS VALIDUS
ALONG A SALINITY GRADIENT .. 57
Introduction . .. 57
Methods . 59
Results .. . 61
Field Measurements .. .61


Page

Sii

viii










Transplants .. 64
Light Extinction .. 67
Discussion . 67

5. THE ROLE OF COMPETITIVE INTERACTIONS, SOIL SALINITY, AND
DISTURBANCE IN THE DISTRIBUTION OF TIDAL MARSH PLANT
SPECIES . 77
Introduction. .. .77
Methods . 81
Field Transplants Between Sites .. .82
Field Transplants Within Sites. 83
Greenhouse Experiments ... 86
Feral Hog Disturbance .. .. 87
Results . 87
Between-site Transplants .. 88
Within-site Transplants. ... 94
Inner S. validus vs. outer control
plots ... 94
Inner S. validus vs. control S. validus
plots ... .... ...... 96
Outer neighbor vs. neighbor control
plots 100
Inner S. validus vs. outer neighbor
plots 101
Greenhouse Experiments .. 103
Feral Hog Disturbance .. 110
Discussion .... 110

6. SUMMARY AND CONCLUSIONS .. .. 125

LITERATURE CITED . .. 131

BIOGRAPHICAL SKETCH . ... ... .ii


vii


Greenhouse Experiments


. 64














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

PLANT DISTRIBUTIONS AND COMPETITIVE INTERACTIONS ALONG A
GRADIENT OF TIDAL FRESHWATER AND BRACKISH MARSHES

By

Pamela J. Latham

August 1990

Chairpersons: Wiley M. Kitchens, G. Ronnie Best
Major Department: Environmental Engineering Sciences

Plant species distributions and their relation to

environmental factors, morphology, competition and

disturbance were examined across a gradient of tidal

freshwater, oligohaline, and mesohaline marshes along the

lower Savannah River. Salinity was significant in

separating major vegetation classes between sites, while

elevation and distance from tidal channel were significant

in separating classes within sites. Overlap among

vegetation classes was high, a result of overlap with

Scirpus validus, which occurred over the widest range of

environmental parameters.

Spatial pattern and relative importance of Scirpus, as

well as composition of co-occurring species, changed

significantly with changing salinity. Patterns were clumped

in the mesohaline marsh, uniform in the strongly. oligohaline


viii








marsh, and nearly random at freshwater and oligohaline

sites. Field and greenhouse experiments indicated more

robust Scirpus under freshwater conditions in the

greenhouse, while this trend was reversed at field sites.

Transplants of Scirpus between sites had morphologies

similar to local populations. Results indicate that

morphologic variation in Scirpus is ecophenic and that

competition, possibly for light, may be more important in

its distribution than environmental factors.

Nested transplants of Scirpus (target species) and

neighbor species were combined with species removals in

transplants. Neighbor and target species were also

transplanted between sites to examine competitive

interactions among species. Greenhouse experiments were

conducted for species at different densities and salinities.

Lastly, feral hog disturbances were documented to determine

the possible effects of disturbance.

Results indicate that effects of competitive

interactions on plant distributions are not exclusively

related to the individual competitive abilities of plant

species, but instead reflect a competitive balance of the

species involved. Although freshwater marsh species have

greater competitive abilities than species of brackish

marshes, changes in environmental conditions strongly

influence species interactions. While the competitive

ability of a weaker competitor itself is not altered,








differences in co-occurring species and environmental

conditions result in altered competitive hierarchies and

species distributions along an environmental gradient. In

addition to environmental gradients, disturbance also

appears to alter species interactions and distributions.














CHAPTER 1
GENERAL INTRODUCTION

Overview

The research findings presented in this dissertation

are organized into 4 main chapters. Introductory and

summary chapters are also included. The introductory

chapter includes a literature review of Southeastern

freshwater tidal marsh structure and function, a brief

description of external factors affecting the system, and

project objectives. Chapter 2 provides a descriptive and

quantitative analysis of the marsh vegetation and

relationships with associated environmental parameters. In

chapters 3 and 4, the population dynamics of the only

cosmopolitan species occurring in the study area, Scirpus

validus, are investigated in regards to its distribution,

changing morphology, and relation to co-occurring species

across a salinity gradient. Research findings from chapters

2, 3, and 4 culminate in chapter 5, which provides a

quantitative analysis of the role of species interactions in

structuring these plant communities. The final chapter

contains a summary of chapters 2 through 5, with emphasis

placed on results and implications from chapter 5.










The System: Freshwater Tidal Marshes

Tidal freshwater marshes, in contrast with coastal salt

marshes and inland freshwater marshes, have been studied

relatively little (Odum 1988, Odum et al. 1984, Simpson et

al. 1983). Unlike inland, non-tidal marshes, they receive

high nutrient and energy pulses characteristic of coastal

salt marshes. Salinity, the dominant factor affecting the

productivity and species richness of salt marshes, is not

imposed upon tidal freshwater marshes. As a result, tidal

freshwater marshes support a high diversity of plants and

animals (Odum 1988, White 1985, Doumlele 1981, Gosselink et

al. 1978, Tiner 1977), are extremely productive (Odum 1988,

Simpson et al. 1983), and are often sensitive to human

impact (Odum et al. 1984, Mitsch and Gosselink 1986).

Tidal freshwater marshes along the South Atlantic Coast

make up a significant portion of tidal marshes of the U.S.

Coastline morphology of Georgia and South Carolina funnel

and amplify tidal waters, creating tides ranging in

amplitude from 2.0 to 2.5 meters (de la Cruz 1981), greater

than most North American marshes. Unlike geologically

younger Pacific coast marshes, there has been time for

extensive development over the broad, gently sloping Coastal

Plains. In addition, the network of rivers feeding the

southeastern marsh system is larger than that of the

northeastern tidal marshes that occur over less erodable

glacial till. Consequently, deposition of river sediments










into south Atlantic coast marshes is greater than those of

northeastern marshes.

Environmental Influences

Tidal freshwater and salt marshes are the extremes of a

gradient that includes oligohaline (0.5 5 ppt) and

mesohaline (5 30 ppt) marshes and comprise a gradient that

coincides with increasing salinities toward the ocean.

Differences in river and tidal influence are associated with

physical gradients of hydroperiod, salinity and soil

characteristics and are reflected in distributions of marsh

species. These physical parameters are generally considered

the primary factors affecting zonation of marsh vegetation

(Cooper 1982), though recent evidence indicates species

interactions are important in salt marsh vegetation

zonation.

Salinity exerts a major influence on tidal marsh

habitats (White 1985, Morris 1978, Parrondo et al., Reid and

Wood 1976, Phleger 1971, Adams 1963) and appears to be the

primary factor in modifying the physical and biological

makeup of the transition zone between brackish and fresh

water (Odum 1988, Haramis and Carter 1983). High salinities

inhibit all but a few halophytes (Phleger 1971) and are

associated with decreased species diversity (White 1983,

Anderson et al. 1968).

Freshwater marsh communities exhibit relatively little

species zonation compared to salt marshes (Odum 1988, Joyce










and Thayer 1986, Carey et al. 1981). Vegetation patterns

(White 1985, Leiffers 1983, De la Cruz 1978, Disraeli and

Fonda 1978), species diversity (Heinselman 1970) and species

ranges (Ferren and Schuler 1980) are, however, strongly

influenced by hydrologic regime and site elevation, and

elevation is probably the most agreed upon factor

influencing marsh zonation (Dawe and White 1982). Major

components associated with hydrologic regime include water,

nutrients, toxins, oxygen availability and spatial

heterogeneity (Gosselink and Turner 1978). Wave exposure

(Wilson and Keddy 1986) and current velocity (Nilsson 1987)

affect plant diversity as well as plant biomass and

vegetative expansion.

Soil Characteristics

Soil characteristics appear less variable than salinity

and hydroperiod in tidal marshes. Most North American

saltmarshes are formed from reworked marine sediments on

marine-dominated coasts (Mitsch and Gosselink 1986),

although sediments of both tidal freshwater and salt marshes

are of riverine and tidal input. Freshwater tidal marshes

are primarily fine textured silts and clays with an organic

content ranging from 10-15% in actively flooded levees to

30-45% in high marshes with minimal tidal influence (Simpson

et al. 1983). Differences in flood velocity due to levee

vegetation also result in the trapping of courser sediments

along creek banks (Kirby and Gosselink 1976) and a decrease










in soil redox (Rowell et al. 1981). Organic content of

creek levees is lower than high or back marsh soils farther

from the creeks. While tidal flushing may be responsible

for the rapid litter loss of up to 80% over 30 days, varying

rates of decomposition in plant species may be due more to

inherent differences in species structure and decomposition

rates than tidal influence (Simpson et al. 1983).

Marsh vegetation is significantly affected by changes

in soil and interstitial water chemistry under flooded

conditions (Ponnamperuma 1972). Except for a thin surface

layer, tidal marsh soils are anaerobic due to the slowed

diffusion of oxygen through water. The oxygen deficit

results in slower organic decomposition and changes in pH,

redox, microbial activity, phosphorous, nitrogen and sulfur,

as well as metals. The cation exchange capacity and pH

increase in acid soils under saturated conditions, while

both cation exchange capacity and pH decrease in alkaline

soils (Ponnamperuma 1972), resulting in nearly neutral soil

pH. An accumulation of toxic metals or solubilization and

absorption of high concentrations of micronutrients may also

occur under reduced conditions. Sediment nitrogen is bound

in an organic form in marshes and is often the primary

limiting nutrient for salt marsh plants (Valiela et al.

1978). Phosphorous, however, is often more available to

plants of saturated or poorly drained soils. Because iron

and aluminum phosphates are more readily reduced, providing










oxygen for the limited decomposition which takes place in

these soils, the phosphorous is released and is seldom

limiting.

Species Interactions

In addition to environmental factors, differences in

distributions among marsh species have been attributed to

species interactions (Bertness and Ellison 1987, Snow and

Vince 1984) and variation in competitive abilities across an

environmental gradient (Wilson and Keddy 1986, Grace and

Wetzel 1981). Competition among and between species of

terrestrial habitats is well documented, although little

experimental data exist for competition among marsh species.

While plant zonation has been highly correlated with

physical factors such as elevation and salinity, these

correlations are not necessarily causal. In addition,

physical and chemical gradients associated with marsh

vegetation are often obscured by the circular zonation due

at least in part to the clonal nature of perennial species.

Salt-tolerant species, e.g. S. alterniflora seedlings

(Mooring et al. 1971), S. foliosa (Phleger 1971) and

Salicornia bigelovii (Webb 1966), are known to grow well,

and in most instances, better, in freshwater. The same

species, however, are outcompeted under freshwater

conditions, and so are confined to brackish and salt marshes

where freshwater species cannot survive (Wainwright 1984).

Results of transplant experiments (Wilson and Keddy 1985,










Snow and Vince 1984, Barbour 1978) provide evidence that

physiological response of plants to environmental gradients

alone does not account for the zonation and patterns

characteristic of marsh vegetation. Both interspecific

competition and physical disturbance have been found to

affect spatial patterns among salt marsh species (Bertness

and Ellison 1987).

Results from several studies (Snow and Vince 1984,

Grace and Wetzel 1981, Sharitz and McCormick 1980) suggest

that along an environmental gradient, competitive

interactions may determine species distributions at one end

of the gradient, while physiological tolerance limits

species distributions at the other. Greater development of

"competitive heirarchies" (Anderson 1986) would be expected

in freshwater marshes, then, based on the relatively benign

environment and greater probability of interactions among

species. A random distribution of individuals would

ultimately result if there was a total lack of response by

individuals to environmental factors or other individuals

(Taylor et al. 1978).

Tidal Marshes of the Lower Savannah River

The Savannah River rises on the southern slope of the

Blue Ridge Mountains in North Carolina and flows through the

Coastal Plain. It is an alluvial, rock-dominated river (as

opposed to precipitation-dominated) with a relatively high

mineral load derived from weathering and leaching of parent










material in the mountains and Piedmont (Wharton et al.

1982). The Chattooga and Tallulah Rivers form the

headwaters of the Savannah River in the Blue Ridge sector of

northeast Georgia (Wharton 1978). Rivers originating in the

Piedmont generally have larger drainage basins and, because

they rise in the mountains where rainfall is heavier, have

larger discharges than Coastal Plain rivers (Rhodes 1949).

The lower portion of the Savannah River is a deltaic plain.

The river dishcarges through the delta into the ocean via

numerous, relatively shallow, interconnected distributary

channels between which are scattered marsh islands of

various sizes (Rhodes 1949).

The head of the Savannah River tidewaters is about 50

miles north of its mouth. Distributary channels of the

delta include: the Little Back, Middle, Back, and Front

Rivers (see Chapter 2 for map). The hydrology of the lower

Savannah River is dependent upon precipitation, run-off,

channel morphology, wind, river discharge, variation in mean

sea level, and seasonal and daily tidal fluctuations.

Importantly, 28 percent of tidal freshwater wetlands of

the eastern U.S. are found in Georgia and South Carolina.

In the lower portion of the Savannah River, approximately

1900 ha, or 21% of the tidal freshwater wetlands in Georgia

and South Carolina are found on the Savannah National

Wildlife Refuge (SNWR), in addition to bottomland hardwoods,

upland forest and artificial impoundments. Artificial










impoundments, left from extensive rice cultivation during

the 18th and 19th century (Baden 1975), make up more than

15% of the the total area of South Carolina and Georgia

coastal marshes. Former rice fields are evidenced in

extensive canal systems traversing much of coastal South

Carolina and Georgia, and are especially notable on the

lower Savannah River. Vegetation zonation associated with

former canal systems is conspicuous. Present vegetation

composition is generally consistent with non-cultivated,

naturally occurring tidal marshes of the southeastern coast

(Odum et al. 1984). Marshes along the lower Savannah River

range from fresh to brackish salinities, commensurate with

changes in soil, hydrology and plant communities.

The Savannah River Tide Gate

The tide gate is a navigational structure designed to

constrict ebbing flows of the Lower Savannah River to the

Savannah Harbor channel. In operation, the structure serves

as a flap gate and is closed on outgoing tides.

Subsequently, flood waters are diverted through connecting

lateral channels to the harbor channel of the Front River.

Because of the increased flow velocities on ebbing tides the

channel is scoured and maintenance dredging in the port is

minimized. Like the Mississippi River deltaic plain

(Pezeshki 1987), man-made alterations appear to be effecting

changes in salinities and hydroperiod, with the potential to

significantly alter plant populations and habitat.










The construction of a tide gate at the mouth of the

north channel of the Savannah River in 1977 has resulted in

increased salinities throughout the refuge (Odum et al.

1977, Pearlstine et al. 1989), and refuge vegetation has

undergone significant alterations (Brown et al. 1987). The

extent of the salinity wedge upstream, a function of both

current and elevation, is farther with the tidegate in

operation (Odum 1977, Pearlstine et al. 1989). The wedge

extended upstream to the confluence of the Front and Back

Rivers in 1943 (USCOE) and 1977 (Odum 1977), and just

upstream from Highway 17-A on the Back River in 1977 (Odum

1977) (Fig. 2-1). Since construction of the tide gate, the

wedge now 2 3 miles farther upstream (Pearlstine et al.

1989). As a result of 15 years of tide gate operations, the

lower Savannah River has undergone a conversion from

freshwater marsh to a compressed gradient of freshwater and

brackish marshes, providing a unique opportunity to document

vegetation response to these changes.

Objectives

This objectives of this study were to 1) characterize

the vegetation features and environmental parameters of the

lower Savannah River tidal marshes; 2) quantify the

relationships among vegetation associations and major

environmental environmental gradients of salinity,

hydroperiod, and soil characteristics; 3) quantify the

population dynamics of Scirpus validus in relation to










environmental gradients and co-occurring species; and 4)

quantify the effect of species interactions as causal

mechanisms for observed plant species distributions and

associations.

For objectives 1) and 2), major physical parameters

associated with vegetation types were identified and

quantified, and correlations among measures of plant species

importance and environmental parameters were used to

characterize plant species distributions in relation to

environmental gradients of salinity, hydroperiod and soil

variation in tidal freshwater and brackish marshes. The

remaining objectives entailed an investigation into causal

mechanisms for species distributions identified in the

gradient analysis. Possible reasons for differences in

spatial patterns of species distributions in fresh and

brackish marsh and development of competitive heirarchies

were experimentally determined. Transplant experiments

allowed determinations of species robustness, not only in

relation to physical and chemical parameters, but also in

regards to other species that occupy the same or overlapping

niches.














CHAPTER 2
SPECIES ASSOCIATION CHANGES ACROSS A GRADIENT OF
FRESH, OLIGOHALINE, AND MESOHALINE TIDAL MARSHES
ALONG THE LOWER SAVANNAH RIVER

Introduction

Contrast in vegetation, animals, and water and soil

chemistry among tidal freshwater and saline marshes is well

documented. Differences in river and tidal influence are

associated with physical gradients of hydroperiod, salinity

and soil characteristics and are reflected in distributions

of marsh species. These physical parameters are generally

considered the primary factors affecting zonation of marsh

plants (Cooper 1982). Salinity exerts a major influence on

tidal marsh habitats (White 1983, Morris 1978, Parrondo et

al. 1978, Reid and Wood 1976, Phleger 1971, Adams 1963) and

may be the principal factor in modifying the physical and

biological makeup of the transition zone between brackish

and fresh water (Odum 1988, Haramis and Carter 1983). In

addition, increasing salinities inhibit all but a few

halophytes (Phleger 1971) and result in a characteristically

low species diversity (White 1983, Anderson 1986) in saline

marshes when compared with freshwater marshes.

Freshwater marsh communities exhibit relatively little

species zonation compared to salt marshes (Odum 1988, Joyce

and Thayer 1986, de la Cruz 1981). Vegetation patterns

12










(White 1983, Leiffers 1983, De la Cruz 1981, Disraeli and

Fonda 1979), species diversity (Heinselman 1970) and species

ranges (Ferren and Schuler 1980) are, however, strongly

influenced by hydrologic regime and site elevation, and

elevation is the most agreed upon factor influencing plant

distributions and donation (Dawe and White 1982). Major

components associated with hydrologic regime include water,

nutrients, toxins, oxygen availability and spatial

heterogeneity (Gosselink and Turner 1978). Wave exposure

(Weisner 1987) and current velocity (Nilsson 1987) also

affect plant diversity, biomass, and vegetative expansion.

Vegetation differences and how those differences relate

to environmental factors across salinity gradient extremes

have received little attention (Odum 1988). Southeastern

tidal marshes, both freshwater and saline, have received

considerable attention on an individual basis, although the

gradient between the two remains poorly studied. A system

of freshwater (< 0.5 ppt), oligohaline (0.5-5.0 ppt), and

mesohaline (5.0-18 ppt) tidal marshes along the lower

portion of the Savannah River provided an opportunity to

examine differences in vegetation and associated

environmental factors across a strong tidal salinity

gradient.










Methods

Study Site

The study area covered 1900 ha of Savannah River tidal

marsh, in Chatham County, Georgia, and Jasper County, South

Carolina, within the boundaries of the Savannah National

Wildlife Refuge (320 10' N., 81008' W.). Local tides

measure 2.0 to 2.5 meters (de la Cruz 1981). Like much of

South Carolina's and Georgia's low country, remnant rice

fields from the 18th and 19th centuries are abundant

throughout the refuge (Baden et al. 1975). Plant zonation

associated with canal drainage is conspicuous, although

present vegetation composition is consistent with non-

cultivated, naturally occurring tidal marshes of the

southeastern coast (Odum et al. 1984).

Four sites were chosen that represented tidal

freshwater (0 .5 ppt), oligohaline (.5 2.5 ppt),

strongly oligohaline (2.5 5 ppt), and mesohaline (5 10)

marsh conditions (Fig. 2-1). Six transects, each with 3

sample points at distances of 20, 70 and 120 m from tidal

channels, were located at each site. Sample points (n = 73)

were also located adjacent to a secondary canal (canals

draining into primary canals) and at approximately equal

distances between two secondary canals for a total of 18

sample points at each study site. Because of a sharp turn

in the primary canal at the oligohaline site, there was no

room for an entire transect at the first sample point. Six






















9'
9'.


40P





Mesohaline





TIDE SATE




0 1
MILES N



Fig. 2-1. Location of study area along the lower Savannah
River, in South Carolina and Georgia. Shaded, numbered
areas represent freshwater (1), oligohaline (2), strongly
oligohaline (3), and mesohaline (4) sites.










transects were positioned in addition to the first sample

point at this site, for a total of 19 points at the

oligohaline site, making a total of 73 sample points.

Sample points equal distances from primary and secondary

canals served as replicates (Fig. 2-2).

Vegetation and Environmental Gradients

Double-nested PVC wells were placed at each sample

point. The outercasing was placed 20 cm into the soil to

prevent surface water from entering the inner sampling well,

and the inner well was placed 30 cm into the soil to collect

soil water through vertical slits below the soil surface.

Soil water samples and vegetation were collected at

sample points at approximate 8-week intervals from June,

1985, to August, 1987. Surface vegetation was harvested

from a randomly placed 0.25 m2 circular plot at each sample

point during each sampling period. Live vegetation was

sorted by species, counted (for a measure of density),

dried, and weighed (for a measure of biomass). Absolute

densities and biomass were multiplied by four to give

density and biomass per square meter for each species.

Importance values (IV) were calculated as the sum of

relative density and relative biomass for each species in

each plot. Dominance-diversity curves, which reflect

differences in patterns of competition and niche

differentiation in communities (Whittaker 1965) were

developed from IV for each species.















*o
0
i
7
91


Fig. 2-2. General arrangement of sample points in relation
to river and tidal channels.










Interstitial water was siphoned from sampling wells

into Nalgene bottles and stored on ice until salinities

could be measured from well samples using a Reichert

refractometer. Elevations relative to mean sea level were

measured with a self-leveling electronic level and verified

by permanent benchmarks along the river.

Analysis

Within-site variation in salinity, organic content, and

elevation was determined by comparing these variables

between low (20 m from channel edge), middle (70 m from

channel edge), and high (120 m from channel edge) marsh.

Between-site variation in salinity, organic matter and

elevation was determined by comparing overall means between

sites for each variable. Because the oligohaline site had 1

more point than the other sites, and because there were

occasions on which either a sample point was missed or the

sample itself inadequately stored, there was not always the

same number of samples to compare. Consequently, Tukey's

studentized range test for unequal cell sizes was used in

means comparisons tests for these variables.

Vegetation classes were chosen based on ordination of

species composition at sample points using detrended

correspondence analysis (DCA) from the program DECORANA

(Hill 1979). DCA is an ordination method which converts

species composition data into vegetation variables, which

are then assigned scores; vegetation and environmental










patterns can then be compared and used to generate

hypotheses about the causes of within-community vegetation

patterns (Bernard et al. 1983). Ordination was first

applied to all 4 sites combined to examine between-community

differences in vegetation. Because of the high diversity of

the freshwater marsh, a second ordination of only freshwater

and oligohaline sites was done so that within-community

vegetation patterns could be examined.

Discriminant function (DF) analysis was then applied to

vegetation classes, both for the 4 sites combined and

separately. Like multiple regression analysis, DF analysis

can be used to predict or describe the relationship between

independent and dependent variables. DF analysis was used,

however, because the dependent variables in this case, i.e.

vegetation classes, are nominal variables, whereas multiple

regression analysis relies on interval or ratio variables

(Afifi and Clark 1984). DF analysis was used to quantify

the contribution of interstitial salinity, elevation, and

distance from tidal channels in defining vegetation classes,

based on relationships between environmental variables and

species composition and dominance at sampling sites. A

"successful" DF analysis is one which results in correct

pairing of vegetation types and environmental parameters

into vegetation classes. Interstitial salinity was used to

discriminate between freshwater, oligohaline, strongly

oligohaline, and mesohaline sites. Once this first level of










classification was achieved, interstitial salinity,

elevation, and distance from tidal channels were used to

differentiate among vegetation classes within sites.

Environmental variables were retained in the DF analysis

based on their significance in predicting vegetation classes

at sites. Results were presented as percent correct

classifications of vegetation observations.

Results

Differences in soil water salinities (Fig. 2-3) were

significant (P = .01) between all sites. Average salinities

were lowest (0.54 ppt 0.63) at the freshwater site and

increased through oligohaline (2.10 1.04 ppt), strongly

oligohaline (4.67 + 1.49 ppt) and mesohaline (9.27 + 1.97

ppt) marshes. No sites exhibited significant within-site

variation in soil water salinities.

Variation in pH was not significant between sites.

Within site variation in pH was also not significant.

Percent soil organic matter (Fig. 2-3) was higher at

freshwater and oligohaline marsh sites (43.62 + 17.2 and

55.28 + 21.2, respectively), when compared with strongly

oligohaline (38.27 + 17.3), and mesohaline (31.62 11.7)

marsh sites. Differences were significant (P = .05) between

all except the strongly oligohaline and mesohaline sites.

Soil organic matter showed a general increase with distance

along transects, i.e. from front to back marsh, at all





































S100-


so-

40--

20-

Freshwater Oligohaline Strongly oligohaline Mesohaline

8O

0 -

c 60
E
*5 50

t so
0


8 20-

10


Freshwater Oligohaline Strongly oligohaline Mesohaline





Fig. 2-3. Means and standard deviations for soil water
salinities (ppt), percent soil organic matter, and elevation
(cm) at 20 (solid bars), 70 (hatched bars), and 120 (open
bars) meters from primary creeks at freshwater, oligohaline,
strongly oligohaline, and mesohaline sites.








22
sites. Differences along transects were significant only at

freshwater and mesohaline sites (P = 0.05).

Mean elevation (Fig. 2-3) was highest at the freshwater

and strongly oligohaline sites, lower at the oligohaline,

and lowest at the mesohaline site (F = 42.75; n = 18; P =

.05). There was an increase in elevation from front to back

marsh along transects at all sites, though differences along

transects were significant at only the oligohaline site (F =

9.45; n = 18; P = .05).

On the basis of the DECORANA ordination of the 4

combined sites (Fig. 2-4) and the re-ordination of the

freshwater and oligohaline sites (Fig. 2-5), 9 vegetation

classes were chosen, the first 6 of which were chosen from

freshwater and oligohaline sites: (1) Eleocharis

montevidensis, (2) E. montevidensis and S. validus, (3)

Zizaniopsis miliaceae, S. validus, and Typha latifolia, (4)

Z. miliaceae, (5) E. montevidensis, S. validus, and Z.

miliaceae, (6) S. validus and Z. miliaceae, (7) S. validus,

(8) Spartina alterniflora and S. validus, and (9) S.

alterniflora and S. robustus. While overlap between

adjacent classes was high, non-neighboring classes did not

appear to overlap and the vegetation classes were considered

more than adequate. Eleocharis montevidensis was the only

dominant species not present in monospecific stands at some

point along the salinity or elevation gradients and only S.

validus occurred at all sites. The first five























FRESH


BRACKISH


*


"



*c
0


1r'
-I

I
I







I

I
-I
I
I
I
-I


I


LI


L.
~-- -- -- -.
I-


. **obe **


C
1 .
0. 0

9O Vi


DCA' Axis I



Fig. 2-4. Ordination of sample vegetation scores at
freshwater, oligohaline, strongly oligohaline, and
mesohaline sites from DECORANA output. Freshwater and
oligohaline sample points are contained within the broken
line.


*0
*
























CU
S*I
S Z. miliacto

< ,* .

qF* 0 0
0 S. validus *


*
0 0
*


S E. montevidensis
I I I I I I

DCA Axis I


Fig. 2-5. Re-ordinations of sample vegetation scores for
freshwater and olighaline sites from DECORANA output.
Arrows indicate decreasing dominance by S. validus and
increasing dominance by the species to which the arrow is
pointing.








25

classes included several relatively rare species, including

Leersia virginica, Hypericum spp., Mikania scandens, Leersia

spp., Aneilema keisak, Aster elliotii, Galium tinctorium,

Panicum spp., Sagittaria spp., Polyqonum spp., Bidens

laevis, Amaranthus cannabinus, and Alternanthera

philoxeroides.

Re-ordination of the freshwater and oligohaline sites

(Fig. 2-5) shows a separation of freshwater species. The

separation of perennials in the freshwater marsh along the

first and second axes corresponded to both elevation and

distance from tidal channels.

DF analysis for the 4 sites combined (Table 2-1) was

highly significant (Wilks-Lambda = 0.05; DF = 24/1529; P <

0.0) in classifying (or separating) vegetation classes and

more than half of the vegetation classes (60%) were

correctly classified. Misclassification (those vegetation

classes which were not successfully matched with their

corresponding environmental factors) fell almost exclusively

into classes containing the same species with different

dominants, or different combinations of species with at

least one species in common. Interstitial salinity,

elevation, and distance from primary channel segregated the

majority of vegetation classes. Means for salinity,

elevation, and distance from tidal channel were

significantly different among vegetation classes (P =

0.0001) and as a result were retained in the DF analysis


















Table 2-1. Discriminant analysis results of vegetation
classes from freshwater (FRESH), oligohaline (OLIGO),
strongly oligohaline (S. OLIGO), and mesohaline (MESO) sites
on the lower Savannah River. A. Classification matrix. B.
Significance of the variable salinity (SAL) in separating
vegetation between sites.



A. Wilk's-Lambda = 0.18; F = 1743.28; DF = 3/1144; P =
0.0000.



Class Percent cases correctly classified



FRESH OLIGO S. OLIGO MESO



FRESH 87 13 0 0
OLIGO 37 47 17 0
S. OLIGO 9 13 68 9
MESO 0 2 19 79



B. Variable R2 F P>F



SAL 0.82 1743.3 0.0000








27
(Table 2-1). Interstitial salinity means were significantly

different between the four vegetation classes (freshwater

marsh, oligohaline marsh, strongly oligohaline marsh, and

mesohaline marsh). Percent of vegetation correctly

classified by environmental variables was 87%, 47%, 68%, and

79%, for freshwater, oligohaline, strongly oligohaline, and

mesohaline sites, respectively.

Within the vegetation classes previously separated by

salinity, vegetation was further differentiated into the

subclasses by elevation and distances from primary and

secondary channels (Tables 2-2 through 2-5). Means for

elevation and distances from primary and secondary channels

were significantly different among vegetation subclasses for

freshwater and mesohaline marsh sites (Tables 2-2 and 2-5).

In the oligohaline and strongly oligohaline marsh sites,

distance from secondary channels did not contribute to

differentiating subclasses; however, means for elevation and

distance from primary channels were significantly different

(Tables 2-3 and 2-4). In several cases, for example the

secondary channels in the freshwater marsh class and

channels in the mesohaline marsh class, the correlation

coefficient was low and the F score indicates that the

variable was significant but a minor contributor to the

discrimination. DF analysis was also significant in

classifying vegetation within sites (Tables 2-2 through 2-

5). Interstitial salinity was not retained as a significant














Table 2-2. Discriminant analysis results of vegetation
classes from freshwater sites on the lower Savannah River.
A. Classification matrix and B. significance values.
Significant variables were elevation (ELEV), distance to
primary channel (CHANNEL), and distance to secondary channel
(SECCHAN).



A. Wilk's-Lambda = 0.24; F = 113.26; DF = 9/1260.83; P =
0.0001.



Class Percent cases correctly classified"



E EX Z ZX



E 78 0 22 0
EF 17 65 18 0
Z 0 0 66 34
ZX 0 0 0 100



B. Variable R2 F P>F



ELEV 0.44 136.5 0.0001
CHANNEL 0.49 168.3 0.0001
SECCHAN 0.10 19.1 0.0001


"E = predominantly E. montevidensis; EX = E. montevidensis
mixed with several freshwater annuals, Zizaniopsis
miliaceae, Scirpus validus, Sagittaria spp., and Hydrocotyle
umbellatum; Z = Z. miliaceae; ZX = predominantly Z.
miliaceae, mixed with annuals, E. montevidensis, Scirpus
validus, Sagittaria spp., and Hydrocotyle umbellatum.
















Table 2-3. Discriminant analysis results of vegetation
classes from oligohaline sites on the lower Savannah River.
A. Classification matrix and B. significance values.
Significant variables were elevation (ELEV), distance to
primary channel (CHANNEL).


A. Wilk's-Lambda = 0.18; F = 1743.28; DF = 3/1144; P =
0.0000.



Class Percent cases correctly classified



FRESH OLIGO S. OLIGO MESO



FRESH 87 13 0 0
OLIGO 37 47 17 0
S. OLIGO 9 13 68 9
MESO 0 2 19 79


B.


Variable R2 F P>F


SAL 0.82 1743.3 0.0000


aES = predominantly E. montevidensis, mixed with annuals,
Zizaniopsis miliaceae, Scirpus validus, Sagittaria spp., and
Hydrocotyle umbellatum; Z = Z. miliaceae; ZX = predominantly
Z. miliaceae, mixed with annuals, E. montevidensis, S.
validus, Sagittaria spp., and Hydrocotyle umbellatum.


















Table 2-4. Discriminant analysis results of vegetation
classes from strongly oligohaline sites on the lower
Savannah River. A. Classification matrix and B.
significance values. Significant variables were elevation
(ELEV) and distance to primary channel (CHANNEL).



A. Wilk's-Lambda = 0.23; F = 61.50; DF = 6/336; P = 0.0001.



Class Percent cases correctly classified



S SZ



S 79 21
SZ 14 86



B. Variable R2 F P>F



ELEV 0.03 5.28 0.0228
CHANNEL 0.34 84.34 0.0001

"S = S. validus; SZ = S. validus and Zizaniopsis miliacea.

















Table 2-5. Discriminant analysis results of vegetation
classes from mesohaline sites on the lower Savannah River.
A. Classification matrix and B. significance values.
Significant variables were elevation (ELEV), distance to
primary channel (CHANNEL), and distance to secondary channel
(SECCHAN).



A. Wilk's-Lambda = 0.23; F = 61.50; DF = 6/336; P = 0.0001.



Class Percent cases correctly classified"



PR PS S



PR 88 0 12
PS 13 67 20
S 0 0 100



B. Variable R2 F P>F



ELEV 0.35 46.9 0.0001
CHANNEL 0.03 2.9 0.0578
SECCHAN 0.57 112.72 0.0001


"PR = Spartina alterniflora and Scirpus robustus; PS = S.
alterniflora and S. validus; S = S. validus.








32
factor in classifying vegetation within sites except at the

freshwater site. Elevation and distance to channel were

significant in the DF analysis at the freshwater,

oligohaline, and mesohaline sites, while only distance to

channel was significant at the strongly oligohaline site.

Dominance-diversity curves (Fig. 2-6) are graphs of

relative importance (y-variable) as a function of species

ranking (x-variable), and represent relative species

dominance, or "dominance hierarchies" of a species

association. In Fig. 2-6, the strongly oligohaline and

mesohaline sites were steeper, with fewer species, and

correspond to Whittaker's (1965) geometric curve.

Freshwater and oligohaline curves exhibit a more gradual

slope and were more similar to lognormal curves

characteristic of higher species diversity and more discrete

levels within the hierarchies.

Discussion

The vegetation of tidal freshwater, oligohaline, and

mesohaline marshes of the lower Savannah River is typical of

that described by Baden et al. (1975) for Georgetown County,

South Carolina, and for southeastern coastal marshes in

general (Odum et al. 1984, Mitsch and Gosselink 1986).

Dominant plant species in the tidal freshwater marsh

included Z. miliaceae and Polygonum spp. along levees, and

Sagittaria spp., Scirpus spp., Typha spp., Eleocharis spp.,

and several others throughout the low and high marsh.

































- Freshwater

1.2-


Strongly T
Oligohaline oligohaline Mesohaline


*


UU
U


U

iU
U
U



a
U-


Species rank













Fig. 2-6. Dominance-diversity curves for species importance
values (IV) at tidal freshwater, oligohaline, strongly
oligohaline, and mesohaline sites. Each species at each
site is represented by a point located by that species' mean
IV on the Y-axis, and its position in the sequence of
species from highest to lowest IV on the X-axis. IV =
relative density + relative biomass.


1-
i



0.6

04-
i


"2.


2I 2z 3 7 z










Dominant species in the more brackish marshes included

Spartina cynosuroides, Baccharis halminifolia, and Myrica

cerifera along levees, and S. alterniflora, S. validus, and

S. robustus in the low and high marsh.

The ordination from DECORANA output successfully

separated freshwater from oligohaline and mesohaline

vegetation. Ordination of the freshwater site further

separated levee (Z. miliaceae), low marsh (S. validus and E.

montevidensis mix), and back marsh (S. validus and T.

latifolia mix) vegetation. Results emphasized the

transition from freshwater species, including Z. miliaceae,

E. montevidensis, S. validus, T. latifolia, and annuals, to

dominance by S. alterniflora at the mesohaline site.

DCA results suggested the main axis of variation in

species composition between the four sites was salinity.

These results support several studies (Odum 1988, Ewing

1983, Phleger 1971, Adams 1963) in which salinity was

considered the driving force in determining differences in

species diversity and plant distributions between tidal

freshwater and saltwater marshes. Salinity differences

between sites corresponded to changes in species

composition, although changes were marked by a gradual

transition of species, rather than distinct community types

usually evident when comparing freshwater with brackish

marshes (Odum 1988). Much of the overlap in vegetation

classes along the salinity gradient can be attributed to the










widespread distribution of S. validus. Not only was S.

validus a component species of 6 of the 10 vegetation

classes, it was also the only species present to tolerate

the range of physical factors present over the entire study

area. Early successional species have been shown to have

broad, overlapping niches in comparison with later

successional species, which show greater niche

differentiation (Parrish and Bazzaz 1982). The extensive

overlap of S. validus with other vegetation classes may

reflect its broad fundamental niche, as well as its

successional status as an early colonizer that may be

displaced.

Continuous overlap in vegetation classes may also

reflect a less abrupt salinity gradient (Beals 1969) from

freshwater to low salinity marshes. Saltmarshes, in

contrast, often have a steep salinity gradient accompanied

by distinct plant zonation that coincides with high and low

marsh (Phleger 1971, Kruczynski et al. 1978). Steeper

environmental gradients place competitors in a smaller area

of optimal environment, and interactions among competiting

individuals may be unavoidable. Over a broader gradient,

optimal environment occurs over a larger area and

individuals may escape competition just by the availability

of more room and less chance for direct contact.

The first DF analysis illustrated between-community

differences. The relationship between vegetation classes










and salinity, elevation, and distance from channel was

significant, although overlap among vegetation classes was

extensive. The second DF illustrated within-community

vegetation differences and indicated that elevation and

distance from channel were important in classifying

vegetation. Elevation largely determines hydroperiod, and

subsequent effects of hydroperiod on vegetation composition

and distribution are well documented (Mitsch and Gosselink

1986). Although there was a total elevational range of only

about 30 cm at each site, low and high marsh are exposed to

different flooding regimes, and vegetation zones reflected

these differences. The distance from a tidal channel

affects the tidal or river current energy reaching different

portions of the marsh, the soils deposited in the marsh, as

well as the hydroperiod. As a result, distance to channels

is also related to plant species richness and community

composition (Nilsson 1987). Distance from a tidal channel

appeared more important in the distribution of Sagittaria

spp. and T. latifolia, for example, which occur

predominantly in the back (or high) marsh of the freshwater

marsh. Distance from channel may reflect differences in

soil consolidation. Because coarser soil materials are

filtered out by vegetation, these soils drop out of the

flooding water near the levee first, and only very fine

silts are transported to the back marsh. Marsh that is not

adjacent to tidal channels often has a partially








37
consolidated surface held together by vegetation (a "quaking

marsh"), or even an unconsolidated surface with nearly

floating vegetation, which may limit the number of species

able to exist in these areas.

Infrequent misclassification of vegetation classes in

the DF analysis occurred when a vegetation class overlapped

with another in regards to where it occurred it terms of

site salinity, elevation, and distance from channel.

Classes that overlap share similar habitats based on the

measured parameters, and have been shown to occupy the same

or similar niches (McNeely 1987). The overlap itself gives

no indication of resource preferences of overlapping

species, although it does indicate the functional or

realized niche of species (Colwell and Futuyma 1971), i.e.,

what habitat or resource is being exploited. While these

results imply overlapping niches among vegetation classes,

the extensive overlap most likely demonstrates the

similarity in resource requirements and fundamental niche

exhibited by most plants (Parrish and Bazzaz 1982, Goldberg

and Werner 1983).

Niche width generally decreases as species numbers

(McNaughton and Wolf 1970, Pianka 1978) increase and

physiological stress decreases (Whittaker 1975, Anderson

1986). This suggests that niche overlap should be least at

sites with greater plant species numbers and no salinity

stress. Our results seemed at first to contradict this: 41










plant species were identified at the freshwater site where

salinities were lowest, compared with 8-12 at each of the

oligohaline and mesohaline sites, yet the greatest overlap

between vegetation classes occurred at the freshwater site

(Table 2-2). The apparent contradiction, however, most

likely reflects the steeper environmental gradient of the

mesohaline marsh which makes the relationship between

vegetation and environmental gradients more discernible.

Importantly, differences in overlap between freshwater

and brackish sites also emphasize the problem of

measurements of different scale. It has recently been

suggested that differences in "hierarchical environmental

structure" may explain differences at the community level

between severe and relatively benign environments;

homogeneity or heterogeneity of a habitat depends on the

resolution with which species experience their microhabitats

(Kolasa and Strayer 1988, Kolasa 1989). Extensive habitat

overlap in the freshwater marsh may indicate a "finer

grained" or more homogeneous habitat in which habitat

differentiation is not discrete at the level measured, while

the steeper environmental gradient of the oligohaline and

mesohaline sites exhibit greater differences over the same

amount of area and can be considered "coarse grained"

habitats (Kolasa 1988). Overlap in vegetation classes is

significant only in regards to the environmental parameters

measured and the resolution with which they were measured.










Results reflect the scale of measurements made and used in

classifying vegetation at each site.

Dominance-diversity curves of species from each site

illustrate the contrast in "hierarchical structure" between

sites. The geometric curve of the mesohaline and strongly

oligohaline marshes are characteristic of severe

environments with small numbers of species (Whittaker 1975).

Here, dominance is strongly developed and niche space is not

evenly divided among several species. Species dominance in

the freshwater and oligohaline marsh correspond more closely

to a lognormal distribution (Preston 1948) and is determined

by several factors that affect the competitive success of

species relative to one another. Species rich communities

and communities in which species occur across a wide

environmental range approach such lognormal distribution.

These curves represent freshwater plant species

distributions across benign environmental gradients which

exhibit only slight changes in regulating factors, thus many

plants compete equally for resources. In contrast, steep

gradients and physiological stress of brackish marshes may

result in well-developed dominance among a few species

competing for the same resources.

Physical parameters such as hydroperiod, salinity, and

soil characteristics have been strongly associated with

plant distributions. While these factors may be. the

dominant factors affecting plant associations in the










Savannah River marshes, competition is often important in

structuring plant communities (Bertness and Ellison 1987,

Barbour 1978), and it has been suggested that overlap in

resource utilization indicates competition among some

prairie plants (Platt and Weis 1977). Several studies have

shown species may be differentially displaced by competition

along an environmental gradient (Barbour 1978, Rabinowitz

1978, Grace and Wetzel 1981, Snow and Vince 1984). Studies

with Jaumea carnosa (Barbour 1978), S. alterniflora (Adams

1963), Spartina foliosa (Phleger 1971), and Typha

angustifolia (McMillan 1959) have found that these species

grow better in freshwater but are confined to higher

salinity marshes by species better specialized to freshwater

conditions. Competitive interactions may be more important

to within-community structure, while salinity and other

physical parameters exert a greater influence on between-

community structure.

The importance of competition cannot be overlooked in

any explanation of plant distributions. While results of

the present study are not sufficient to address the role of

competition in structuring these plant associations,

competition will be addressed in subsequent chapters.














CHAPTER 3
SPATIAL DISTRIBUTIONS OF THE SOFTSTEM BULRUSH,
SCIRPUS VALIDUS, ACROSS A SALINITY GRADIENT


Introduction

Plant species distributions in tidal marshes have been

attributed predominantly to environmental gradients of

salinity (Phleger 1971, Morris et al. 1978, Leiffers 1983,

Haramis and Carter 1983, White 1983), site elevation and

hydroperiod (Ferren and Schuler 1980, Lieffers 1983, Dawe

and White 1982, White 1983), and soil characteristics (Kirby

and Gosselink 1976). In salt marshes, where environmental

gradients are often steep and correlated with one another,

they may be strongly associated with community structure and

species distribution patterns (Snedaker 1982, Hoover 1984).

Clumped distribution pattern or zonation of salt marshes may

indicate a species response to environmental gradients

(Pielou and Routledge 1976, Cooper 1982, Vince and Snow

1984), competition (Snow and Vince 1984, Bertness and

Ellison 1987), or physical disturbance (Turner 1987,

Bertness and Ellison 1987).

Zonation in tidal freshwater marshes is much less

distinct than in salt marshes (Odum 1988). There is greater

overlap in habitat (de la Cruz 1981, Joyce and Thayer 1986,

Odum 1988), and relations among species and environmental

41








42
factors are more complex (Hoover 1984, Odum 1988), obscuring

species zonation.

Although the importance of pattern in plant species

distributions has received considerable attention (Pielou

and Routledge 1976, Mack and Harper 1977, Dale 1986,

Shaltout 1987), comparative studies are generally restricted

to species distributions within a community type (e.g. salt

marsh) and do not address community distribution patterns

between community types (e.g. tidal freshwater to salt

marshes). Because species interactions occur at the level

of individual neighbors (Mack and Harper 1977, Fowler and

Antonovics 1981) the distribution patterns of a single

species may affect larger scale community structure (Harper

1977, Fowler and Antonovics 1981, Dale 1986).

In this study, the distribution patterns of Scirpus

validus, a dominant species in both freshwater and brackish

marshes along the lower Savannah River, were examined. The

objectives were to (1) determine the distribution patterns

of S. validus, and (2) examine the role of environmental

factors in the distribution of S. validus.

Methods

Mean number of species at each site and species

similarity between sites (Whittaker 1975; I, = (total

species at site x plus the total species at site y)/ number

of species common to both sites) were calculated.








43
Between-site differences in environmental variables and

species numbers were compared using Waller-Duncan multiple

means comparison tests. The same test was used to compare

changes in soil organic matter associated with distances

from primary creeks along transects. Elevation changes

along transects were compared using Tukey's standardized

range test.

Coefficients of dispersion (CD) were calculated for

each site, for each sampling period, as the variance to mean

ratio of standing densities of S. validus. The coefficients

were used as a measure of the amount of nonrandom dispersion

of stems (see Sokal and Rohlf 1981, Whittaker 1975, Kershaw

1973). A CD value equal to 1 indicates a random

distribution of individuals. Clumped distributions have a

CD > 1, and a CD < 1 represents a uniform distribution.

Paired t-tests were used to determine departures from

randomness (i.e., CD values significantly different from 1).

Between-site differences in CD values were compared using

Waller-Duncan multiple means comparison tests.

Multiple regression analysis was used to examine the

relations of S. validus importance values (IV) with measured

environmental parameters at sites (see Methods and Results,

Chapter 1). Regression analysis was also used to examine

the relationship of species numbers and IV with the salinity

gradient across all 4 sites.










Results

Species Associations

The average number of species was significantly greater

at the freshwater site (F = 28.73; N=27; P = 0.05). The

number of species was not significantly different among the

oligohaline, strongly oligohaline, and mesohaline sites.

Freshwater marsh had an average of 18 species, compared to

less than 7 in the oligohaline, 3 in the strongly

oligohaline, and 5 in the mesohaline marsh (Fig. 3-1). The

large number of species in the freshwater marsh was due

mostly to the presence of annual species. Similarity in

species composition was greatest between the strongly

oligohaline and mesohaline and least between freshwater and

mesohaline marsh sites (Table 3-1).

The highest mean IV for S. validus (Fig. 3-2) occurred

in the strongly oligohaline marsh (Mean = 1.16), and the

lowest at freshwater (Mean = 0.39) and mesohaline (Mean =

0.41) sites. Scirpus validus was surpassed in IV by

Zizaniopsis miliaceae and Eleocharis montevidensis at the

freshwater site, and by Spartina alterniflora in the

mesohaline marsh. Scirpus validus had the highest IVs at

oligohaline and strongly oligohaline sites.

Spatial Pattern

Differences in interstitial salinities were accompanied

by differences in spatial pattern in S. validus (Fig. 3-3).

Coefficients of dispersion (CD) exceeded 1.0 (P = .05) at





























I 15



10 -
z
--I-
5--4 -



0
Freshwater Oligohaline S. Oligohaline Mesohaline




Fig. 3-1. Mean number of species ( 1 standard error) at
freshwater, oligohaline, strongly oligohaline (S.
Oligohaline), and mesohaline sites. Means falling within
the same vertical line to the right of graph are not
significantly different (Waller-Duncan, P = 0.05).



























Table 3-1. Similarity in species composition (Is) between
freshwater, mildly oligohaline, strongly oligohaline, and
mesohaline sites.



Site Fresh Oligo S. Oligo Meso


Fresh 1.00

Oligo 0.51 1.00

S. Oligo 0.20 0.43 1.00

Meso 0.12 0.25 0.71 1.00
















1.2-


0.8-

0.6

0.4

0.2


+ I




I I



1 -11


Freshwater Oligohaline S. Oligohaline Mesohaline




Fig. 3-2. Mean Importance Values ( 1 standard error) for S.
validus at freshwater, oligohaline, strongly oligohaline (S.
Oligohaline), and mesohaline sites. Means falling within
the same vertical line to the right of graph are not
significantly different (Waller-Duncan, P = 0.05).


n' -

















22
2- +
1.8
1.6-
1.4 -
1 1 I


E 08
0.6 I
0.4 -
0.2-
0
Freshwater Oligohaline S. Oligohaline Mesohaline





Fig. 3-3. Mean Coefficients of Dispersion ( 1 standard
error) for S. validus over a gradient of increasing soil
water salinities at freshwater, oligohaline, strongly
oligohaline (S. Oligohaline), and mesohaline sites. Means
falling within the same vertical line to the right of graph
are not significantly different (Waller-Duncan, P = 0.05).










the freshwater (Mean = 1.41; STD = 0.28; n = 7), and

mesohaline (Mean = 1.84; STD = 0.55; n = 7) sites,

indicating clumped distributions. CD values of S. validus

at the strongly oligohaline site (Mean = 0.62; STD = 0.28; n

= 7) were significantly less than 1.0, indicating a uniform

distribution of S. validus. Only the oligohaline site (Mean

= 1.15; STD = 0.34; n = 7) exhibited a random distribution

of S. validus (P = 0.24). CD values indicated

significantly greater clumping in the mesohaline marsh when

compared to the freshwater and oligohaline sites (F = 12.73;

P = .05).

Relationship of IV with Environmental Variables

Importance values for S. validus (Fig. 3-1) showed a

significant quadratic relationship with increasing soil

water salinities (y = 0.35 + 0.4x 0.03x2; R2 = 0.16; n =

448; P = 0.0001). Scirpus validus IVs were significantly

related to soil water salinities and elevation at all but

the oligohaline site (Table 3-2). R-square values for

multiple regressions were less than 0.30 for all 4 sites.

Salinity and elevation added significantly to the regression

model in determining S. validus IV at the freshwater site (P

= .05). Elevation, but not salinity, was significantly

related to IV at the mildly oligohaline (P = .05) and

mesohaline (P = .05) sites. Only salinity was significantly

related to IV at the strongly oligohaline site (P = .05).


























Table 3-2. R-square, n, F, and P values for multiple
regression analysis of S. validus IVs with interstitial
salinities (ppt), elevation (cm) and percent soil organic
matter at freshwater, mildly oligohaline, strongly
oligohaline, and mesohaline sites.



Site R2 N F


Fresh 0.23 103 4.80"

Oligo 0.13 87 1.96'

S. Oligo 0.25 100 5.23"

Meso 0.27 91 5.11"

*P = 0.05; "P = 0.01










Soil organic content was not significant in determining S.

validus IVs at any site.

Discussion

Species Diversity

The decrease in species numbers and replacement by

other, more salt tolerant species across the 4 sites were

typical of community changes from freshwater to brackish

marshes. Species similarity was greatest for those sites

with similar salinity regimes and least for those with

greatest differences in soil water salinities. Seasonal

differences in species composition due to annuals strongly

influenced species diversity in freshwater marshes. High

species similarity between adjacent sites along the

continuum from freshwater to mesohaline marsh, and the

overlap of S. validus throughout the gradient emphasize the

gradual shift in species composition from freshwater to

brackish marsh plant communities in this study.

Distribution Patterns

Coefficients of dispersion indicate a significant

change in spatial pattern for S. validus across the salinity

gradient. Scirpus validus was clumped in monospecific

stands at the mesohaline site. Significantly less clumping

occurs in freshwater and very low salinity marshes, where S.

validus was interspersed with freshwater plant species. At

the strongly oligohaline site, S. validus had a uniform

spatial distribution, rather than the discrete clumps found










52
clumps found in the mesohaline marsh. Species diversity was

low and IVs of S. validus high at intermediate salinities.

Uniform and clumped distributions, while not proof of

competition or environmental specialization, do indicate

that more than random chance is at work in producing or

maintaining the pattern. The uniform, or regular,

distribution of individuals occurs when available space is

almost fully occupied (Pielou 1959), allowing each

individual the same amount of space. While changes in

pattern among most wetland species have not been examined,

some desert shrubs exhibit uniform distributions as a result

of intra-specific competition (King and Woodell 1973,

Shaltout 1987). Intraspecific competition may explain the

distribution of S. validus at the strongly oligohaline site

in this study, where freshwater species are physiologically

intolerant and Spartina alterniflora is infrequent except at

the edges of canals and the river. With few co-occurring

species and high densities of S. validus, neighborhood

competition among individuals may be limited to competing

individuals of S. validus, resulting in a uniform

distribution pattern.

Results from several studies (Sharitz and McCormick

1973, Grace and Wetzel 1981, Snow and Vince 1984, Wilson and

Keddy 1986) suggest that along an environmental gradient,

competitive interactions may determine species distributions

at one end of the gradient where tolerance ranges overlap,










while physiological tolerance limits species distributions

at the other end of the gradient. Separate multiple

regressions of S. validus IV with environmental factors at

each site indicate a stronger relationship at the mesohaline

site. This suggests that environmental gradients are

primarily responsible for the clumping of S. validus at the

mesohaline site, where salinities are highest. The clumping

pattern reflects dominance of single species in zones, and

possibly greater habitat differentiation in the more saline

marsh between a limited number of species. Competition,

environmental tolerance, disturbance, and vegetative growth,

all probably play a role in explaining the clumped

distribution of S. validus.

Given the wide environmental tolerances of S. validus

and the large number of species at the freshwater site, the

distribution pattern at this site may be influenced more

strongly by competitive interactions. Resources must be

shared with neighboring individuals and individual stems

occur much farther apart. Variations in light, created by

differences in plant architecture and growth form (Jones

1983), and subsequent competition for light and space, may

account for variation in densities of S. validus in the

freshwater marsh.

While physiological mechanisms alone do not explain the

quadratic relationship of importance values for S. validus

with increasing soil water salinities, the relationship may










be explained by a combination of physiological and

environmental mechanisms. Scirpus validus is distributed

across a broad environmental gradient along the lower

Savannah River. A wide range of environmental tolerances

has been reported for this wetland plant (Beal 1977, Barko

and Smart 1978, Langeland 1981, Barclay and Crawford 1982,

Joyce and Thayer 1986), which reproduces readily both vege-

tatively and by seed (Godfrey and Wooten 1979). Like

saltwater and brackish marsh plants that generally grow best

in freshwater but are limited by competition (Mooring et al.

1971, Phleger 1971, Barbour 1978), S. validus may also be

limited by competition to intermediate salinities where

freshwater species cannot survive and extreme salt tolerant

species cannot compete well.

The quadratic relationship between IV and salinities

also reflects pattern differences among sites. At the

strongly oligohaline site, where the CD value was lowest, S.

validus exhibited the highest stem densities and occurred

with few other species. Salinities at this site appear to

be high enough to limit freshwater species without

inhibiting S. validus.

Lower IVs in the freshwater site reflect low densities

of S. validus mixed with many other species in a clumped

distribution, but less clumped than at the mesohaline site.

Generally, a. validus occurred in very high densities or not










at all in the mesohaline marsh due to increased species

donation.

Elevation, but not salinity was significantly related

to S. validus IVs in mildly oligohaline marsh. Higher

salinities extend farther upstream in the Middle River than

in the Little Back River. Consequently, interstitial

salinities at the mildly oligohaline site reflect higher

salinities of the Middle River on the northwest side of the

marsh and lower Back River salinities on the southeast side.

While variation in salinities was greatest at this site, it

was still significantly different from other sites.

Significant changes in spatial distribution along

physical gradients for S. validus in this study suggest that

salinity strongly influences its distribution, but not

directly. Scirpus validus is a generalist species, capable

of growing well over a wide range of salinities. Many other

species perform as well or better at low salinities and in

freshwater, but resources must be shared by all species

under these conditions. As salinities increase, less

species are physiologically tolerant, and S. validus occurs

as a dominant species. As salinities increase across the

salinity gradient, Spartina alterniflora and Scirpus

robustus are able to better compete for resources, and

dominance by S. validus decreases.

Within individual sites, the influence of a salinity

gradient appears to be replaced by less distinct elevational








56
differences, and changes in species distributions also

become less distinct. In this study, elevation was a

significant factor in determining IVs of S. validus within a

site. Low elevations at the mesohaline site resulted in

increased hydroperiod which, combined with increased

salinities, may account for the increase in dominance by S.

alterniflora and zonation of S. validus in the higher marsh.














CHAPTER 4
MORPHOLOGICAL PLASTICITY IN SCIRPUS VALIDUS
ALONG A SALINITY GRADIENT


Introduction

Results of several transplant experiments have

demonstrated both ecotypic and ecophenic variation in plant

morphology along environmental gradients. After being

transplanted to different habitats, morphological

characteristics of glasswort (Salicornia europaea) (Ungar

1987, Jefferies 1981), cinquefoils (Potentilla erecta)

(Watson and Fyfe 1975), and yarrow (Achillea millefolium)

(Clausen et al. 1948), were shown to remain constant,

exhibiting ecotypic variation.

Species from contrasting habitats may also have a

morphology that changes in response to the environment. In

experiments with Spartina alterniflora (Shea et al. 1975),

and several other salt marsh species (Seliskar 1985), plants

that were moved from one end of a salinity gradient to

another became morphologically similar to their new

neighbors, exhibiting ecophenic, rather than ecotypic,

variation in contrasting habitats.

Salt marshes often exhibit gradients of increasing soil

salinities from low to high marsh as a result of less

frequent tidal inundation and increased evaporation and

57










plant evapotranspiration in the high marsh. Decreases in

density, biomass, and/or height for Sporobolus virginicus

(Donovan and Gallagher 1985), Spartina foliosa (Phleger

1971), Juncus roemerianus and Spartina alterniflora

(Kruczynski et al. 1978) have been associated with the

increased salinities from low to high marsh in field and

greenhouse experiments.

Tidal amplitudes along the lower Savannah River in

South Carolina and Georgia range from 2.5 to 3 meters,

inundating most of the marsh, and eliminating any salinity

gradient from low to high marsh at a given site. Marshes

are, however, associated with the first 28 miles of the

river from the ocean, and there is a wide salinity gradient

downstream to upstream. In this region, Scirpus validus is

the only species consistently present throughout tidal

freshwater, oligohaline, and mesohaline marshes that

otherwise differ strongly in species composition.

The objectives of this study were to test 1) whether

morphological variation in S. validus is significant between

contrasting habitats, 2) whether morphological variation in

S. validus is ecotypic or ecophenic, and 3) whether measured

differences in morphology are directly related to salinity.

Light extinction was also measured to determine whether it

may have a more direct influence than salinity on

morphological variation in S. validus. Because different

growth morphologies may influence macrophyte community








59
composition (Barko and Smart 1981), variation in S. validus

morphology may further understanding of factors which help

structure plant communities in this area.

Methods

The study area included freshwater (0 ppt), oligohaline

(5-7 ppt), and mesohaline (9-10 ppt) tidal marshes of the

lower Savannah River, in Chatham County, Georgia, and Jasper

County, South Carolina. Vegetation composition in the study

area is similar to other non-cultivated, naturally occurring

tidal marshes of the southeastern coast (Odum et al. 1984, and

see Ch. 1 and 2).

In July 1987, S. validus stem densities, internode

lengths of rhizomes, and stem heights were measured.

Measurements were made on 10 replicate S. validus stems from

10 random, 0.25 m2 plots at each site. Variation in S.

validus stem heights and internode lengths were calculated

using the coefficient of variation, CV = (standard deviation

/ mean) x 100. CV values indicate the relative amount of

variation in stem height and internode length within each

site.

In December 1987, S. validus in 0.25 m2 plots,

approximately 15 cm deep, were randomly collected from the

oligohaline marsh site. Twenty of these plots were

transplanted to the freshwater site and 20 to the mesohaline

site. Five additional plots at the donor site were removed

and then replaced as controls to determine if transplanting










effects were significant. Eighteen undisturbed plots

previously established at each site were used for comparison

with transplants and controls (A Reichert refractometer was

used to measure and monitor soil water salinities at field

sites and in the greenhouse). Above-ground vegetation from

transplants, controls, and undisturbed plots was harvested

in June 1989. Stem heights and densities were measured for

each plot. Because there was no way to determine which

parts of rhizomes were produced during the 1.5 years at

transplant sites, internode distances were not measured for

transplants.

In December 1988 a total of 60 rhizome sections of S.

validus were collected randomly from freshwater,

oligohaline, and mesohaline sites. Sections were washed

free of soil and debris, cut into node sections 2.5 cm in

length, and planted in shallow greenhouse trays of potting

soil. Three treatments, each consisting of 2 trays from

each site and 10 sections per tray, were watered daily with

0, 5, and 10 ppt seawater solution for 6 months.

Differences in light extinction between sites and

between vegetation types were also measured in December

1988. Light intensity (uE x m2 x sec-1) was measured at the

top of the vegetation canopy and at ground level using a

LICOR quantum radiometer (Model # LI-185B) and a flat

quantum sensor. Light extinction coefficients through the

vegetation were calculated from the LICOR measurements.










Three replicate readings were taken at 10 random points

within each vegetation type and percent cover of vegetation

was recorded at each site. Vegetation types at the

mesohaline site were based on dominant species. Vegetation

types were S. validus, Spartina alterniflora, and Spartina

cynosuroides in the mesohaline marsh. Vegetation types in

the freshwater marsh were Eleocharis montevidensis, S.

validus mixed with Zizaniopsis miliaceae, and S. validus

mixed with Eleocharis montevidensis.

Waller-Duncan multiple means comparison tests (P =

0.05) were used to compare greenhouse measurements, field

measurements, and light extinction treatments. Means

comparisons were also used to determine if differences were

significant (P = 0.05) between transplanted plots, control

plots, and undisturbed plots, within treatments. Student's

t-test (P = 0.05) was used to compare transplanted densities

and stem heights between freshwater and mesohaline sites.

Results

Field Measurements

Scirpus validus had shorter internode lengths, taller

stems, and greater densities at the oligohaline and

mesohaline sites when compared with the freshwater site

(Fig. 4-1). These traits were not significantly different

between oligohaline and mesohaline marsh sites, although

both differed significantly from the freshwater site.

Variation in stem height of S. validus was lowest at the
































Fig. 4-1. Mean ( 1 SE) internode lengths, stem heights,
and stem densities for S. validus from field sites and
greenhouse. Means falling within the same vertical line to
the right of graph are not significantly different (Waller-
Duncan, P = 0.05).








Field Greenhouse
190 500-





i-i
S+ + +

l40 350
140
I rc


100 200

300 160




S150 so + 1
a


P L

0 0

6 +
i E






0' 0
F O M
-3





F 0 M


- II


F 0 M










freshwater site for stem height (CV = 6.3), and higher at

the mesohaline (CV = 10.98) and oligohaline (CV = 12.4)

sites. Variation in internode distance was highest at the

oligohaline site (CV = 79.9), and lower at the freshwater

(CV = 48.4) and mesohaline (CV = 46.1) sites.

Greenhouse Experiments

Stems/m2 (and node survival) were greater overall for

freshwater treatments (Fig. 4-1). Stem densities and

surviving nodes did not differ between freshwater and

oligohaline treatments. The olighaline and mesohaline

treatments were not significantly different from one another

with respect to stem density. Stem heights of S. validus

were also significantly greater for freshwater and

oligohaline treatments when compared with mesohaline

treatments (Fig. 4-1), though stems did not grow to heights

measured in the field. Differences in internode lengths

between treatments were not significant.

Transplants

Scirpus validus transplanted from the oligohaline site

to freshwater and mesohaline sites changed significantly in

stem height and density after 18 months. In contrast with

greenhouse results, S. validus transplanted to higher

salinities had taller stems and greater stem densities when

compared with freshwater transplants (Fig. 4-2).

Stem heights of S. validus transplants from the

oligohaline site converged with values characteristic of the

site to which they were transplanted (Fig. 4-2). Stems of








140
4nn 7r


100-


Ir


60


20 -


TCU
TCU


CU
c u


TCU


TCU CU TCU
Freshwater Oligohaline Mesohaline


Fig. 4-2. Mean ( 1 SE) stem heights and stem densities
for S. validus transplanted from oligohaline marsh to tidal
freshwater and mesohaline marsh. Hatched bars represent
mean values for transplanted plots (T), solid bars represent










transplants were significantly shorter at the freshwater

site when compared with the mesohaline site. Differences in

stem heights were significant between freshwater transplants

and oligohaline control plots, while differences between

mesohaline transplants and oligohaline control plots were

insignificant.

Differences in S. validus stem heights between

transplant, control, and undisturbed plots were significant

at only the freshwater site (Fig. 4-2). Transplants at the

freshwater site had significantly smaller stem heights than

undisturbed plots and were significantly smaller than the

mean stem height at oligohaline and mesohaline sites. At

both freshwater and mesohaline sites, control plots had

significantly greater stem heights than S. validus

transplanted from the oligohaline site.

Stem densities of transplants to freshwater and

mesohaline sites decreased to densities that were not

significantly different from those of the control plots at

freshwater and mesohaline sites. Differences in stem

densities between freshwater transplants that originated at

the oligohaline site and control plots at the oligohaline

site were significant. Differences between mesohaline

transplants and oligohaline controls were not

significant. Undisturbed plots of S. validus at the

oligohaline site also had greater densities when compared

with plots transplanted to freshwater and mesohaline sites.










Differences in mean stem densities of S. validus between

transplant, control, and undisturbed plots were not

significant at any of the 3 sites (Fig. 4-2).

Transplants to the freshwater site were invaded

predominantly by Eleocharis montevidensis, Hydrocotyle

umbellatum, and Aster spp. Zizaniopsis miliaceae and Typha

latifolia, commonly co-occurring species with S. validus at

the freshwater site, were rarely present in the transplants.

Mesohaline transplants were invaded almost exclusively by

Spartina alterniflora.

Light Extinction

Light extinction through the vegetation to the soil

surface was significantly greater at the freshwater site

when compared with the oligohaline and mesohaline sites

(Fig. 4-3). Plant species and percent cover accounted for a

significant amount of variation in light extinction within

each marsh site (ANOVA; F = 2.43; P = 0.01; n = 107) (Table

4-1). Internode lengths were greatest, and stem heights and

densities lowest for S. validus at sites where light

penetration through the canopy was lowest.

Discussion

Detrimental effects of increased salinities on plant

growth were evident in decreased stem heights and densities

of S. validus in greenhouse experiments. These results were

similar to those for other studies in which salt tolerant

species were found to grow best under freshwater or low


























4


3.


5.5 1


5

.5

4-

.5 -


25 F


Freshwater


Oligohaline


Mesohaline


Fig. 4-3. Mean ( 1 SE) light extinction coefficients for
freshwater, oligohaline, and mesohaline sites. Means
falling within the same vertical line to the right of graph
are not significantly different (Waller-Duncan, P = 0.05).


























Table 4-1. Variation in light extinction coefficient
between and within sites.


Source of variation D.F. F value


Marsh 2 5.76"

% Open canopy 10 2.79*

Species (marsh) 5 2.42"

Species (% open) 2 6.49"

Replicates (marsh) 18 1.42(N.S.)

*Significant at P = 0.05 level; **Significant at P = 0.01
level; N.S. = not significant.










salinity conditions (Phleger 1971, Jackson 1952, Parrondo

1978, Cooper 1982, Seneca 1969), indicating that higher

salinities inhibit growth in these species.

Transplants of S. validus growing at the freshwater

site exhibited a significant decrease in stem heights and

densities compared with the mesohaline transplant site and

oligohaline donor site. In both greenhouse and transplant

experiments, however, variation in morphology was not

related to the salinity of the area from which the plants

were taken, suggesting that morphological variation in S.

validus is not genetically fixed. Variation was dependent

upon local conditions rather than plant origin, and

demonstrates that changes in S. validus morphology across a

salinity gradient are ecophenic, rather than ecotypic,

responses.

Salinity has been found to have an overriding effect on

growth and survival in some experiments with Salicornia

europeae (Ungar 1987). In contrast, several studies have

concluded that competition, disturbance, light penetration,

and herbivory were more important in the growth and

distribution of this particular species (Ellison 1987), as

well as others (Bertness and Ellison 1987). While S.

validus exhibited plastic responses to environmental

conditions in both greenhouse and transplant experiments,

growth patterns were reversed between the two. .Greenhouse

plants grew better at 0 ppt, while field transplants did








71
better at 5 and 10 ppt. Salinity had no effect on internode

length in greenhouse treatments, but field measurements of

internode lengths differed between freshwater, oligohaline,

and mesohaline field sites.

Discrepancies in stem heights, stem densities, and

internode lengths between greenhouse experiments and field

transplants indicate morphological plasticity observed in S.

validus was not solely attributable to salinity. Factors

other than salinity appeared to be important in the actual

distribution and growth morphology of S. validus. Scirpus

validus transplanted to the freshwater site was unable to

expand into immediately adjacent areas, where other species

were already present and overall stem densities were high.

Internode lengths at field sites were greater, averaging 2.2

cm, compared with an average of 0.5 cm in greenhouse plants.

Greenhouse plants, because they were planted at low

densities without other species present, could expand into

open areas where there was plenty of light and produce

shorter internode lengths and closely arranged stems.

Transplants of S. validus were invaded at both

freshwater and mesohaline sites by neighboring species until

densities and species composition reached those of

undisturbed plots at their respective transplant sites.

These changes suggested that interspecific competition may

be important in the morphology and distribution of S.

validus. Decreases in stem heights and densities, and










invasion of S. validus transplants by salt intolerant

species at the freshwater site may be due to greater

competition from freshwater species under these conditions.

Physiological specialization on a widely distributed

habitat type (e.g., freshwater marsh) results in ecological

generalization, whereas physiological generalization to

several habitat types (e.g., freshwater and brackish

marshes) results in ecological specialization (McNaughton

and Wolf 1970). Scirpus validus, a physiological

generalist, may be outcompeted at the freshwater site by

physiological specialists (e.g., Eleocharis montevidensis

and Typha latifolia) because it cannot compete with the

specialist within the specialist's range of physiological

tolerance.

Salinities at the oligohaline site were high enough to

inhibit freshwater species and low enough that stem heights

and densities of S. validus were highest, internode lengths

were shortest, and Spartina alterniflora was unable to

expand into the site any farther than river and creek edges.

Transplants at the mesohaline site exhibited decreased stem

heights and densities, and were invaded by S. alterniflora.

It may be that S. validus is not as great a physiological

generalist as S. alterniflora. Spartina alterniflora, more

tolerant of higher salinities, was able to invade and limit

the growth of S. validus, probably due to inhibitory effects

of higher salinities on S. validus.










Previous studies (Barbour 1978, Rabinowitz 1978) have

noted that plant species' distributions often do not

coincide with their physiological tolerances and that

competitive interactions may be responsible for displacement

of species from a habitat. Species' physiological

tolerances and competitive interactions have also been shown

to differ along an environmental gradient; physiological

tolerance may limit a species at one end of an environmental

gradient where conditions are relatively harsh, while

competitive interactions may be of greater importance along

the other, more benign portion of the gradient (Grace and

Wetzel 1981, Snow and Vince 1984, Wilson and Keddy 1986).

Differences in morphology and densities for S. validus

appear to be a result of differences in relative competitive

abilities and salt tolerance between S. validus and other

species. Scirpus validus was outcompeted at the freshwater

site due to greater competitive abilities of other species,

while it was limited by its own salt intolerance and

decreased competitive ability at the mesohaline site. More

specifically, differences in transplanted S. validus may be

a result of light extinction differences between sites and

the ability of S. validus to compete for available light by

morphological adaptation to differential light regimes.

Light-related morphological changes such as internode

elongation may confer a competitive advantage on a species

by enabling the plant to expand into less light-limiting










vegetation or gaps. Morphological variation and decreased

survival have been associated with decreased light

penetration in Salicornia europaea (Ellison 1987), and

decreases in mean internode length have resulted from

shading in submersed freshwater macrophytes (Spence and Dale

1978, Barko and Smart 1981, Barko et al. 1982). Numbers of

species and subsequent differences in canopy layers and

complexity accounted for a significant amount of the

variation in light extinction within each marsh site.

Differences in morphology at field sites may be a result of

responses to light limitations placed on S. validus by

neighboring individuals.

Morphological variation has been used to illustrate

competitive interactions among animal species. Data on the

morphology of birds (Van Valen 1965), lizards (Lister

1976a), and harvesting ants (Davidson 1978) provide evidence

that variation in morphology increases as interspecific

competition and the number of species decreases. This may

further explain why morphological differences in S. validus

coincide with species numbers more so than salinity

gradients. Coefficients of variation for field measurements

of S. validus stem heights and internode lengths were lowest

at the freshwater marsh, where species numbers were

greatest. While examples of morphological variation have

most often been described for species in relatively harsh

environments, for example, salt marshes, deserts, and








75

exposed mountain slopes, and may reflect steep environmental

gradients in these habitats, results of this study

demonstrate that morphological variation observed for S.

validus is not directly due to a strong environmental

gradient. A reduction in the range of morphological

variation expressed by S. validus in the freshwater marsh

may be due to increased species numbers and competitive

interactions and may account for the lower variation in stem

height and internode length in S. validus at this site.

Conversely, decreased species numbers and fewer competitive

interactions at higher salinities may allow a greater range

of morphological expression at the mesohaline site.

In conclusion, different growth morphologies in S.

validus are ecophenic responses to local abiotic and biotic

conditions. While salinity alone is not responsible for

variation in measured plant characteristics, it greatly

affects species composition, which in turn may reflect the

degree of physiological specialization and competitive

ability of species in freshwater and low salinity marshes.

Species composition also affects light availability. Not

only is light an important resource for which plants

compete, but it also influences morphological variation in

several species, and light appears important in S. validus

as well. Morphological variation in S. validus further

indicates differences in the degree of competitive









76
interactions between plants of tidal freshwater and brackish

marshes along the lower Savannah River.














CHAPTER 5

THE ROLE OF COMPETITIVE INTERACTIONS, SOIL SALINITY, AND
DISTURBANCE IN THE DISTRIBUTION OF TIDAL MARSH PLANT SPECIES

Introduction

Plant species of tidal freshwater marshes are numerous

and have narrow, well differentiated niches (Odum 1988), in

comparison with saline marshes, suggesting that competitive

hierarchies are well developed and competition among species

is diffuse rather than species-specific. Freshwater species

distributions may reflect better developed and more complex

competitive interactions characteristic of late successional,

environmentally benign communities. In contrast, brackish

marsh vegetation is more strongly influenced by salinity-

induced physiological stress and steeper environmental

gradients (Phleger 1971, Kruczynski et al. 1978)

characteristic of relatively early successional communities.

Species in such harsh environments generally have broadly

overlapping resource requirements, resulting in more intense

competitive interactions (Parrish and Bazzaz 1982), as well

as altered dominance hierarchies (Anderson 1986).

Few experiments have directly addressed the role of

competitive interactions in species distributions along

environmental gradients (see Chapter 4). Pair-wise species










competition experiments across a gradient of stress

and disturbance have demonstrated lower competitive

abilities for species where environmental stress is greater.

Conversely, where conditions are more benign, competitive

abilities are greater (Wilson and Keddy 1986). The combined

consequences of competition and environmental gradients on

plant interactions have not been directly examined. Studies

of salt marsh species distributions have shown that, in

addition to physical factors, species interactions (Bertness

and Ellison 1987, Bertness and Ellison 1987, Snow and Vince

1984) are important to plant community structure. Several

of these studies have also implied that along an

environmental gradient, competitive interactions may

determine species distributions at one end of the gradient,

while physiological tolerance may limit species

distributions at the other.

In addition to the effects of physical factors and

species interactions, plant species abundance and

composition in mixed species salt marshes can be altered

(Bakker 1985), plant regeneration inhibited (Chabreck 1968),

and net aboveground primary production reduced (Turner 1987)

as a result of trampling by foraging animals. Soil

disturbance from rooting and trampling by feral hogs often

results in a reduction of herbaceous cover and local

extinction of some plant species (Bratton 1974). These

changes have not, however, been documented in freshwater










tidal marshes. A large feral hog population inhabits

freshwater and oligohaline regions of the study area and may

significantly affect plant community structure. Vegetation

response to such disturbances will help define the role of

disturbance in structuring freshwater tidal marshes.

The objective of this study was to examine the role of

competition in structuring plant communities across a

salinity gradient of freshwater, oligohaline, and mesohaline

tidal marshes and to document possible effects of animal

disturbance. Species interactions occur at the level of

individual neighbors (Mack and Harper 1977, Fowler and

Antonovics 1981) and the distribution of a single species

may affect larger scale community structure (Mack and Harper

1977, Fowler and Antonovics 1981, Dale 1986). Because of

this, I chose to use Scirpus validus, the only species which

occurred throughout the salinity gradient, as the primary

indicator species in examining the distribution of species

as a function of competitive variation along this gradient.

Two general hypotheses were proposed: (1) S. validus,

the species with the widest range of occurrence, was

restricted under freshwater conditions by more specialized

species better able to compete for resources, (2) at higher

salinities, physiological tolerance is more important in

limiting the distribution of S. validus. Removal and

addition of species in both field and greenhouse

experiments, combined with reciprocal transplants of species










across the salinity gradient, were used to test these

hypotheses. Competitive ability was measured as relative

increases in biomass and density. A third hypothesis

proposes that disturbances created by feral hog activities

will result in a local reduction in species diversity.

Disturbance was measured as the change in abundance of

species following feral hog activities.

Differences in competitive ability along an

environmental gradient should be apparent for a species such

as S. validus because of its wide distribution. The range

of salinity tolerance exhibited by S. validus indicated that

it is a physiological generalist, and should therefore be

competitively unsuccessful in combination with physiological

specialists within the range of the specialistss. In the

freshwater marsh, removal of neighbors would be expected to

enhance the growth and expansion of S. validus, while

removal of S. validus may or may not have a significant

effect on neighboring freshwater species. Transplants of S.

validus into freshwater marsh from monospecific stands in

oligohaline marsh should succumb to the greater competitive

abilities of freshwater species.

Increased salinities of oligohaline and mesohaline

marshes place greater physiological stress on plant species

and zonation of species was conspicuous. Scirpus validus

may have occurred in high marsh zones because of its

competitive superiority over other marsh species at the less








81
stressful higher elevations, and be restricted from the low

marsh by a physiological intolerance of frequent flooding.

Competition under these conditions was expected to favor the

species with the broader salinity tolerance, e.g. S.

alterniflora, and result in greater competitive interactions

between S. validus and other species.

Methods

The study sites included freshwater (<1 ppt),

mildly oligohaline (2-4 ppt), strongly oligohaline (5-7

ppt), and mesohaline (8-10 ppt) tidal marshes of the lower

Savannah River, in Chatham County, Georgia, and Jasper

County, South Carolina (see Chapter 2). The high energy,

dynamic conditions of tidal marshes maintain these systems

in an early successional stage (de la Cruz, A.A. 1981),

while the salinity gradient from freshwater to brackish

marshes creates a natural stress gradient. Tidal marshes of

the lower Savannah River provided an opportunity to examine

competitive interactions among plant species across a

salinity gradient.

Eleocharis montevidensis was a dominant species co-

occuring with S. validus in the freshwater marsh along with

several freshwater annuals. Spartina alterniflora is the

co-dominant species under mesohaline conditions. At mildly

oligohaline conditions, the dominant species are Zizaniopsis

miliaceae and S. validus. Strongly oligohaline conditions

support very few species other than S. validus.










All statistical analyses were carried out using SAS

(Statistical Analysis System, Cary, North Carolina, 1988).

Field Transplants Between Sites

To determine the response of vegetation to changing

soil salinities and species associations, between-site

transplants of dominant species from freshwater,

oligohaline, and mesohaline marshes were made between

marshes. In December 1987, transplant, control, and

undisturbed plots were established at freshwater, mildly and

strongly oligohaline, and mesohaline sites to be

transplanted between sites. Eighteen previously

established, undisturbed plots at each site were used for

comparison with controls. A Reichert refractometer was used

to measure and monitor soil water salinities at field sites.

At random points, plant contents of 0.25 m X 0.25 m plots,

approximately 15 cm deep, were collected from donor sites

and transplanted to random points at recipient marsh sites.

The same size control plots were removed and then replaced

at each donor site to determine if transplanting effects

were significant. Rubber lawn edging (10 cm deep) was

placed around all between site transplants to inhibit

belowground competition. PVC flags were placed 0.25 m deep

into the marsh at corners of transplants to delineate plots.

Twenty S. validus plots were transplanted from the

donor oligohaline site to the freshwater site, and 20 to the

mesohaline site. Twenty control plots were established.










Twenty plots from the donor freshwater marsh were

transplanted to the strongly oligohaline site, and 10 to the

mesohaline site. Twenty five control plots were established

at the freshwater donor site. Twenty plots were

transplanted from the mesohaline donor site to the

oligohaline site, and 10 to the freshwater site. Twenty

five control plots were established at the mesohaline site.

Above-ground vegetation from transplants and controls

was harvested in June, 1989. Undisturbed plots were

harvested in June, 1988. Plants were identified, counted,

dried, and weighed.

Analysis of variance was used to determine if

differences among donor plots transplanted to different

marsh sites were significant. LSD tests, which are pairwise

t-tests equivalent to Fisher's least-significant difference

test for unequal sample sizes, were used to compare biomass

and density for donor marsh plots transplanted to

freshwater, oligohaline, and mesohaline sites.

Field Transplants Within Sites

To examine differences in competitive ability among

species within a site, within-site transplant comparisons

were made between inner and outer plots and their control

plots. Changes in within-site transplants were also

compared between marsh sites to determine whether

differences in competitive interactions varied along the

salinity gradient.








84

At freshwater, mildly oligohaline, and mesohaline marsh

sites, 75 sets of transplants, at 25 randomly chosen points

at each site, were established in December, 1987. Each set

of transplants consisted of a S. validus control plot, a

neighbor control plot, and a S. validus experimental plot

nested into the larger neighbor plot.

Control and experimental S. validus plots were 0.25 m

long X 0.25 m wide X 0.15 m deep. Neighbor plots were 1.0 m

long X 1.0 m wide X 0.15 m deep, with a section removed from

the center; experimental S. validus plots were nested in the

center of neighbor plots. Control plots were removed and

replaced for both S. validus and neighbor plots. Rubber

garden edging was placed 0.10 m deep around 1.0 m X 1.0 m

neighbor plots to inhibit interactions between experimental

plots and surrounding vegetation. PVC flags were placed

0.25 m deep into the marsh at corners of all plots so that

vegetation could be harvested separately for controls,

neighbor, and experimental S. validus plots.

All species except S. validus were clipped and removed

from inner S. validus plots and all S. validus was removed

from outer neighbor plots in April, 1988. Clipping and

removal of species was repeated in June and August, 1988.

Above-ground vegetation was harvested from all plots in

June, 1989. Vegetation was sorted by species, and stems

were counted and dried to a constant weight. Relative

proportions by weight and density were calculated for S.








85

validus, S. alterniflora, Z. miliaceae, and E. montevidensis

and an "other" category for transplants at each site.

Differences between outer neighbor controls and inner

S. validus plots enabled a determination of relative

increase, decrease, or no change with the removal of

neighbor species. Control S. validus plots were compared

with inner S. validus plots to determine changes in S,

especially for mildly oligohaline and mesohaline sites where

control S. validus plots rarely contained any other species.

Differences between neighbor controls and outer neighbor

plots were used to determine effects of the removal of S.

validus on neighbor species. Differences between inner S.

validus and outer neighbor plots at each marsh site were

calculated and compared to determine the overall effects of

the combined removal of neighbor species from inner plots

and S. validus from outer plots.

Paired t-tests were used to determine if differences

among controls, neighbor plots, and experimental S. validus

plots were significant. Pairwise t-tests were used to

compare differences between freshwater, mildly oligohaline,

and mesohaline sites (P < .05).

Greenhouse Experiments

To examine the potential for competition between S.

validus and freshwater species at varying salinities, a

series of DeWitt experiments (Silvertown 1982) were set up

in a greenhouse at the Center for Wetlands, Univ. of










Florida, in December, 1988. Randomly chosen plots of

freshwater marsh were collected in 0.25 cm diam. X 0.20 cm

deep pots. Ramets of S. validus were also randomly

collected. Fifteen pots contained no S. validus, 15

contained 10% S. validus, 15 contained 30% S. validus, 15

contained 50% S. validus, and 15 contained 100% S. validus.

Five pots from each group were placed in each of 3 treatment

pools, and watered daily with 0, 5, and 10 ppt seawater

solution, respectively, for 6 months. Soil water salinities

were monitored using a Reichert refractometer. The five

pots containing freshwater species but no S. validus were

also used to determine the effects of increasing salinity on

freshwater marsh species.

Above-ground vegetation was harvested from all plots in

July, 1989. Vegetation was sorted by species and stems were

counted and weighed. Total and individual relative yield by

weight and density were calculated for S. validus and the

combination of other species for each pot.

Two-way analysis of variance was used to test for

significant effects of salinity and initial proportions of

S. validus on relative and total yields of S. validus and

the combined other species. The T-method for multiple

comparisons among nearly equal sample sizes (Sokal and Rohlf

1981) was used to compare differences in effects of initial

S. validus proportions between freshwater, olighaline, and

mesohaline treatments.










Feral Hog Disturbance

A 0.5 km2 study area was established in an area

dominated by giant cutgrass, softstem bulrush, cattail

(Typha angustifolia), and spikerush (Eleocharis

montevidensis). Cypress, tupelo, and sweetgum (Liquidambar

styraciflua) grew along adjacent river levees. Interstitial

soil salinities averaged 4 ppt.

Thirty 1.0 m2 sample plots initially intended for plant

competition experiments were established in the study area

in December 1987. Vegetation sampling plots were located in

stands with an even mix of Z. miliaceae and S. validus (50%

cutgrass, 50% bulrush) at 6 random points along each of 5

transects in the study area.

Six of 12 study plots showed signs of trampling or

rooting by feral hogs in January 1988. Percent cover of

cutgrass and species composition of remaining vegetation was

measured in disturbed and undisturbed plots during March and

April 1988. Student's T-test was used to compare percent

cover of disturbed and undisturbed plots.

Results

Control plots did not differ significantly from

undisturbed plots at any transplant sites, indicating that

transplanting within sites had no effect on vegetation.

Subsequently, only comparisons between controls and

experimental plots are presented.










Between-site Transplants

Changes in proportions of dominant species for plots

transplanted to different sites are given in Figure 5-1.

Relative density and biomass of E. montevidensis decreased

significantly after being transplanted to the mesohaline

site, but remained unchanged relative to the control when

transplanted to the olighaline site (P = 0.05). Relative

density of S. validus transplants at the mesohaline marsh

decreased significantly (P = 0.05), while relative biomass

decreased significantly at both freshwater and mesohaline

sites. Spartina alterniflora decreased significantly (P

0.05) in relative density and biomass when transplanted to

the freshwater site, but not the oligohaline site.

Donor transplants at the freshwater marsh site were

invaded by local species and showed no difference in species

composition, biomass, or density when compared with

freshwater marsh controls (Fig. 5-2). Donor transplants

from the oligohaline marsh, composed predominantly of S.

validus, decreased significantly in S. validus biomass (F =

6.43; DF = 2; P < .05) and density (F = 38.44; DF = 2; P <

.05) after transplanting to the freshwater marsh site.

Spartina alterniflora from the mesohaline marsh decreased

significantly in biomass (F = 9.26; DF = 2; P < .05) and

density (F = 3.45; DF = 2; P < .05). Eight of the 10

initial transplants of S. alterniflora from the mesohaline

marsh were still intact when harvested; two were lost to








89



E. montevidensis S.validus S. alterniflora



E y
E |_





C M C M F C F








and b aE. montevidensis S.validus S. alterniflora

I I



4



E
validus transplanted from oligohaline to mesohaline and
C O M C M F C O F


a E. montevidensis S.validus S. alterniflora























freshwater marsh; and S. alterniflora transplanted from
mesohaline to oligohaline and freshwater marsh (see methods
section for details). C = control, F = freshwater, 0 =
oligohaline, M = mesohaline site.
cLr
~ 024 /

/O C F O

Fig ,/A. Aen n tnaderrsfrrltv est
andbioas of E moteidesistrnspaned7/o























S250
200-





5C

0
Control Oligohaline Mesohaline
60 1




60 -- -------i-------,----- -----









Control Oligohaline Mesohaline

IS Total alterifora

D E. montevidensis E Other

e validus



Fig. 5-2. Total and individual means and standard errors
for species density and biomass for oligohaline and
mesohaline donor transplants and controls at freshwater
site.




Full Text
PLANT DISTRIBUTIONS AND COMPETITIVE INTERACTIONS ALONG A
GRADIENT OF TIDAL FRESHWATER AND BRACKISH MARSHES
By
PAMELA J. LATHAM
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
TO THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1990
UNIVERSITY OF FLORIDA LI

ACKNOWLEDGEMENTS
I would like to extend my thanks to all the volunteers
that braved the Savannah River to help me with my work.
They faced alligators, feral hogs, water moccasins, an
extraordinary insect diversity, sucking muds, intense heat
and cold, rain storms and sleet, failed boats, and small
(and medium-sized) mammals in the sleeping accommodations -
- all for someone else's project. I sincerely appreciate
the help of Rick Bartleson, Lance Peterson, Kelly Latham,
Clay Phillips, Tim Harris, Brad Mace, and Monica Maynard for
their help in transplanting 390 vegetation plots. Kathy
Hallman provided invaluable lab assistance and she and Brad
Mace spent innumerable hours counting, drying, and weighing
plants.
My committee members all provided advice during various
phases of my field work and writing. Wiley Kitchens was
always there to provide anything I needed to carry out my
work and encouraged me to pursue interests beyond those of
the project. Ronnie Best and Jack Stout are responsible for
my work in wetlands ecology. Jack had enough faith in me
after my Master's work to send me off to the University of
Florida to take Ronnie's Wetlands Ecology class. Ronnie
helped establish my interest in wetlands ecology. Clay
11

Montague gave me the perspective I needed to be comfortable
with systems modelling and computers. Don Graetz somehow
made wetland soils biochemistry both interesting and
applicable to ecological systems. All my committee members
have shown an interest in my work that has kept me
motivated.
Rick Bartleson and Lance Peterson provided more help
and support than I could have ever asked for. Rick worked
harder in the field than anyone I've ever known, has an
unbelievable sense of direction, edited page after page of
manuscript, provided insightful discussion during the entire
project, and was willing to swim across the Savannah River
with me when airboats broke down. Lance began the project
with me, and together we tried and dismissed various forms
of transportation: helicopters, airboats, the Whaler, and
even a canoe. We settled on the johnboat, in which Lance
was able to attain speeds up to Warp 4. Through it all,
Rick and Lance provided support, enthusiasm, encouragement,
stability, and friendships I'll always cherish.
Jack Putz provided me with the scientific tools to
carry out this work; most of the hypotheses and design were
all generated from insightful discussions with him. Jack
did his best to keep me on the straight and narrow path of
science and I greatly appreciate the friendship that came
along with the science. Leonard Pearlstine was unfortunate
enough to be the closest to my work and was patient enough
in

to listen to and read nearly every idea I've had in the past
5 years; he was always willing to help me work out problems
by listening to me. Susan Vince's experience with tidal
salt marshes and transplant experiments was invaluable in
setting up my own experiments. Ken Portier, the only
statistician I know who can successfully integrate
statistics and biology and make it look easy, provided early
statistical designs for my transplant experiments.
In addition to volunteers, there were some folks who
just made things easier for me during the 4 years of field
work. Monica Maynard of the Georgia Coop Unit always
welcomed us at the Refuge trailer during her striped bass
surveys and made what were otherwise barren sleeping
quarters a temporary home. Daryl Hendricks, a Refuge
employee, was always there to help get things ready, warn us
about potential problems or get us out of a jam, repair
anything mechanical, and somehow manage to like us anyway.
Ray Porter was always around for us "just in case." The
Savannah National Wildlife Refuge personnel at the refuge
and the Savannah Office were all helpful in finishing this
project. John Davis has been a staunch supporter of this
project.
My Chesapeake Bay Retriever, Mac, was also instrumental
in my work. Mac was a valuable field assistant, carrying
equipment for me in the field and keeping a watchful eye for
IV

anything unusual. He diligently watched over me when I was
alone in the field or working late at the office.
Throughout the entire project, Clay Phillips provided a
source of support and encouragement on which I could depend.
I could not have accomplished the field work, the project
goals, the qualifiers, the writing, or the defense without
him.
v

TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
ABSTRACT viii
1. GENERAL INTRODUCTION 1
Overview 1
The System: Freshwater Tidal Marshes 2
Environmental Influences 3
Soil Characteristics 5
Species Interactions 6
Tidal Marshes of the Lower Savannah River 8
The Savannah River Tide 10
Objectives 11
2. SPECIES ASSOCIATION CHANGES ACROSS A GRADIENT OF
FRESH, OLIGOHALINE, AND MESOHALINE TIDAL MARSHES
ALONG THE LOWER SAVANNAH RIVER 12
Introduction 12
Methods 14
Study Site 14
Vegetation and Environmental Gradients 17
Analysis 19
Results 21
Discussion 32
3. SPATIAL DISTRIBUTIONS OF THE SOFTSTEM BULRUSH,
SCIRPUS VALIDUS, ACROSS A SALINITY GRADIENT .... 41
Introduction 41
Methods 42
Results 44
Species Associations 44
Spatial Pattern 44
Relationship of IV with Environmental Variables . . 49
Discussion 51
Species Diversity 51
Distribution Patterns 51
4. MORPHOLOGICAL PLASTICITY IN SCIRPUS VALIDUS
ALONG A SALINITY GRADIENT 57
Introduction 57
Methods 59
Results 61
Field Measurements 61
vi

Greenhouse Experiments 64
Transplants 64
Light Extinction 67
Discussion 67
5. THE ROLE OF COMPETITIVE INTERACTIONS, SOIL SALINITY, AND
DISTURBANCE IN THE DISTRIBUTION OF TIDAL MARSH PLANT
SPECIES 77
Introduction 77
Methods 81
Field Transplants Between Sites 82
Field Transplants Within Sites 83
Greenhouse Experiments 86
Feral Hog Disturbance 87
Results 87
Between-site Transplants 88
Within-site Transplants 94
Inner S. validus vs. outer control
plots 94
Inner S. validus vs. control S. validus
plots 96
Outer neighbor vs. neighbor control
plots 100
Inner S. validus vs. outer neighbor
plots 101
Greenhouse Experiments 103
Feral Hog Disturbance 110
. Discussion 110
6. SUMMARY AND CONCLUSIONS 125
LITERATURE CITED 131
BIOGRAPHICAL SKETCH ii
vil

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
PLANT DISTRIBUTIONS AND COMPETITIVE INTERACTIONS ALONG A
GRADIENT OF TIDAL FRESHWATER AND BRACKISH MARSHES
By
Pamela J. Latham
August 1990
Chairpersons: Wiley M. Kitchens, G. Ronnie Best
Major Department: Environmental Engineering Sciences
Plant species distributions and their relation to
environmental factors, morphology, competition and
disturbance were examined across a gradient of tidal
freshwater, oligohaline, and mesohaline marshes along the
lower Savannah River. Salinity was significant in
separating major vegetation classes between sites, while
elevation and distance from tidal channel were significant
in separating classes within sites. Overlap among
vegetation classes was high, a result of overlap with
Scirpus validus, which occurred over the widest range of
environmental parameters.
Spatial pattern and relative importance of Scirpus. as
well as composition of co-occurring species, changed
significantly with changing salinity. Patterns were clumped
in the mesohaline marsh, uniform in the strongly, oligohaline
Vlll

marsh, and nearly random at freshwater and oligohaline
sites. Field and greenhouse experiments indicated more
robust Scirpus under freshwater conditions in the
greenhouse, while this trend was reversed at field sites.
Transplants of Scirpus between sites had morphologies
similar to local populations. Results indicate that
morphologic variation in Scirpus is ecophenic and that
competition, possibly for light, may be more important in
its distribution than environmental factors.
Nested transplants of Scirpus (target species) and
neighbor species were combined with species removals in
transplants. Neighbor and target species were also
transplanted between sites to examine competitive
interactions among species. Greenhouse experiments were
conducted for species at different densities and salinities.
Lastly, feral hog disturbances were documented to determine
the possible effects of disturbance.
Results indicate that effects of competitive
interactions on plant distributions are not exclusively
related to the individual competitive abilities of plant
species, but instead reflect a competitive balance of the
species involved. Although freshwater marsh species have
greater competitive abilities than species of brackish
marshes, changes in environmental conditions strongly
influence species interactions. While the competitive
ability of a weaker competitor itself is not altered,
IX

differences in co-occurring species and environmental
conditions result in altered competitive hierarchies and
species distributions along an environmental gradient. In
addition to environmental gradients, disturbance also
appears to alter species interactions and distributions.
x

CHAPTER 1
GENERAL INTRODUCTION
Overview
The research findings presented in this dissertation
are organized into 4 main chapters. Introductory and
summary chapters are also included. The introductory
chapter includes a literature review of Southeastern
freshwater tidal marsh structure and function, a brief
description of external factors affecting the system, and
project objectives. Chapter 2 provides a descriptive and
guantitative analysis of the marsh vegetation and
relationships with associated environmental parameters. In
chapters 3 and 4, the population dynamics of the only
cosmopolitan species occurring in the study area, Scirpus
validus, are investigated in regards to its distribution,
changing morphology, and relation to co-occurring species
across a salinity gradient. Research findings from chapters
2, 3, and 4 culminate in chapter 5, which provides a
guantitative analysis of the role of species interactions in
structuring these plant communities. The final chapter
contains a summary of chapters 2 through 5, with emphasis
placed on results and implications from chapter 5.
1

2
The System: Freshwater Tidal Marshes
Tidal freshwater marshes, in contrast with coastal salt
marshes and inland freshwater marshes, have been studied
relatively little (Odum 1988, Odum et al. 1984, Simpson et
al. 1983). Unlike inland, non-tidal marshes, they receive
high nutrient and energy pulses characteristic of coastal
salt marshes. Salinity, the dominant factor affecting the
productivity and species richness of salt marshes, is not
imposed upon tidal freshwater marshes. As a result, tidal
freshwater marshes support a high diversity of plants and
animals (Odum 1988, White 1985, Doumlele 1981, Gosselink et
al. 1978, Tiner 1977), are extremely productive (Odum 1988,
Simpson et al. 1983), and are often sensitive to human
impact (Odum et al. 1984, Mitsch and Gosselink 1986).
Tidal freshwater marshes along the South Atlantic Coast
make up a significant portion of tidal marshes of the U.S.
Coastline morphology of Georgia and South Carolina funnel
and amplify tidal waters, creating tides ranging in
amplitude from 2.0 to 2.5 meters (de la Cruz 1981), greater
than most North American marshes. Unlike geologically
younger Pacific coast marshes, there has been time for
extensive development over the broad, gently sloping Coastal
Plains. In addition, the network of rivers feeding the
southeastern marsh system is larger than that of the
northeastern tidal marshes that occur over less erodable
glacial till. Consequently, deposition of river sediments

3
into south Atlantic coast marshes is greater than those of
northeastern marshes.
Environmental Influences
Tidal freshwater and salt marshes are the extremes of a
gradient that includes oligohaline (0.5 - 5 ppt) and
mesohaline (5-30 ppt) marshes and comprise a gradient that
coincides with increasing salinities toward the ocean.
Differences in river and tidal influence are associated with
physical gradients of hydroperiod, salinity and soil
characteristics and are reflected in distributions of marsh
species. These physical parameters are generally considered
the primary factors affecting zonation of marsh vegetation
(Cooper 1982), though recent evidence indicates species
interactions are important in salt marsh vegetation
zonation.
Salinity exerts a major influence on tidal marsh
habitats (White 1985, Morris 1978, Parrondo et al., Reid and
Wood 1976, Phleger 1971, Adams 1963) and appears to be the
primary factor in modifying the physical and biological
makeup of the transition zone between brackish and fresh
water (Odum 1988, Haramis and Carter 1983). High salinities
inhibit all but a few halophytes (Phleger 1971) and are
associated with decreased species diversity (White 1983,
Anderson et al. 1968).
Freshwater marsh communities exhibit relatively little
species zonation compared to salt marshes (Odum 1988, Joyce

4
and Thayer 1986, Carey et al. 1981). Vegetation patterns
(White 1985, Leiffers 1983, De la Cruz 1978, Disraeli and
Fonda 1978), species diversity (Heinselman 1970) and species
ranges (Ferren and Schuler 1980) are, however, strongly
influenced by hydrologic regime and site elevation, and
elevation is probably the most agreed upon factor
influencing marsh zonation (Dawe and White 1982). Major
components associated with hydrologic regime include water,
nutrients, toxins, oxygen availability and spatial
heterogeneity (Gosselink and Turner 1978). Wave exposure
(Wilson and Keddy 1986) and current velocity (Nilsson 1987)
affect plant diversity as well as plant biomass and
vegetative expansion.
Soil Characteristics
Soil characteristics appear less variable than salinity
and hydroperiod in tidal marshes. Most North American
saltmarshes are formed from reworked marine sediments on
marine-dominated coasts (Mitsch and Gosselink 1986),
although sediments of both tidal freshwater and salt marshes
are of riverine and tidal input. Freshwater tidal marshes
are primarily fine textured silts and clays with an organic
content ranging from 10-15% in actively flooded levees to
30-45% in high marshes with minimal tidal influence (Simpson
et al. 1983). Differences in flood velocity due to levee
vegetation also result in the trapping of courser sediments
along creek banks (Kirby and Gosselink 1976) and a decrease

5
in soil redox (Rowell et al. 1981). Organic content of
creek levees is lower than high or back marsh soils farther
from the creeks. While tidal flushing may be responsible
for the rapid litter loss of up to 80% over 30 days, varying
rates of decomposition in plant species may be due more to
inherent differences in species structure and decomposition
rates than tidal influence (Simpson et al. 1983).
Marsh vegetation is significantly affected by changes
in soil and interstitial water chemistry under flooded
conditions (Ponnamperuma 1972). Except for a thin surface
layer, tidal marsh soils are anaerobic due to the slowed
diffusion of oxygen through water. The oxygen deficit
results in slower organic decomposition and changes in pH,
redox, microbial activity, phosphorous, nitrogen and sulfur,
as well as metals. The cation exchange capacity and pH
increase in acid soils under saturated conditions, while
both cation exchange capacity and pH decrease in alkaline
soils (Ponnamperuma 1972), resulting in nearly neutral soil
pH. An accumulation of toxic metals or solubilization and
absorption of high concentrations of micronutrients may also
occur under reduced conditions. Sediment nitrogen is bound
in an organic form in marshes and is often the primary
limiting nutrient for salt marsh plants (Valiela et al.
1978). Phosphorous, however, is often more available to
plants of saturated or poorly drained soils. Because iron
and aluminum phosphates are more readily reduced, providing

6
oxygen for the limited decomposition which takes place in
these soils, the phosphorous is released and is seldom
limiting.
Species Interactions
In addition to environmental factors, differences in
distributions among marsh species have been attributed to
species interactions (Bertness and Ellison 1987, Snow and
Vince 1984) and variation in competitive abilities across an
environmental gradient (Wilson and Keddy 1986, Grace and
Wetzel 1981). Competition among and between species of
terrestrial habitats is well documented, although little
experimental data exist for competition among marsh species.
While plant zonation has been highly correlated with
physical factors such as elevation and salinity, these
correlations are not necessarily causal. In addition,
physical and chemical gradients associated with marsh
vegetation are often obscured by the circular zonation due
at least in part to the clonal nature of perennial species.
Salt-tolerant species, e.g. S. alterniflora seedlings
(Mooring et al. 1971), S. foliosa (Phleger 1971) and
Salicornia biaelovii (Webb 1966), are known to grow well,
and in most instances, better, in freshwater. The same
species, however, are outcompeted under freshwater
conditions, and so are confined to brackish and salt marshes
where freshwater species cannot survive (Wainwright 1984).
Results of transplant experiments (Wilson and Keddy 1985,

7
Snow and Vince 1984, Barbour 1978) provide evidence that
physiological response of plants to environmental gradients
alone does not account for the zonation and patterns
characteristic of marsh vegetation. Both interspecific
competition and physical disturbance have been found to
affect spatial patterns among salt marsh species (Bertness
and Ellison 1987).
Results from several studies (Snow and Vince 1984,
Grace and Wetzel 1981, Sharitz and McCormick 1980) suggest
that along an environmental gradient, competitive
interactions may determine species distributions at one end
of the gradient, while physiological tolerance limits
species distributions at the other. Greater development of
"competitive heirarchies" (Anderson 1986) would be expected
in freshwater marshes, then, based on the relatively benign
environment and greater probability of interactions among
species. A random distribution of individuals would
ultimately result if there was a total lack of response by
individuals to environmental factors or other individuals
(Taylor et al. 1978).
Tidal Marshes of the Lower Savannah River
The Savannah River rises on the southern slope of the
Blue Ridge Mountains in North Carolina and flows through the
Coastal Plain. It is an alluvial, rock-dominated river (as
opposed to precipitation-dominated) with a relatively high
mineral load derived from weathering and leaching of parent

8
material in the mountains and Piedmont (Wharton et al.
1982). The Chattooga and Tallulah Rivers form the
headwaters of the Savannah River in the Blue Ridge sector of
northeast Georgia (Wharton 1978). Rivers originating in the
Piedmont generally have larger drainage basins and, because
they rise in the mountains where rainfall is heavier, have
larger discharges than Coastal Plain rivers (Rhodes 1949).
The lower portion of the Savannah River is a deltaic plain.
The river dishcarges through the delta into the ocean via
numerous, relatively shallow, interconnected distributary
channels between which are scattered marsh islands of
various sizes (Rhodes 1949).
The head of the Savannah River tidewaters is about 50
miles north of its mouth. Distributary channels of the
delta include: the Little Back, Middle, Back, and Front
Rivers (see Chapter 2 for map). The hydrology of the lower
Savannah River is dependent upon precipitation, run-off,
channel morphology, wind, river discharge, variation in mean
sea level, and seasonal and daily tidal fluctuations.
Importantly, 28 percent of tidal freshwater wetlands of
the eastern U.S. are found in Georgia and South Carolina.
In the lower portion of the Savannah River, approximately
1900 ha, or 21% of the tidal freshwater wetlands in Georgia
and South Carolina are found on the Savannah National
Wildlife Refuge (SNWR), in addition to bottomland hardwoods,
upland forest and artificial impoundments. Artificial

9
impoundments, left from extensive rice cultivation during
the 18th and 19th century (Baden 1975), make up more than
15% of the the total area of South Carolina and Georgia
coastal marshes. Former rice fields are evidenced in
extensive canal systems traversing much of coastal South
Carolina and Georgia, and are especially notable on the
lower Savannah River. Vegetation zonation associated with
former canal systems is conspicuous. Present vegetation
composition is generally consistent with non-cultivated,
naturally occurring tidal marshes of the southeastern coast
(Odum et al. 1984). Marshes along the lower Savannah River
range from fresh to brackish salinities, commensurate with
changes in soil, hydrology and plant communities.
The Savannah River Tide Gate
The tide gate is a navigational structure designed to
constrict ebbing flows of the Lower Savannah River to the
Savannah Harbor channel. In operation, the structure serves
as a flap gate and is closed on outgoing tides.
Subseguently, flood waters are diverted through connecting
lateral channels to the harbor channel of the Front River.
Because of the increased flow velocities on ebbing tides the
channel is scoured and maintenance dredging in the port is
minimized. Like the Mississippi River deltaic plain
(Pezeshki 1987), man-made alterations appear to be effecting
changes in salinities and hydroperiod, with the potential to
significantly alter plant populations and habitat.

10
The construction of a tide gate at the mouth of the
north channel of the Savannah River in 1977 has resulted in
increased salinities throughout the refuge (Odum et al.
1977, Pearlstine et al. 1989), and refuge vegetation has
undergone significant alterations (Brown et al. 1987). The
extent of the salinity wedge upstream, a function of both
current and elevation, is farther with the tidegate in
operation (Odum 1977, Pearlstine et al. 1989). The wedge
extended upstream to the confluence of the Front and Back
Rivers in 1943 (USCOE) and 1977 (Odum 1977), and just
upstream from Highway 17-A on the Back River in 1977 (Odum
1977) (Fig. 2-1). Since construction of the tide gate, the
wedge now 2-3 miles farther upstream (Pearlstine et al.
1989). As a result of 15 years of tide gate operations, the
lower Savannah River has undergone a conversion from
freshwater marsh to a compressed gradient of freshwater and
brackish marshes, providing a unique opportunity to document
vegetation response to these changes.
Objectives
This objectives of this study were to 1) characterize
the vegetation features and environmental parameters of the
lower Savannah River tidal marshes; 2) quantify the
relationships among vegetation associations and major
environmental environmental gradients of salinity,
hydroperiod, and soil characteristics; 3) quantify the
population dynamics of Scirous validus in relation to

11
environmental gradients and co-occurring species; and 4)
guantify the effect of species interactions as causal
mechanisms for observed plant species distributions and
associations.
For objectives 1) and 2), major physical parameters
associated with vegetation types were identified and
quantified, and correlations among measures of plant species
importance and environmental parameters were used to
characterize plant species distributions in relation to
environmental gradients of salinity, hydroperiod and soil
variation in tidal freshwater and brackish marshes. The
remaining objectives entailed an investigation into causal
mechanisms for species distributions identified in the
gradient analysis. Possible reasons for differences in
spatial patterns of species distributions in fresh and
brackish marsh and development of competitive heirarchies
were experimentally determined. Transplant experiments
allowed determinations of species robustness, not only in
relation to physical and chemical parameters, but also in
regards to other species that occupy the same or overlapping
niches.

CHAPTER 2
SPECIES ASSOCIATION CHANGES ACROSS A GRADIENT OF
FRESH, OLIGOHALINE, AND MESOHALINE TIDAL MARSHES
ALONG THE LOWER SAVANNAH RIVER
Introduction
Contrast in vegetation, animals, and water and soil
chemistry among tidal freshwater and saline marshes is well
documented. Differences in river and tidal influence are
associated with physical gradients of hydroperiod, salinity
and soil characteristics and are reflected in distributions
of marsh species. These physical parameters are generally
considered the primary factors affecting zonation of marsh
plants (Cooper 1982). Salinity exerts a major influence on
tidal marsh habitats (White 1983, Morris 1978, Parrondo et
al. 1978, Reid and Wood 1976, Phleger 1971, Adams 1963) and
may be the principal factor in modifying the physical and
biological makeup of the transition zone between brackish
and fresh water (Odum 1988, Haramis and Carter 1983). In
addition, increasing salinities inhibit all but a few
halophytes (Phleger 1971) and result in a characteristically
low species diversity (White 1983, Anderson 1986) in saline
marshes when compared with freshwater marshes.
Freshwater marsh communities exhibit relatively little
species zonation compared to salt marshes (Odum 1988, Joyce
and Thayer 1986, de la Cruz 1981). Vegetation patterns
12

13
(White 1983, Leiffers 1983, De la Cruz 1981, Disraeli and
Fonda 1979), species diversity (Heinselman 1970) and species
ranges (Ferren and Schuler 1980) are, however, strongly
influenced by hydrologic regime and site elevation, and
elevation is the most agreed upon factor influencing plant
distributions and zonation (Dawe and White 1982). Major
components associated with hydrologic regime include water,
nutrients, toxins, oxygen availability and spatial
heterogeneity (Gosselink and Turner 1978). Wave exposure
(Weisner 1987) and current velocity (Nilsson 1987) also
affect plant diversity, biomass, and vegetative expansion.
Vegetation differences and how those differences relate
to environmental factors across salinity gradient extremes
have received little attention (Odum 1988). Southeastern
tidal marshes, both freshwater and saline, have received
considerable attention on an individual basis, although the
gradient between the two remains poorly studied. A system
of freshwater (< 0.5 ppt), oligohaline (0.5-5.0 ppt), and
mesohaline (5.0-18 ppt) tidal marshes along the lower
portion of the Savannah River provided an opportunity to
examine differences in vegetation and associated
environmental factors across a strong tidal salinity
gradient.

14
Methods
Study Site
The study area covered 1900 ha of Savannah River tidal
marsh, in Chatham County, Georgia, and Jasper County, South
Carolina, within the boundaries of the Savannah National
Wildlife Refuge (32° 10' N. , 81°08' W.). Local tides
measure 2.0 to 2.5 meters (de la Cruz 1981). Like much of
South Carolina's and Georgia's low country, remnant rice
fields from the 18th and 19th centuries are abundant
throughout the refuge (Baden et al. 1975). Plant zonation
associated with canal drainage is conspicuous, although
present vegetation composition is consistent with non-
cultivated, naturally occurring tidal marshes of the
southeastern coast (Odum et al. 1984).
Four sites were chosen that represented tidal
freshwater (0 - .5 ppt), oligohaline (.5 - 2.5 ppt),
strongly oligohaline (2.5 - 5 ppt), and mesohaline (5 - 10)
marsh conditions (Fig. 2-1). Six transects, each with 3
sample points at distances of 20, 70 and 120 m from tidal
channels, were located at each site. Sample points (n = 73)
were also located adjacent to a secondary canal (canals
draining into primary canals) and at approximately egual
distances between two secondary canals for a total of 18
sample points at each study site. Because of a sharp turn
in the primary canal at the oligohaline site, there was no
room for an entire transect at the first sample point. Six

15
MILES
Fig. 2-1. Location of study area along the lower Savannah
River, in South Carolina and Georgia. Shaded, numbered
areas represent freshwater (1), oligohaline (2), strongly
oligohaline (3), and mesohaline (4) sites.

16
transects were positioned in addition to the first sample
point at this site, for a total of 19 points at the
oligohaline site, making a total of 73 sample points.
Sample points equal distances from primary and secondary
canals served as replicates (Fig. 2-2).
Vegetation and Environmental Gradients
Double-nested PVC wells were placed at each sample
point. The outercasing was placed 20 cm into the soil to
prevent surface water from entering the inner sampling well,
and the inner well was placed 30 cm into the soil to collect
soil water through vertical slits below the soil surface.
Soil water samples and vegetation were collected at
sample points at approximate 8-week intervals from June,
1985, to August, 1987. Surface vegetation was harvested
from a randomly placed 0.25 m2 circular plot at each sample
point during each sampling period. Live vegetation was
sorted by species, counted (for a measure of density),
dried, and weighed (for a measure of biomass). Absolute
densities and biomass were multiplied by four to give
density and biomass per square meter for each species.
Importance values (IV) were calculated as the sum of
relative density and relative biomass for each species in
each plot. Dominance-diversity curves, which reflect
differences in patterns of competition and niche
differentiation in communities (Whittaker 1965) were
developed from IV for each species.

PRIMARY CANAL

18
Interstitial water was siphoned from sampling wells
into Nalgene bottles and stored on ice until salinities
could be measured from well samples using a Reichert
refractometer. Elevations relative to mean sea level were
measured with a self-leveling electronic level and verified
by permanent benchmarks along the river.
Analysis
Within-site variation in salinity, organic content, and
elevation was determined by comparing these variables
between low (20 m from channel edge), middle (70 m from
channel edge), and high (120 m from channel edge) marsh.
Between-site variation in salinity, organic matter and
elevation was determined by comparing overall means between
sites for each variable. Because the oligohaline site had 1
more point than the other sites, and because there were
occasions on which either a sample point was missed or the
sample itself inadequately stored, there was not always the
same number of samples to compare. Consequently, Tukey's
studentized range test for unequal cell sizes was used in
means comparisons tests for these variables.
Vegetation classes were chosen based on ordination of
species composition at sample points using detrended
correspondence analysis (DCA) from the program DECORANA
(Hill 1979). DCA is an ordination method which converts
species composition data into vegetation variables, which
are then assigned scores; vegetation and environmental

19
patterns can then be compared and used to generate
hypotheses about the causes of within-community vegetation
patterns (Bernard et al. 1983). Ordination was first
applied to all 4 sites combined to examine between-community
differences in vegetation. Because of the high diversity of
the freshwater marsh, a second ordination of only freshwater
and oligohaline sites was done so that within-community
vegetation patterns could be examined.
Discriminant function (DF) analysis was then applied to
vegetation classes, both for the 4 sites combined and
separately. Like multiple regression analysis, DF analysis
can be used to predict or describe the relationship between
independent and dependent variables. DF analysis was used,
however, because the dependent variables in this case, i.e.
vegetation classes, are nominal variables, whereas multiple
regression analysis relies on interval or ratio variables
(Afifi and Clark 1984). DF analysis was used to quantify
the contribution of interstitial salinity, elevation, and
distance from tidal channels in defining vegetation classes,
based on relationships between environmental variables and
species composition and dominance at sampling sites. A
"successful" DF analysis is one which results in correct
pairing of vegetation types and environmental parameters
into vegetation classes. Interstitial salinity was used to
discriminate between freshwater, oligohaline, strongly
oligohaline, and mesohaline sites. Once this first level of

20
classification was achieved, interstitial salinity,
elevation, and distance from tidal channels were used to
differentiate among vegetation classes within sites.
Environmental variables were retained in the DF analysis
based on their significance in predicting vegetation classes
at sites. Results were presented as percent correct
classifications of vegetation observations.
Results
Differences in soil water salinities (Fig. 2-3) were
significant (P = .01) between all sites. Average salinities
were lowest (0.54 ppt ±0.63) at the freshwater site and
increased through oligohaline (2.10 + 1.04 ppt), strongly
oligohaline (4.67 + 1.49 ppt) and mesohaline (9.27 ± 1.97
ppt) marshes. No sites exhibited significant within-site
variation in soil water salinities.
Variation in pH was not significant between sites.
Within site variation in pH was also not significant.
Percent soil organic matter (Fig. 2-3) was higher at
freshwater and oligohaline marsh sites (43.62 ± 17.2 and
55.28 + 21.2, respectively), when compared with strongly
oligohaline (38.27 + 17.3), and mesohaline (31.62 ± 11.7)
marsh sites. Differences were significant (P = .05) between
all except the strongly oligohaline and mesohaline sites.
Soil organic matter showed a general increase with distance
along transects, i.e. from front to back marsh, at all

21
E
CJ
C
o
>
ju
W
S3
E
u
'c
CO
«>0
c
u
u
Cu
Fig. 2-3. Means and standard deviations for soil water
salinities (ppt), percent soil organic matter, and elevation
(cm) at 20 (solid bars), 70 (hatched bars), and 120 (open
bars) meters from primary creeks at freshwater, oligohaline,
strongly oligohaline, and mesohaline sites.

22
sites. Differences along transects were significant only at
freshwater and mesohaline sites (P = 0.05).
Mean elevation (Fig. 2-3) was highest at the freshwater
and strongly oligohaline sites, lower at the oligohaline,
and lowest at the mesohaline site (F = 42.75; n = 18; P =
.05). There was an increase in elevation from front to back
marsh along transects at all sites, though differences along
transects were significant at only the oligohaline site (F =
9.45; n = 18; P = .05).
On the basis of the DECORANA ordination of the 4
combined sites (Fig. 2-4) and the re-ordination of the
freshwater and oligohaline sites (Fig. 2-5), 9 vegetation
classes were chosen, the first 6 of which were chosen from
freshwater and oligohaline sites: (1) Eleocharis
montevidensis. (2) E. montevidensis and S. validus, (3)
Zizaniopsis miliaceae. S. validus. and Typha latifolia. (4)
Z. miliaceae. (5) E. montevidensis, S. validus. and Z.
miliaceae. (6) S. validus and Z. miliaceae f (7) S. validus.
(8) Soartina alterniflora and S. validus. and (9) S.
alterniflora and £L_ robustus. While overlap between
adjacent classes was high, non-neighboring classes did not
appear to overlap and the vegetation classes were considered
more than adequate. Eleocharis montevidensis was the only
dominant species not present in monospecific stands at some
point along the salinity or elevation gradients and only S.
validus occurred at all sites. The first five

DC A Axis
23
Fig. 2-4. Ordination of sample vegetation scores at
freshwater, oligohaline, strongly oligohaline, and
mesohaline sites from DECORANA output. Freshwater and
oligohaline sample points are contained within the broken
line.

D C A Axis
24
Fig. 2-5. Re-ordinations of sample vegetation scores for
freshwater and olighaline sites from DECORANA output.
Arrows indicate decreasing dominance by S. validus and
increasing dominance by the species to which the arrow is
pointing.

25
classes included several relatively rare species, including
Leersia virainica. Hypericum spp., Mikania scandens. Leersia
spp., Aneilema keisak. Aster elliotii. Galium tinctorium.
Panicum spp., Sagittaria spp., Polygonum spp., Bidens
laevis. Amaranthus cannabinus. and Alternanthera
philoxeroides.
Re-ordination of the freshwater and oligohaline sites
(Fig. 2-5) shows a separation of freshwater species. The
separation of perennials in the freshwater marsh along the
first and second axes corresponded to both elevation and
distance from tidal channels.
DF analysis for the 4 sites combined (Table 2-1) was
highly significant (Wilks-Lambda = 0.05; DF = 24/1529; P <
0.0) in classifying (or separating) vegetation classes and
more than half of the vegetation classes (60%) were
correctly classified. Misclassification (those vegetation
classes which were not successfully matched with their
corresponding environmental factors) fell almost exclusively
into classes containing the same species with different
dominants, or different combinations of species with at
least one species in common. Interstitial salinity,
elevation, and distance from primary channel segregated the
majority of vegetation classes. Means for salinity,
elevation, and distance from tidal channel were
significantly different among vegetation classes (P =
0.0001) and as a result were retained in the DF analysis

26
Table 2-1. Discriminant analysis results of vegetation
classes from freshwater (FRESH), oligohaline (OLIGO),
strongly oligohaline (S. OLIGO), and mesohaline (MESO) sites
on the lower Savannah River. A. Classification matrix. B.
Significance of the variable salinity (SAL) in separating
vegetation between sites.
A. Wilk's-Lambda
0.0000.
= 0.18; F
= 1743.28;
DF =
3/1144; P =
Class
Percent
cases correctly
classified
FRESH
OLIGO
S. OLIGO MESO
FRESH
87
13
0
0
OLIGO
37
47
17
0
S. OLIGO
9
13
68
9
MESO
0
2
19
79
B.
Variable
R2
F
P>F
SAL
0.82
1743
.3 0.0000

27
(Table 2-1). Interstitial salinity means were significantly
different between the four vegetation classes (freshwater
marsh, oligohaline marsh, strongly oligohaline marsh, and
mesohaline marsh). Percent of vegetation correctly
classified by environmental variables was 87%, 47%, 68%, and
79%, for freshwater, oligohaline, strongly oligohaline, and
mesohaline sites, respectively.
Within the vegetation classes previously separated by
salinity, vegetation was further differentiated into the
subclasses by elevation and distances from primary and
secondary channels (Tables 2-2 through 2-5). Means for
elevation and distances from primary and secondary channels
were significantly different among vegetation subclasses for
freshwater and mesohaline marsh sites (Tables 2-2 and 2-5).
In the oligohaline and strongly oligohaline marsh sites,
distance from secondary channels did not contribute to
differentiating subclasses; however, means for elevation and
distance from primary channels were significantly different
(Tables 2-3 and 2-4). In several cases, for example the
secondary channels in the freshwater marsh class and
channels in the mesohaline marsh class, the correlation
coefficient was low and the F score indicates that the
variable was significant but a minor contributor to the
discrimination. DF analysis was also significant in
classifying vegetation within sites (Tables 2-2 through 2-
5). Interstitial salinity was not retained as a significant

28
Table 2-2. Discriminant analysis results of vegetation
classes from freshwater sites on the lower Savannah River.
A. Classification matrix and B. significance values.
Significant variables were elevation (ELEV), distance to
primary channel (CHANNEL), and distance to secondary channel
(SECCHAN).
A. Wilk's-Lambda = 0.24; F = 113.26; DF = 9/1260.83; P =
0.0001.
Class
Percent cases
correctly
classified'1
E
EX
Z
ZX
E
78
0
22
0
EF
17
65
18
0
Z
0
0
66
34
ZX
0
0
0
100
B.
Variable
R2
F
P>F
ELEV
0.44
136.5
0.0001
CHANNEL
0.49
168.3
0.0001
SECCHAN
0.10
19.1
0.0001
aE = predominantly E. montevidensis; EX = E. montevidensis
mixed with several freshwater annuals, Zizaniopsis
miliaceae. Scirpus validus. Saaittaria spp., and Hydrocotyle
umbellatum; Z = Z. miliaceae; ZX = predominantly Z.
miliaceae. mixed with annuals, E. montevidensis. Scirpus
validus. Saaittaria spp., and Hydrocotyle umbellatum.

29
Table 2-3. Discriminant analysis results of vegetation
classes from oligohaline sites on the lower Savannah River.
A. Classification matrix and B. significance values.
Significant variables were elevation (ELEV), distance to
primary channel (CHANNEL).
A. Wilk's-Lambda = 0.18;
F = 1743.28;
DF = 3/1144; P =
0.0000.
Class
Percent cases correctly classified
FRESH
OLIGO
S. OLIGO
MESO
FRESH
87
13
0
0
OLIGO
37
47
17
0
S. OLIGO
9
13
68
9
MESO
0
2
19
79
B.
Variable
R2
F
P>F
SAL
0.82
1743.3
0.0000
aES = predominantly E. montevidensis. mixed with annuals,
Zizaniopsis miliaceae. Scirous validus. Saaittaria spp., and
Hvdrocotyle umbellatum; Z = Z. miliaceae; ZX = predominantly
Z. miliaceae. mixed with annuals, E. montevidensis. S.
validus, Saaittaria spp., and Hydrocotyle umbellatum.

30
Table 2-4. Discriminant analysis results of vegetation
classes from strongly oligohaline sites on the lower
Savannah River. A. Classification matrix and B.
significance values. Significant variables were elevation
(ELEV) and distance to primary channel (CHANNEL).
A. Wilk's-Lambda
= 0.23; F
= 61.50; DF = 6/336;
P = 0.0001.
Class
Percent
cases correctly classified3
S
SZ
S
79
21
SZ
14
86
B. Variable
R2
F
P>F
ELEV
0.03
5.28
0.0228
CHANNEL
0.34
84.34
0.0001
3S = S. validus; SZ = S. validus and Zizaniopsis miliacea.

31
Table 2-5. Discriminant analysis results of vegetation
classes from mesohaline sites on the lower Savannah River.
A. Classification matrix and B. significance values.
Significant variables were elevation (ELEV), distance to
primary channel (CHANNEL), and distance to secondary channel
(SECCHAN).
A. Wilk's-Lambda
= 0.23; F
= 61.50; DF = 6/336;
P = 0.0001.
Class
Percent
cases correctly classified3
PR
PS
S
PR
88
0
12
PS
13
67
20
S
0
0
100
B. Variable
R2
F
P>F
ELEV
0.35
46.9
0.0001
CHANNEL
0.03
2.9
0.0578
SECCHAN
0.57
112.72
0.0001
aPR = Spartina alterniflora and Scirpus robustus; PS = S.
alterniflora and S. validus; S = S. validus.

32
factor in classifying vegetation within sites except at the
freshwater site. Elevation and distance to channel were
significant in the DF analysis at the freshwater,
oligohaline, and mesohaline sites, while only distance to
channel was significant at the strongly oligohaline site.
Dominance-diversity curves (Fig. 2-6) are graphs of
relative importance (y-variable) as a function of species
ranking (x-variable), and represent relative species
dominance, or "dominance hierarchies" of a species
association. In Fig. 2-6, the strongly oligohaline and
mesohaline sites were steeper, with fewer species, and
correspond to Whittaker's (1965) geometric curve.
Freshwater and oligohaline curves exhibit a more gradual
slope and were more similar to lognormal curves
characteristic of higher species diversity and more discrete
levels within the hierarchies.
Discussion
The vegetation of tidal freshwater, oligohaline, and
mesohaline marshes of the lower Savannah River is typical of
that described by Baden et al. (1975) for Georgetown County,
South Carolina, and for southeastern coastal marshes in
general (Odum et al. 1984, Mitsch and Gosselink 1986).
Dominant plant species in the tidal freshwater marsh
included Z. miliaceae and Polygonum spp. along levees, and
Saaittaria spp., Scirpus spp., Typha spp., Eleocharis spp.,
and several others throughout the low and high marsh.

im|x>rtance value
33
1 .Sr
I
i6r
i
' i- Freshwater
Oligohaline
Strongly *
oligohaline
30
Mesohaline
Species rank
Fig. 2-6. Dominance-diversity curves for species importance
values (IV) at tidal freshwater, oligohaline, strongly
oligohaline, and mesohaline sites. Each species at each
site is represented by a point located by that species' mean
IV on the Y-axis, and its position in the sequence of
species from highest to lowest IV on the X-axis. IV =
relative density + relative biomass.

34
Dominant species in the more brackish marshes included
Spartina cvnosuroides. Baccharis halminifolia, and Myrica
cerifera along levees, and S. alterniflora. S. validus. and
S. robustus in the low and high marsh.
The ordination from DECORANA output successfully
separated freshwater from oligohaline and mesohaline
vegetation. Ordination of the freshwater site further
separated levee (Z. miliaceae), low marsh (S. validus and E.
montevidensis mix), and back marsh (S. validus and T.
latifolia mix) vegetation. Results emphasized the
transition from freshwater species, including Z. miliaceae.
E. montevidensis. S. validus. T. latifolia. and annuals, to
dominance by S. alterniflora at the mesohaline site.
DCA results suggested the main axis of variation in
species composition between the four sites was salinity.
These results support several studies (Odum 1988, Ewing
1983, Phleger 1971, Adams 1963) in which salinity was
considered the driving force in determining differences in
species diversity and plant distributions between tidal
freshwater and saltwater marshes. Salinity differences
between sites corresponded to changes in species
composition, although changes were marked by a gradual
transition of species, rather than distinct community types
usually evident when comparing freshwater with brackish
marshes (Odum 1988). Much of the overlap in vegetation
classes along the salinity gradient can be attributed to the

35
widespread distribution of S. validus. Not only was S.
validus a component species of 6 of the 10 vegetation
classes, it was also the only species present to tolerate
the range of physical factors present over the entire study
area. Early successional species have been shown to have
broad, overlapping niches in comparison with later
successional species, which show greater niche
differentiation (Parrish and Bazzaz 1982). The extensive
overlap of S. validus with other vegetation classes may
reflect its broad fundamental niche, as well as its
successional status as an early colonizer that may be
displaced.
Continuous overlap in vegetation classes may also
reflect a less abrupt salinity gradient (Beals 1969) from
freshwater to low salinity marshes. Saltmarshes, in
contrast, often have a steep salinity gradient accompanied
by distinct plant zonation that coincides with high and low
marsh (Phleger 1971, Kruczynski et al. 1978). Steeper
environmental gradients place competitors in a smaller area
of optimal environment, and interactions among competiting
individuals may be unavoidable. Over a broader gradient,
optimal environment occurs over a larger area and
individuals may escape competition just by the availability
of more room and less chance for direct contact.
The first DF analysis illustrated between-community
differences. The relationship between vegetation classes

36
and salinity, elevation, and distance from channel was
significant, although overlap among vegetation classes was
extensive. The second DF illustrated within-community
vegetation differences and indicated that elevation and
distance from channel were important in classifying
vegetation. Elevation largely determines hydroperiod, and
subsequent effects of hydroperiod on vegetation composition
and distribution are well documented (Mitsch and Gosselink
1986). Although there was a total elevational range of only
about 30 cm at each site, low and high marsh are exposed to
different flooding regimes, and vegetation zones reflected
these differences. The distance from a tidal channel
affects the tidal or river current energy reaching different
portions of the marsh, the soils deposited in the marsh, as
well as the hydroperiod. As a result, distance to channels
is also related to plant species richness and community
composition (Nilsson 1987). Distance from a tidal channel
appeared more important in the distribution of Saoittaria
spp. and T. latifolia. for example, which occur
predominantly in the back (or high) marsh of the freshwater
marsh. Distance from channel may reflect differences in
soil consolidation. Because coarser soil materials are
filtered out by vegetation, these soils drop out of the
flooding water near the levee first, and only very fine
silts are transported to the back marsh. Marsh that is not
adjacent to tidal channels often has a partially

37
consolidated surface held together by vegetation (a "quaking
marsh"), or even an unconsolidated surface with nearly
floating vegetation, which may limit the number of species
able to exist in these areas.
Infrequent misclassification of vegetation classes in
the DF analysis occurred when a vegetation class overlapped
with another in regards to where it occurred it terms of
site salinity, elevation, and distance from channel.
Classes that overlap share similar habitats based on the
measured parameters, and have been shown to occupy the same
or similar niches (McNeely 1987). The overlap itself gives
no indication of resource preferences of overlapping
species, although it does indicate the functional or
realized niche of species (Colwell and Futuyma 1971), i.e.,
what habitat or resource is being exploited. While these
results imply overlapping niches among vegetation classes,
the extensive overlap most likely demonstrates the
similarity in resource requirements and fundamental niche
exhibited by most plants (Parrish and Bazzaz 1982, Goldberg
and Werner 1983).
Niche width generally decreases as species numbers
(McNaughton and Wolf 1970, Pianka 1978) increase and
physiological stress decreases (Whittaker 1975, Anderson
1986). This suggests that niche overlap should be least at
sites with greater plant species numbers and no salinity
stress. Our results seemed at first to contradict this: 41

38
plant species were identified at the freshwater site where
salinities were lowest, compared with 8-12 at each of the
oligohaline and mesohaline sites, yet the greatest overlap
between vegetation classes occurred at the freshwater site
(Table 2-2). The apparent contradiction, however, most
likely reflects the steeper environmental gradient of the
mesohaline marsh which makes the relationship between
vegetation and environmental gradients more discernible.
Importantly, differences in overlap between freshwater
and brackish sites also emphasize the problem of
measurements of different scale. It has recently been
suggested that differences in "hierarchical environmental
structure" may explain differences at the community level
between severe and relatively benign environments;
homogeneity or heterogeneity of a habitat depends on the
resolution with which species experience their microhabitats
(Kolasa and Strayer 1988, Kolasa 1989). Extensive habitat
overlap in the freshwater marsh may indicate a "finer
grained" or more homogeneous habitat in which habitat
differentiation is not discrete at the level measured, while
the steeper environmental gradient of the oligohaline and
mesohaline sites exhibit greater differences over the same
amount of area and can be considered "coarse grained"
habitats (Kolasa 1988). Overlap in vegetation classes is
significant only in regards to the environmental parameters
measured and the resolution with which they were measured.

39
Results reflect the scale of measurements made and used in
classifying vegetation at each site.
Dominance-diversity curves of species from each site
illustrate the contrast in "hierarchical structure" between
sites. The geometric curve of the mesohaline and strongly
oligohaline marshes are characteristic of severe
environments with small numbers of species (Whittaker 1975).
Here, dominance is strongly developed and niche space is not
evenly divided among several species. Species dominance in
the freshwater and oligohaline marsh correspond more closely
to a lognormal distribution (Preston 1948) and is determined
by several factors that affect the competitive success of
species relative to one another. Species rich communities
and communities in which species occur across a wide
environmental range approach such lognormal distribution.
These curves represent freshwater plant species
distributions across benign environmental gradients which
exhibit only slight changes in regulating factors, thus many
plants compete equally for resources. In contrast, steep
gradients and physiological stress of brackish marshes may
result in well-developed dominance among a few species
competing for the same resources.
Physical parameters such as hydroperiod, salinity, and
soil characteristics have been strongly associated with
plant distributions. While these factors may be. the
dominant factors affecting plant associations in the

40
Savannah River marshes, competition is often important in
structuring plant communities (Bertness and Ellison 1987,
Barbour 1978), and it has been suggested that overlap in
resource utilization indicates competition among some
prairie plants (Platt and Weis 1977). Several studies have
shown species may be differentially displaced by competition
along an environmental gradient (Barbour 1978, Rabinowitz
1978, Grace and Wetzel 1981, Snow and Vince 1984). Studies
with Jaumea carnosa (Barbour 1978), S. alterniflora (Adams
1963), Spartina foliosa (Phleger 1971), and Typha
anaustifolia (McMillan 1959) have found that these species
grow better in freshwater but are confined to higher
salinity marshes by species better specialized to freshwater
conditions. Competitive interactions may be more important
to within-community structure, while salinity and other
physical parameters exert a greater influence on between-
community structure.
The importance of competition cannot be overlooked in
any explanation of plant distributions. While results of
the present study are not sufficient to address the role of
competition in structuring these plant associations,
competition will be addressed in subseguent chapters.

CHAPTER 3
SPATIAL DISTRIBUTIONS OF THE SOFTSTEM BULRUSH,
SCIRPUS VALIDUS, ACROSS A SALINITY GRADIENT
Introduction
Plant species distributions in tidal marshes have been
attributed predominantly to environmental gradients of
salinity (Phleger 1971, Morris et al. 1978, Leiffers 1983,
Haramis and Carter 1983, White 1983), site elevation and
hydroperiod (Ferren and Schuler 1980, Lieffers 1983, Dawe
and White 1982, White 1983), and soil characteristics (Kirby
and Gosselink 1976). In salt marshes, where environmental
gradients are often steep and correlated with one another,
they may be strongly associated with community structure and
species distribution patterns (Snedaker 1982, Hoover 1984).
Clumped distribution pattern or zonation of salt marshes may
indicate a species response to environmental gradients
(Pielou and Routledge 1976, Cooper 1982, Vince and Snow
1984), competition (Snow and Vince 1984, Bertness and
Ellison 1987), or physical disturbance (Turner 1987,
Bertness and Ellison 1987).
Zonation in tidal freshwater marshes is much less
distinct than in salt marshes (Odum 1988). There is greater
overlap in habitat (de la Cruz 1981, Joyce and Thayer 1986,
Odum 1988), and relations among species and environmental
41

42
factors are more complex (Hoover 1984, Odum 1988), obscuring
species zonation.
Although the importance of pattern in plant species
distributions has received considerable attention (Pielou
and Routledge 1976, Mack and Harper 1977, Dale 1986,
Shaltout 1987), comparative studies are generally restricted
to species distributions within a community type (e.g. salt
marsh) and do not address community distribution patterns
between community types (e.g. tidal freshwater to salt
marshes). Because species interactions occur at the level
of individual neighbors (Mack and Harper 1977, Fowler and
Antonovics 1981) the distribution patterns of a single
species may affect larger scale community structure (Harper
1977, Fowler and Antonovics 1981, Dale 1986).
In this study, the distribution patterns of Scirpus
validus. a dominant species in both freshwater and brackish
marshes along the lower Savannah River, were examined. The
objectives were to (1) determine the distribution patterns
of S. validus. and (2) examine the role of environmental
factors in the distribution of S. validus.
Methods
Mean number of species at each site and species
similarity between sites (Whittaker 1975; Is = (total
species at site x plus the total species at site y)/ number
of species common to both sites) were calculated.

43
Between-site differences in environmental variables and
species numbers were compared using Waller-Duncan multiple
means comparison tests. The same test was used to compare
changes in soil organic matter associated with distances
from primary creeks along transects. Elevation changes
along transects were compared using Tukey's standardized
range test.
Coefficients of dispersion (CD) were calculated for
each site, for each sampling period, as the variance to mean
ratio of standing densities of S. validus. The coefficients
were used as a measure of the amount of nonrandom dispersion
of stems (see Sokal and Rohlf 1981, Whittaker 1975, Kershaw
1973). A CD value egual to 1 indicates a random
distribution of individuals. Clumped distributions have a
CD > 1, and a CD < 1 represents a uniform distribution.
Paired t-tests were used to determine departures from
randomness (i.e., CD values significantly different from 1).
Between-site differences in CD values were compared using
Waller-Duncan multiple means comparison tests.
Multiple regression analysis was used to examine the
relations of S. validus importance values (IV) with measured
environmental parameters at sites (see Methods and Results,
Chapter 1). Regression analysis was also used to examine
the relationship of species numbers and IV with the salinity
gradient across all 4 sites.

44
Results
Species Associations
The average number of species was significantly greater
at the freshwater site (F = 28.73? N=27; P = 0.05). The
number of species was not significantly different among the
oligohaline, strongly oligohaline, and mesohaline sites.
Freshwater marsh had an average of 18 species, compared to
less than 7 in the oligohaline, 3 in the strongly
oligohaline, and 5 in the mesohaline marsh (Fig. 3-1). The
large number of species in the freshwater marsh was due
mostly to the presence of annual species. Similarity in
species composition was greatest between the strongly
oligohaline and mesohaline and least between freshwater and
mesohaline marsh sites (Table 3-1).
The highest mean IV for S. validus (Fig. 3-2) occurred
in the strongly oligohaline marsh (Mean = 1.16), and the
lowest at freshwater (Mean = 0.39) and mesohaline (Mean =
0.41) sites. Scirpus validus was surpassed in IV by
Zizaniopsis miliaceae and Eleocharis montevidensis at the
freshwater site, and by Spartina alterniflora in the
mesohaline marsh. Scirpus validus had the highest IVs at
oligohaline and strongly oligohaline sites.
Spatial Pattern
Differences in interstitial salinities were accompanied
by differences in spatial pattern in S. validus (Fig. 3-3).
Coefficients of dispersion (CD) exceeded 1.0 (P = .05) at

45
Freshwater Oligohaline S. Oiigohaline Mesohaline
Fig. 3-1. Mean number of species (± 1 standard error) at
freshwater, oligohaline, strongly oligohaline (S.
Oligohaline), and mesohaline sites. Means falling within
the same vertical line to the right of graph are not
significantly different (Waller-Duncan, P = 0.05).

46
Table 3-1. Similarity in species composition (Is) between
freshwater, mildly oligohaline, strongly oligohaline, and
mesohaline sites.
Site
Fresh
Oligo
S. Oligo
Fresh
1.00
Oligo
0.51
1.00
S. Oligo
0.20
0.43
1.00
Meso
0.12
0.25
0.71
1.00

47
Fig. 3-2. Mean Importance Values (± 1 standard error) for S.
validus at freshwater, oligohaline, strongly oligohaline (S.
Oligohaline), and mesohaline sites. Means falling within
the same vertical line to the right of graph are not
significantly different (Waller-Duncan, P = 0.05).

48
Freshwater Oligohalinc S. Oligohaline Mesohaline
Fig. 3-3. Mean Coefficients of Dispersion (+ 1 standard
error) for £. validus over a gradient of increasing soil
water salinities at freshwater, oligohaline, strongly
oligohaline (S. Oligohaline), and mesohaline sites. Means
falling within the same vertical line to the right of graph
are not significantly different (Waller-Duncan, P = 0.05).

49
the freshwater (Mean = 1.41; STD = 0.28; n = 7), and
mesohaline (Mean = 1.84; STD = 0.55; n = 7) sites,
indicating clumped distributions. CD values of S. validus
at the strongly oligohaline site (Mean = 0.62; STD = 0.28; n
= 7) were significantly less than 1.0, indicating a uniform
distribution of S. validus. Only the oligohaline site (Mean
= 1.15; STD = 0.34; n = 7) exhibited a random distribution
of S. validus (P = 0.24). CD values indicated
significantly greater clumping in the mesohaline marsh when
compared to the freshwater and oligohaline sites (F = 12.73;
P = .05) .
Relationship of IV with Environmental Variables
Importance values for S. validus (Fig. 3-1) showed a
significant quadratic relationship with increasing soil
water salinities (y = 0.35 + 0.4x - 0.03x2; R2 = 0.16; n =
448; P = 0.0001). Scirous validus IVs were significantly
related to soil water salinities and elevation at all but
the oligohaline site (Table 3-2). R-square values for
multiple regressions were less than 0.30 for all 4 sites.
Salinity and elevation added significantly to the regression
model in determining S. validus IV at the freshwater site (P
= .05). Elevation, but not salinity, was significantly
related to IV at the mildly oligohaline (P = .05) and
mesohaline (P = .05) sites. Only salinity was significantly
related to IV at the strongly oligohaline site (P = .05).

50
Table 3-2. R-square, n, F, and P values for multiple
regression analysis of S. validus IVs with interstitial
salinities (ppt), elevation (cm) and percent soil organic
matter at freshwater, mildly oligohaline, strongly
oligohaline, and mesohaline sites.
Site
R2
N
F
Fresh
0.23
103
4.80“
Oligo
0.13
87
1.96*
S. Oligo
0.25
100
5.23"
Meso
0.27
91
5.11"
*P = 0.05; **P =
0.01

51
Soil organic content was not significant in determining S.
validus IVs at any site.
Discussion
Species Diversity
The decrease in species numbers and replacement by
other, more salt tolerant species across the 4 sites were
typical of community changes from freshwater to brackish
marshes. Species similarity was greatest for those sites
with similar salinity regimes and least for those with
greatest differences in soil water salinities. Seasonal
differences in species composition due to annuals strongly
influenced species diversity in freshwater marshes. High
species similarity between adjacent sites along the
continuum from freshwater to mesohaline marsh, and the
overlap of S. validus throughout the gradient emphasize the
gradual shift in species composition from freshwater to
brackish marsh plant communities in this study.
Distribution Patterns
Coefficients of dispersion indicate a significant
change in spatial pattern for S. validus across the salinity
gradient. Scirpus validus was clumped in monospecific
stands at the mesohaline site. Significantly less clumping
occurs in freshwater and very low salinity marshes, where S.
validus was interspersed with freshwater plant species. At
the strongly oligohaline site, S. validus had a uniform
spatial distribution, rather than the discrete clumps found

52
clumps found in the mesohaline marsh. Species diversity was
low and IVs of S. validus high at intermediate salinities.
Uniform and clumped distributions, while not proof of
competition or environmental specialization, do indicate
that more than random chance is at work in producing or
maintaining the pattern. The uniform, or regular,
distribution of individuals occurs when available space is
almost fully occupied (Pielou 1959), allowing each
individual the same amount of space. While changes in
pattern among most wetland species have not been examined,
some desert shrubs exhibit uniform distributions as a result
of intra-specific competition (King and Woodell 1973,
Shaltout 1987). Intraspecific competition may explain the
distribution of S. validus at the strongly oligohaline site
in this study, where freshwater species are physiologically
intolerant and Spartina alterniflora is infrequent except at
the edges of canals and the river. With few co-occurring
species and high densities of S. validus. neighborhood
competition among individuals may be limited to competing
individuals of S. validus. resulting in a uniform
distribution pattern.
Results from several studies (Sharitz and McCormick
1973, Grace and Wetzel 1981, Snow and Vince 1984, Wilson and
Keddy 1986) suggest that along an environmental gradient,
competitive interactions may determine species distributions
at one end of the gradient where tolerance ranges overlap,

53
while physiological tolerance limits species distributions
at the other end of the gradient. Separate multiple
regressions of S. validus IV with environmental factors at
each site indicate a stronger relationship at the mesohaline
site. This suggests that environmental gradients are
primarily responsible for the clumping of S. validus at the
mesohaline site, where salinities are highest. The clumping
pattern reflects dominance of single species in zones, and
possibly greater habitat differentiation in the more saline
marsh between a limited number of species. Competition,
environmental tolerance, disturbance, and vegetative growth,
all probably play a role in explaining the clumped
distribution of S. validus.
Given the wide environmental tolerances of S. validus
and the large number of species at the freshwater site, the
distribution pattern at this site may be influenced more
strongly by competitive interactions. Resources must be
shared with neighboring individuals and individual stems
occur much farther apart. Variations in light, created by
differences in plant architecture and growth form (Jones
1983), and subseguent competition for light and space, may
account for variation in densities of S. validus in the
freshwater marsh.
While physiological mechanisms alone do not explain the
quadratic relationship of importance values for S. validus
with increasing soil water salinities, the relationship may

54
be explained by a combination of physiological and
environmental mechanisms. Scirpus validus is distributed
across a broad environmental gradient along the lower
Savannah River. A wide range of environmental tolerances
has been reported for this wetland plant (Beal 1977, Barko
and Smart 1978, Langeland 1981, Barclay and Crawford 1982,
Joyce and Thayer 1986), which reproduces readily both vege-
tatively and by seed (Godfrey and Wooten 1979). Like
saltwater and brackish marsh plants that generally grow best
in freshwater but are limited by competition (Mooring et al.
1971, Phleger 1971, Barbour 1978), S. validus may also be
limited by competition to intermediate salinities where
freshwater species cannot survive and extreme salt tolerant
species cannot compete well.
The quadratic relationship between IV and salinities
also reflects pattern differences among sites. At the
strongly oligohaline site, where the CD value was lowest, S.
validus exhibited the highest stem densities and occurred
with few other species. Salinities at this site appear to
be high enough to limit freshwater species without
inhibiting S. validus.
Lower IVs in the freshwater site reflect low densities
of S. validus mixed with many other species in a clumped
distribution, but less clumped than at the mesohaline site.
Generally, S. validus occurred in very high densities or not

55
at all in the mesohaline marsh due to increased species
zonation.
Elevation, but not salinity was significantly related
to S. validus IVs in mildly oligohaline marsh. Higher
salinities extend farther upstream in the Middle River than
in the Little Back River. Consequently, interstitial
salinities at the mildly oligohaline site reflect higher
salinities of the Middle River on the northwest side of the
marsh and lower Back River salinities on the southeast side.
While variation in salinities was greatest at this site, it
was still significantly different from other sites.
Significant changes in spatial distribution along
physical gradients for S. validus in this study suggest that
salinity strongly influences its distribution, but not
directly. Scirpus validus is a generalist species, capable
of growing well over a wide range of salinities. Many other
species perform as well or better at low salinities and in
freshwater, but resources must be shared by all species
under these conditions. As salinities increase, less
species are physiologically tolerant, and S. validus occurs
as a dominant species. As salinities increase across the
salinity gradient, Spartina alterniflora and Scirpus
robustus are able to better compete for resources, and
dominance by S. validus decreases.
Within individual sites, the influence of a salinity
gradient appears to be replaced by less distinct elevational

56
differences, and changes in species distributions also
become less distinct. In this study, elevation was a
significant factor in determining IVs of S. validus within a
site. Low elevations at the mesohaline site resulted in
increased hydroperiod which, combined with increased
salinities, may account for the increase in dominance by S.
alterniflora and zonation of S. validus in the higher marsh.

CHAPTER 4
MORPHOLOGICAL PLASTICITY IN SCIRPUS VALIDUS
ALONG A SALINITY GRADIENT
Introduction
Results of several transplant experiments have
demonstrated both ecotypic and ecophenic variation in plant
morphology along environmental gradients. After being
transplanted to different habitats, morphological
characteristics of glasswort (Salicornia europaea) (Ungar
1987, Jefferies 1981), cinquefoils C Potentilla erecta)
(Watson and Fyfe 1975), and yarrow (Achillea millefolium)
(Clausen et al. 1948), were shown to remain constant,
exhibiting ecotypic variation.
Species from contrasting habitats may also have a
morphology that changes in response to the environment. In
experiments with Spartina alterniflora (Shea et al. 1975),
and several other salt marsh species (Seliskar 1985), plants
that were moved from one end of a salinity gradient to
another became morphologically similar to their new
neighbors, exhibiting ecophenic, rather than ecotypic,
variation in contrasting habitats.
Salt marshes often exhibit gradients of increasing soil
salinities from low to high marsh as a result of less
frequent tidal inundation and increased evaporation and
57

58
plant evapotranspiration in the high marsh. Decreases in
density, biomass, and/or height for Sporobolus virainicus
(Donovan and Gallagher 1985), Spartina foliosa (Phleger
1971), Juncus roemerianus and Spartina alterniflora
(Kruczynski et al. 1978) have been associated with the
increased salinities from low to high marsh in field and
greenhouse experiments.
Tidal amplitudes along the lower Savannah River in
South Carolina and Georgia range from 2.5 to 3 meters,
inundating most of the marsh, and eliminating any salinity
gradient from low to high marsh at a given site. Marshes
are, however, associated with the first 28 miles of the
river from the ocean, and there is a wide salinity gradient
downstream to upstream. In this region, Scirpus validus is
the only species consistently present throughout tidal
freshwater, oligohaline, and mesohaline marshes that
otherwise differ strongly in species composition.
The objectives of this study were to test 1) whether
morphological variation in S. validus is significant between
contrasting habitats, 2) whether morphological variation in
S. validus is ecotypic or ecophenic, and 3) whether measured
differences in morphology are directly related to salinity.
Light extinction was also measured to determine whether it
may have a more direct influence than salinity on
morphological variation in S. validus. Because different
growth morphologies may influence macrophyte community

59
composition (Barko and Smart 1981), variation in S. validus
morphology may further understanding of factors which help
structure plant communities in this area.
Methods
The study area included freshwater (0 ppt), oligohaline
(5-7 ppt), and mesohaline (9-10 ppt) tidal marshes of the
lower Savannah River, in Chatham County, Georgia, and Jasper
County, South Carolina. Vegetation composition in the study
area is similar to other non-cultivated, naturally occurring
tidal marshes of the southeastern coast (Odum et al. 1984, and
see Ch. 1 and 2).
In July 1987, S. validus stem densities, internode
lengths of rhizomes, and stem heights were measured.
Measurements were made on 10 replicate S. validus stems from
10 random, 0.25 m2 plots at each site. Variation in S.
validus stem heights and internode lengths were calculated
using the coefficient of variation, CV = (standard deviation
/ mean) x 100. CV values indicate the relative amount of
variation in stem height and internode length within each
site.
In December 1987, S. validus in 0.25 m2 plots,
approximately 15 cm deep, were randomly collected from the
oligohaline marsh site. Twenty of these plots were
transplanted to the freshwater site and 20 to the mesohaline
site. Five additional plots at the donor site were removed
and then replaced as controls to determine if transplanting

60
effects were significant. Eighteen undisturbed plots
previously established at each site were used for comparison
with transplants and controls (A Reichert refractometer was
used to measure and monitor soil water salinities at field
sites and in the greenhouse). Above-ground vegetation from
transplants, controls, and undisturbed plots was harvested
in June 1989. Stem heights and densities were measured for
each plot. Because there was no way to determine which
parts of rhizomes were produced during the 1.5 years at
transplant sites, internode distances were not measured for
transplants.
In December 1988 a total of 60 rhizome sections of S.
validus were collected randomly from freshwater,
oligohaline, and mesohaline sites. Sections were washed
free of soil and debris, cut into node sections 2.5 cm in
length, and planted in shallow greenhouse trays of potting
soil. Three treatments, each consisting of 2 trays from
each site and 10 sections per tray, were watered daily with
0, 5, and 10 ppt seawater solution for 6 months.
Differences in light extinction between sites and
between vegetation types were also measured in December
1988. Light intensity (uE x m2 x sec-1) was measured at the
top of the vegetation canopy and at ground level using a
LICOR quantum radiometer (Model # LI-185B) and a flat
quantum sensor. Light extinction coefficients through the
vegetation were calculated from the LICOR measurements.

61
Three replicate readings were taken at 10 random points
within each vegetation type and percent cover of vegetation
was recorded at each site. Vegetation types at the
mesohaline site were based on dominant species. Vegetation
types were S. validus, Spartina alterniflora. and Spartina
cynosuroides in the mesohaline marsh. Vegetation types in
the freshwater marsh were Eleocharis montevidensis. S.
validus mixed with Zizaniopsis miliaceae. and S. validus
mixed with Eleocharis montevidensis.
Waller-Duncan multiple means comparison tests (P =
0.05) were used to compare greenhouse measurements, field
measurements, and light extinction treatments. Means
comparisons were also used to determine if differences were
significant (P = 0.05) between transplanted plots, control
plots, and undisturbed plots, within treatments. Student's
t-test (P = 0.05) was used to compare transplanted densities
and stem heights between freshwater and mesohaline sites.
Results
Field Measurements
Scirpus validus had shorter internode lengths, taller
stems, and greater densities at the oligohaline and
mesohaline sites when compared with the freshwater site
(Fig. 4-1). These traits were not significantly different
between oligohaline and mesohaline marsh sites, although
both differed significantly from the freshwater site.
Variation in stem height of S. validus was lowest at the

Fig. 4-1. Mean (± 1 SE) internode lengths, stem heights,
and stem densities for S. validus from field sites and
greenhouse. Means falling within the same vertical line to
the right of graph are not significantly different (Waller-
Duncan, P = 0.05).

Inlernode length (cm)
Stem density (no. / m2)
O'
Ol
o
-n
O
S
o
Internode length (mm) Stem density (no./m2)
Stem height (cm)
8 I §
+
+
+
Stem height (cm)
Field Greenhouse

64
freshwater site for stem height (CV = 6.3), and higher at
the mesohaline (CV = 10.98) and oligohaline (CV = 12.4)
sites. Variation in internode distance was highest at the
oligohaline site (CV = 79.9), and lower at the freshwater
(CV = 48.4) and mesohaline (CV = 46.1) sites.
Greenhouse Experiments
Stems/m2 (and node survival) were greater overall for
freshwater treatments (Fig. 4-1). Stem densities and
surviving nodes did not differ between freshwater and
oligohaline treatments. The olighaline and mesohaline
treatments were not significantly different from one another
with respect to stem density. Stem heights of S. validus
were also significantly greater for freshwater and
oligohaline treatments when compared with mesohaline
treatments (Fig. 4-1), though stems did not grow to heights
measured in the field. Differences in internode lengths
between treatments were not significant.
Transplants
Scirpus validus transplanted from the oligohaline site
to freshwater and mesohaline sites changed significantly in
stem height and density after 18 months. In contrast with
greenhouse results, S. validus transplanted to higher
salinities had taller stems and greater stem densities when
compared with freshwater transplants (Fig. 4-2).
Stem heights of S. validus transplants from the
oligohaline site converged with values characteristic of the
site to which they were transplanted (Fig. 4-2). Stems of

Stem density (no. / m2) Stem heights (cm)
65
140
120
100
80
60
40
20
0
300
250
200
150
100
50
0
TCU CU TCU
Freshwater Oligohaline Mesohaline
Fig. 4-2. Mean (± 1 SE) stem heights and stem densities
for S. validus transplanted from oligohaline marsh to tidal
freshwater and mesohaline marsh. Hatched bars represent
mean values for transplanted plots (T), solid bars represent

66
transplants were significantly shorter at the freshwater
site when compared with the mesohaline site. Differences in
stem heights were significant between freshwater transplants
and oligohaline control plots, while differences between
mesohaline transplants and oligohaline control plots were
insignificant.
Differences in S. validus stem heights between
transplant, control, and undisturbed plots were significant
at only the freshwater site (Fig. 4-2) . Transplants at the
freshwater site had significantly smaller stem heights than
undisturbed plots and were significantly smaller than the
mean stem height at oligohaline and mesohaline sites. At
both freshwater and mesohaline sites, control plots had
significantly greater stem heights than S. validus
transplanted from the oligohaline site.
Stem densities of transplants to freshwater and
mesohaline sites decreased to densities that were not
significantly different from those of the control plots at
freshwater and mesohaline sites. Differences in stem
densities between freshwater transplants that originated at
the oligohaline site and control plots at the oligohaline
site were significant. Differences between mesohaline
transplants and oligohaline controls were not
significant. Undisturbed plots of S. validus at the
oligohaline site also had greater densities when compared
with plots transplanted to freshwater and mesohaline sites.

67
Differences in mean stem densities of S. validus between
transplant, control, and undisturbed plots were not
significant at any of the 3 sites (Fig. 4-2).
Transplants to the freshwater site were invaded
predominantly by Eleocharis montevidensis. Hydrocotyle
umbellatum, and Aster spp. Zizaniopsis miliaceae and Typha
latifolia. commonly co-occurring species with S. validus at
the freshwater site, were rarely present in the transplants.
Mesohaline transplants were invaded almost exclusively by
Spartina alterniflora.
Light Extinction
Light extinction through the vegetation to the soil
surface was significantly greater at the freshwater site
when compared with the oligohaline and mesohaline sites
(Fig. 4-3). Plant species and percent cover accounted for a
significant amount of variation in light extinction within
each marsh site (ANOVA; F = 2.43; P = 0.01; n = 107) (Table
4-1). Internode lengths were greatest, and stem heights and
densities lowest for S. validus at sites where light
penetration through the canopy was lowest.
Discussion
Detrimental effects of increased salinities on plant
growth were evident in decreased stem heights and densities
of S. validus in greenhouse experiments. These results were
similar to those for other studies in which salt tolerant
species were found to grow best under freshwater or low

68
Fig. 4-3. Mean (± 1 SE) light extinction coefficients for
freshwater, oligohaline, and mesohaline sites. Means
falling within the same vertical line to the right of graph
are not significantly different (Waller-Duncan, P = 0.05).

69
Table 4-1. Variation in light extinction coefficient
between and within sites.
Source of variation
D.F.
F value
Marsh
2
5.76**
% Open canopy
10
2.79*
Species (marsh)
5
2.42**
Species (% open)
2
6.49**
Replicates (marsh)
18
1.42(N.S.)
*Significant at P = 0.05 level; **Significant at P = 0.01
level; N.S. = not significant.

70
salinity conditions (Phleger 1971, Jackson 1952, Parrondo
1978, Cooper 1982, Seneca 1969), indicating that higher
salinities inhibit growth in these species.
Transplants of S. validus growing at the freshwater
site exhibited a significant decrease in stem heights and
densities compared with the mesohaline transplant site and
oligohaline donor site. In both greenhouse and transplant
experiments, however, variation in morphology was not
related to the salinity of the area from which the plants
were taken, suggesting that morphological variation in S.
validus is not genetically fixed. Variation was dependent
upon local conditions rather than plant origin, and
demonstrates that changes in S. validus morphology across a
salinity gradient are ecophenic, rather than ecotypic,
responses.
Salinity has been found to have an overriding effect on
growth and survival in some experiments with Salicornia
europeae (Ungar 1987). In contrast, several studies have
concluded that competition, disturbance, light penetration,
and herbivory were more important in the growth and
distribution of this particular species (Ellison 1987), as
well as others (Bertness and Ellison 1987). While S.
validus exhibited plastic responses to environmental
conditions in both greenhouse and transplant experiments,
growth patterns were reversed between the two. .Greenhouse
plants grew better at 0 ppt, while field transplants did

71
better at 5 and 10 ppt. Salinity had no effect on internode
length in greenhouse treatments, but field measurements of
internode lengths differed between freshwater, oligohaline,
and mesohaline field sites.
Discrepancies in stem heights, stem densities, and
internode lengths between greenhouse experiments and field
transplants indicate morphological plasticity observed in S.
validus was not solely attributable to salinity. Factors
other than salinity appeared to be important in the actual
distribution and growth morphology of S. validus. Scirpus
validus transplanted to the freshwater site was unable to
expand into immediately adjacent areas, where other species
were already present and overall stem densities were high.
Internode lengths at field sites were greater, averaging 2.2
cm, compared with an average of 0.5 cm in greenhouse plants.
Greenhouse plants, because they were planted at low
densities without other species present, could expand into
open areas where there was plenty of light and produce
shorter internode lengths and closely arranged stems.
Transplants of S. validus were invaded at both
freshwater and mesohaline sites by neighboring species until
densities and species composition reached those of
undisturbed plots at their respective transplant sites.
These changes suggested that interspecific competition may
be important in the morphology and distribution of S.
validus. Decreases in stem heights and densities, and

72
invasion of S. validus transplants by salt intolerant
species at the freshwater site may be due to greater
competition from freshwater species under these conditions.
Physiological specialization on a widely distributed
habitat type (e.g., freshwater marsh) results in ecological
generalization, whereas physiological generalization to
several habitat types (e.g., freshwater and brackish
marshes) results in ecological specialization (McNaughton
and Wolf 1970). Scirpus validus. a physiological
generalist, may be outcompeted at the freshwater site by
physiological specialists (e.g., Eleocharis montevidensis
and Typha latifolia) because it cannot compete with the
specialist within the specialist's range of physiological
tolerance.
Salinities at the oligohaline site were high enough to
inhibit freshwater species and low enough that stem heights
and densities of S. validus were highest, internode lengths
were shortest, and Spartina alterniflora was unable to
expand into the site any farther than river and creek edges.
Transplants at the mesohaline site exhibited decreased stem
heights and densities, and were invaded by S. alterniflora.
It may be that S. validus is not as great a physiological
generalist as S. alterniflora. Spartina alterniflora. more
tolerant of higher salinities, was able to invade and limit
the growth of S. validus. probably due to inhibitory effects
of higher salinities on S. validus.

73
Previous studies (Barbour 1978, Rabinowitz 1978) have
noted that plant species' distributions often do not
coincide with their physiological tolerances and that
competitive interactions may be responsible for displacement
of species from a habitat. Species' physiological
tolerances and competitive interactions have also been shown
to differ along an environmental gradient; physiological
tolerance may limit a species at one end of an environmental
gradient where conditions are relatively harsh, while
competitive interactions may be of greater importance along
the other, more benign portion of the gradient (Grace and
Wetzel 1981, Snow and Vince 1984, Wilson and Keddy 1986).
Differences in morphology and densities for S. validus
appear to be a result of differences in relative competitive
abilities and salt tolerance between S. validus and other
species. Scirpus validus was outcompeted at the freshwater
site due to greater competitive abilities of other species,
while it was limited by its own salt intolerance and
decreased competitive ability at the mesohaline site. More
specifically, differences in transplanted S. validus may be
a result of light extinction differences between sites and
the ability of S. validus to compete for available light by
morphological adaptation to differential light regimes.
Light-related morphological changes such as internode
elongation may confer a competitive advantage on a species
by enabling the plant to expand into less light-limiting

74
vegetation or gaps. Morphological variation and decreased
survival have been associated with decreased light
penetration in Salicornia europaea (Ellison 1987), and
decreases in mean internode length have resulted from
shading in submersed freshwater macrophytes (Spence and Dale
1978, Barko and Smart 1981, Barko et al. 1982). Numbers of
species and subsequent differences in canopy layers and
complexity accounted for a significant amount of the
variation in light extinction within each marsh site.
Differences in morphology at field sites may be a result of
responses to light limitations placed on S. validus by
neighboring individuals.
Morphological variation has been used to illustrate
competitive interactions among animal species. Data on the
morphology of birds (Van Valen 1965), lizards (Lister
1976a), and harvesting ants (Davidson 1978) provide evidence
that variation in morphology increases as interspecific
competition and the number of species decreases. This may
further explain why morphological differences in S. validus
coincide with species numbers more so than salinity
gradients. Coefficients of variation for field measurements
of S. validus stem heights and internode lengths were lowest
at the freshwater marsh, where species numbers were
greatest. While examples of morphological variation have
most often been described for species in relatively harsh
environments, for example, salt marshes, deserts, and

75
exposed mountain slopes, and may reflect steep environmental
gradients in these habitats, results of this study
demonstrate that morphological variation observed for S.
validus is not directly due to a strong environmental
gradient. A reduction in the range of morphological
variation expressed by S. validus in the freshwater marsh
may be due to increased species numbers and competitive
interactions and may account for the lower variation in stem
height and internode length in S. validus at this site.
Conversely, decreased species numbers and fewer competitive
interactions at higher salinities may allow a greater range
of morphological expression at the mesohaline site.
In conclusion, different growth morphologies in S.
validus are ecophenic responses to local abiotic and biotic
conditions. While salinity alone is not responsible for
variation in measured plant characteristics, it greatly
affects species composition, which in turn may reflect the
degree of physiological specialization and competitive
ability of species in freshwater and low salinity marshes.
Species composition also affects light availability. Not
only is light an important resource for which plants
compete, but it also influences morphological variation in
several species, and light appears important in S. validus
as well. Morphological variation in S. validus further
indicates differences in the degree of competitive

76
interactions between plants of tidal freshwater and brackish
marshes along the lower Savannah River.

CHAPTER 5
THE ROLE OF COMPETITIVE INTERACTIONS, SOIL SALINITY, AND
DISTURBANCE IN THE DISTRIBUTION OF TIDAL MARSH PLANT SPECIES
Introduction
Plant species of tidal freshwater marshes are numerous
and have narrow, well differentiated niches (Odum 1988), in
comparison with saline marshes, suggesting that competitive
hierarchies are well developed and competition among species
is diffuse rather than species-specific. Freshwater species
distributions may reflect better developed and more complex
competitive interactions characteristic of late successional,
environmentally benign communities. In contrast, brackish
marsh vegetation is more strongly influenced by salinity-
induced physiological stress and steeper environmental
gradients (Phleger 1971, Kruczynski et al. 1978)
characteristic of relatively early successional communities.
Species in such harsh environments generally have broadly
overlapping resource requirements, resulting in more intense
competitive interactions (Parrish and Bazzaz 1982), as well
as altered dominance hierarchies (Anderson 1986).
Few experiments have directly addressed the role of
competitive interactions in species distributions along
environmental gradients (see Chapter 4). Pair-wise species
77

78
competition experiments across a gradient of stress
and disturbance have demonstrated lower competitive
abilities for species where environmental stress is greater.
Conversely, where conditions are more benign, competitive
abilities are greater (Wilson and Keddy 1986). The combined
consequences of competition and environmental gradients on
plant interactions have not been directly examined. Studies
of salt marsh species distributions have shown that, in
addition to physical factors, species interactions (Bertness
and Ellison 1987, Bertness and Ellison 1987, Snow and Vince
1984) are important to plant community structure. Several
of these studies have also implied that along an
environmental gradient, competitive interactions may
determine species distributions at one end of the gradient,
while physiological tolerance may limit species
distributions at the other.
In addition to the effects of physical factors and
species interactions, plant species abundance and
composition in mixed species salt marshes can be altered
(Bakker 1985), plant regeneration inhibited (Chabreck 1968),
and net aboveground primary production reduced (Turner 1987)
as a result of trampling by foraging animals. Soil
disturbance from rooting and trampling by feral hogs often
results in a reduction of herbaceous cover and local
extinction of some plant species (Bratton 1974). These
changes have not, however, been documented in freshwater

79
tidal marshes. A large feral hog population inhabits
freshwater and oligohaline regions of the study area and may
significantly affect plant community structure. Vegetation
response to such disturbances will help define the role of
disturbance in structuring freshwater tidal marshes.
The objective of this study was to examine the role of
competition in structuring plant communities across a
salinity gradient of freshwater, oligohaline, and mesohaline
tidal marshes and to document possible effects of animal
disturbance. Species interactions occur at the level of
individual neighbors (Mack and Harper 1977, Fowler and
Antonovics 1981) and the distribution of a single species
may affect larger scale community structure (Mack and Harper
1977, Fowler and Antonovics 1981, Dale 1986). Because of
this, I chose to use Scirpus validus. the only species which
occurred throughout the salinity gradient, as the primary
indicator species in examining the distribution of species
as a function of competitive variation along this gradient.
Two general hypotheses were proposed: (1) S. validus.
the species with the widest range of occurrence, was
restricted under freshwater conditions by more specialized
species better able to compete for resources, (2) at higher
salinities, physiological tolerance is more important in
limiting the distribution of S. validus. Removal and
addition of species in both field and greenhouse
experiments, combined with reciprocal transplants of species

80
across the salinity gradient, were used to test these
hypotheses. Competitive ability was measured as relative
increases in biomass and density. A third hypothesis
proposes that disturbances created by feral hog activities
will result in a local reduction in species diversity.
Disturbance was measured as the change in abundance of
species following feral hog activities.
Differences in competitive ability along an
environmental gradient should be apparent for a species such
as S. validus because of its wide distribution. The range
of salinity tolerance exhibited by S. validus indicated that
it is a physiological generalist, and should therefore be
competitively unsuccessful in combination with physiological
specialists within the range of the specialist(s). In the
freshwater marsh, removal of neighbors would be expected to
enhance the growth and expansion of S. validus. while
removal of S. validus may or may not have a significant
effect on neighboring freshwater species. Transplants of S.
validus into freshwater marsh from monospecific stands in
oligohaline marsh should succumb to the greater competitive
abilities of freshwater species.
Increased salinities of oligohaline and mesohaline
marshes place greater physiological stress on plant species
and zonation of species was conspicuous. Scirpus validus
may have occurred in high marsh zones because of its
competitive superiority over other marsh species at the less

81
stressful higher elevations, and be restricted from the low
marsh by a physiological intolerance of frequent flooding.
Competition under these conditions was expected to favor the
species with the broader salinity tolerance, e.g. S.
alterniflora. and result in greater competitive interactions
between S. validus and other species.
Methods
The study sites included freshwater (<1 ppt),
mildly oligohaline (2-4 ppt), strongly oligohaline (5-7
ppt), and mesohaline (8-10 ppt) tidal marshes of the lower
Savannah River, in Chatham County, Georgia, and Jasper
County, South Carolina (see Chapter 2). The high energy,
dynamic conditions of tidal marshes maintain these systems
in an early successional stage (de la Cruz, A.A. 1981),
while the salinity gradient from freshwater to brackish
marshes creates a natural stress gradient. Tidal marshes of
the lower Savannah River provided an opportunity to examine
competitive interactions among plant species across a
salinity gradient.
Eleocharis montevidensis was a dominant species co-
occuring with S. validus in the freshwater marsh along with
several freshwater annuals. Soartina alterniflora is the
co-dominant species under mesohaline conditions. At mildly
oligohaline conditions, the dominant species are Zizaniopsis
miliaceae and S. validus. Strongly oligohaline conditions
support very few species other than S. validus.

82
All statistical analyses were carried out using SAS
(Statistical Analysis System, Cary, North Carolina, 1988).
Field Transplants Between Sites
To determine the response of vegetation to changing
soil salinities and species associations, between-site
transplants of dominant species from freshwater,
oligohaline, and mesohaline marshes were made between
marshes. In December 1987, transplant, control, and
undisturbed plots were established at freshwater, mildly and
strongly oligohaline, and mesohaline sites to be
transplanted between sites. Eighteen previously
established, undisturbed plots at each site were used for
comparison with controls. A Reichert refractometer was used
to measure and monitor soil water salinities at field sites.
At random points, plant contents of 0.25 m X 0.25 m plots,
approximately 15 cm deep, were collected from donor sites
and transplanted to random points at recipient marsh sites.
The same size control plots were removed and then replaced
at each donor site to determine if transplanting effects
were significant. Rubber lawn edging (10 cm deep) was
placed around all between site transplants to inhibit
belowground competition. PVC flags were placed 0.25 m deep
into the marsh at corners of transplants to delineate plots.
Twenty S. validus plots were transplanted from the
donor oligohaline site to the freshwater site, and 20 to the
mesohaline site. Twenty control plots were established.

83
Twenty plots from the donor freshwater marsh were
transplanted to the strongly oligohaline site, and 10 to the
mesohaline site. Twenty five control plots were established
at the freshwater donor site. Twenty plots were
transplanted from the mesohaline donor site to the
oligohaline site, and 10 to the freshwater site. Twenty
five control plots were established at the mesohaline site.
Above-ground vegetation from transplants and controls
was harvested in June, 1989. Undisturbed plots were
harvested in June, 1988. Plants were identified, counted,
dried, and weighed.
Analysis of variance was used to determine if
differences among donor plots transplanted to different
marsh sites were significant. LSD tests, which are pairwise
t-tests equivalent to Fisher's least-significant difference
test for unequal sample sizes, were used to compare biomass
and density for donor marsh plots transplanted to
freshwater, oligohaline, and mesohaline sites.
Field Transplants Within Sites
To examine differences in competitive ability among
species within a site, within-site transplant comparisons
were made between inner and outer plots and their control
plots. Changes in within-site transplants were also
compared between marsh sites to determine whether
differences in competitive interactions varied along the
salinity gradient.

84
At freshwater, mildly oligohaline, and mesohaline marsh
sites, 75 sets of transplants, at 25 randomly chosen points
at each site, were established in December, 1987. Each set
of transplants consisted of a S. validus control plot, a
neighbor control plot, and a S. validus experimental plot
nested into the larger neighbor plot.
Control and experimental S. validus plots were 0.25 m
long X 0.25 m wide X 0.15 m deep. Neighbor plots were 1.0 m
long X 1.0 m wide X 0.15 m deep, with a section removed from
the center; experimental S. validus plots were nested in the
center of neighbor plots. Control plots were removed and
replaced for both S. validus and neighbor plots. Rubber
garden edging was placed 0.10 m deep around 1.0 m X 1.0 m
neighbor plots to inhibit interactions between experimental
plots and surrounding vegetation. PVC flags were placed
0.25 m deep into the marsh at corners of all plots so that
vegetation could be harvested separately for controls,
neighbor, and experimental S. validus plots.
All species except S. validus were clipped and removed
from inner S. validus plots and all S. validus was removed
from outer neighbor plots in April, 1988. Clipping and
removal of species was repeated in June and August, 1988.
Above-ground vegetation was harvested from all plots in
June, 1989. Vegetation was sorted by species, and stems
were counted and dried to a constant weight. Relative
proportions by weight and density were calculated for S.

85
validus. S. alterniflora. Z. miliaceae. and E. montevidensis
and an "other" category for transplants at each site.
Differences between outer neighbor controls and inner
S. validus plots enabled a determination of relative
increase, decrease, or no change with the removal of
neighbor species. Control S. validus plots were compared
with inner S. validus plots to determine changes in S,
especially for mildly oligohaline and mesohaline sites where
control S. validus plots rarely contained any other species.
Differences between neighbor controls and outer neighbor
plots were used to determine effects of the removal of S.
validus on neighbor species. Differences between inner S.
validus and outer neighbor plots at each marsh site were
calculated and compared to determine the overall effects of
the combined removal of neighbor species from inner plots
and S. validus from outer plots.
Paired t-tests were used to determine if differences
among controls, neighbor plots, and experimental S. validus
plots were significant. Pairwise t-tests were used to
compare differences between freshwater, mildly oligohaline,
and mesohaline sites (P < .05).
Greenhouse Experiments
To examine the potential for competition between S.
validus and freshwater species at varying salinities, a
series of DeWitt experiments (Silvertown 1982) were set up
in a greenhouse at the Center for Wetlands, Univ. of

86
Florida, in December, 1988. Randomly chosen plots of
freshwater marsh were collected in 0.25 cm diam. X 0.20 cm
deep pots. Ramets of S. validus were also randomly
collected. Fifteen pots contained no S. validus. 15
contained 10% S. validus. 15 contained 30% S. validus, 15
contained 50% S. validus. and 15 contained 100% S. validus.
Five pots from each group were placed in each of 3 treatment
pools, and watered daily with 0, 5, and 10 ppt seawater
solution, respectively, for 6 months. Soil water salinities
were monitored using a Reichert refractometer. The five
pots containing freshwater species but no S. validus were
also used to determine the effects of increasing salinity on
freshwater marsh species.
Above-ground vegetation was harvested from all plots in
July, 1989. Vegetation was sorted by species and stems were
counted and weighed. Total and individual relative yield by
weight and density were calculated for S. validus and the
combination of other species for each pot.
Two-way analysis of variance was used to test for
significant effects of salinity and initial proportions of
S. validus on relative and total yields of S. validus and
the combined other species. The T-method for multiple
comparisons among nearly egual sample sizes (Sokal and Rohlf
1981) was used to compare differences in effects of initial
S. validus proportions between freshwater, olighaline, and
mesohaline treatments.

87
Feral Hog Disturbance
A 0.5 km2 study area was established in an area
dominated by giant cutgrass, softstem bulrush, cattail
ÍTypha anaustifolia). and spikerush (Eleocharis
montevidensis)♦ Cypress, túpelo, and sweetgum (Liquidambar
stvraciflua) grew along adjacent river levees. Interstitial
soil salinities averaged 4 ppt.
Thirty 1.0 m2 sample plots initially intended for plant
competition experiments were established in the study area
in December 1987. Vegetation sampling plots were located in
stands with an even mix of Z. miliaceae and S. validus (50%
cutgrass, 50% bulrush) at 6 random points along each of 5
transects in the study area.
Six of 12 study plots showed signs of trampling or
rooting by feral hogs in January 1988. Percent cover of
cutgrass and species composition of remaining vegetation was
measured in disturbed and undisturbed plots during March and
April 1988. Student's T-test was used to compare percent
cover of disturbed and undisturbed plots.
Results
Control plots did not differ significantly from
undisturbed plots at any transplant sites, indicating that
transplanting within sites had no effect on vegetation.
Subseguently, only comparisons between controls and
experimental plots are presented.

88
Betveen-site Transplants
Changes in proportions of dominant species for plots
transplanted to different sites are given in Figure 5-1.
Relative density and biomass of E. montevidensis decreased
significantly after being transplanted to the mesohaline
site, but remained unchanged relative to the control when
transplanted to the olighaline site (P = 0.05). Relative
density of S. validus tranplants at the mesohaline marsh
decreased significantly (P = 0.05), while relative biomass
decreased significantly at both freshwater and mesohaline
sites. Spartina alterniflora decreased significantly (P =
0.05) in relative density and biomass when transplanted to
the freshwater site, but not the oligohaline site.
Donor transplants at the freshwater marsh site were
invaded by local species and showed no difference in species
composition, biomass, or density when compared with
freshwater marsh controls (Fig. 5-2). Donor transplants
from the oligohaline marsh, composed predominantly of S.
validus. decreased significantly in S. validus biomass (F =
6.43; DF = 2; P < .05) and density (F = 38.44; DF = 2; P <
.05) after transplanting to the freshwater marsh site.
Spartina alterniflora from the mesohaline marsh decreased
significantly in biomass (F = 9.26? DF = 2; P < .05) and
density (F = 3.45; DF = 2? P < .05). Eight of the 10
initial transplants of S. alterniflora from the mesohaline
marsh were still intact when harvested; two were lost to

Relative biomass (grams dry wcight/m2) Relative density (stems/m*)
89
COM CMF COF
Fig. 5-1. Means and standard errors for relative densities
and biomass of: E. montevidensis transplanted from
freshwater marsh to olighaline and mesohaline marsh; S.
validus transplanted from oligohaline to mesohaline and
freshwater marsh; and S. alterniflora transplanted from
mesohaline to oligohaline and freshwater marsh (see methods
section for details). C = control, F = freshwater, 0 =
oligohaline, M = mesohaline site.

90
Control
Oligohaline
Mesohaline
SS8 Total
â–  S- altemiflora
1 1 E. raontevidensis
Other
S. validus
Fig. 5-2. Total and individual means and standard errors
for species density and biomass for oligohaline and
mesohaline donor transplants and controls at freshwater
site.

91
feral hog disturbance. Three of the 8 plots had 2-4 small
S. alterniflora remaining.
Plant biomass was significantly less in donor plots
transplanted to the oligohaline site (F = 54.18; DF = 2; P <
.05) when compared with the oligohaline controls (Fig. 5-3).
Unlike plots transplanted to the freshwater marsh, donor
transplants at the oligohaline marsh were not as readily
replaced by local vegetation. Scirpus validus and the
"other" category made up the biomass in the control plots;
the "other" category contained almost exclusively Typha
anaustifolia and Zizaniopsis miliaceae. Neither E.
montevidensis from the freshwater marsh nor S. alterniflora
from the mesohaline marsh decreased in biomass or density
after being transplanted to the oligohaline marsh when
compared with their respective controls. Plant densities
were greatest for donor freshwater marsh plots (F = 12.08;
DF = 2; P < .05), where E. montevidensis reached densities
twice that of total densities in the oligohaline control
plots, although it was restricted to donor freshwater
transplants. In addition to it's survival in donor
mesohaline transplants, S. alterniflora invaded donor
freshwater transplants. Oligohaline control plots were not
invaded by S. alterniflora. The oligohaline site was the
only site at which edging around plots appeared to inhibit
invasion by neighbor species.

ggsfr-
■r-*—
Control
Freshwater
Mesohaline
Total â–  S. altemiflora
â–¡ E. montevidensis Y/A1 Other
S. validus
Fig. 5-3. Total and individual means and standard errors
for species density and biomass for freshwater and
mesohaline donor transplants and controls at oligohaline
site.

93
All donor plots at the mesohaline marsh were invaded
successfully by S. alterniflora as well as small amounts of
Scirpus robustus. Donor transplants at the mesohaline marsh
did not differ in density or biomass of invading S.
alterniflora at the end of the experiment, although total
density (F = 4.94; df = 2; P < .05) and biomass (F = 3.20;
DF = 2; P < .05) were significantly less in freshwater donor
transplants (Fig. 5-4). Donor transplants did not differ in
the biomass or density of S. validus remaining when compared
to mesohaline control plots. Donor freshwater transplants
had significantly less biomass and density compared with
oligohaline donor plots and mesohaline controls, a result of
less S. validus in these transplants. Variance associated
with these plots was high, however, due to the relatively
small number of transplants (10), and differences in S.
validus between these and other plots were not significant.
The initially dominant species in freshwater donor
transplants, E. montevidensis. significantly declined when
transplanted from freshwater to mesohaline marsh in both
biomass (F = 11.93; DF = 2) and density (F = 23.67; DF = 2);
only one donor freshwater plot had E. montevidensis
remaining.
Within-site Transplants
Inner S. validus vs. outer control plots. A comparison
of inner S. validus plots and outer controls was made to
determine if S. validus had increased beyond the abundance

94
80 r
70 '
Control Oligohaline Freshwater
140
Control
Oligohaline
Freshwater
Total
S. altemiflora
1 1 E. raontevidensis
¡£3 0ther
WA S. validus
Fig. 5-4. Total and individual means and standard errors
for species density and biomass for freshwater and
oligohaline donor transplants and controls at mesohaline
site.

95
expected in mixed plots (rather than monospecific S. validus
plots). Clipping and removal of neighbor species from inner
plots resulted in increases in S. validus biomass at
freshwater and oligohaline sites, but not mesohaline (Fig.
5-5). While densities of initially monospecific S. validus
plots remained greater than for neighbor control plots only
at the mesohaline site (t = 3.10; N = 25; P < .005), the
difference in density between inner plots and neighbor
controls at the mesohaline site was no greater than
differences at freshwater and oligohaline sites.
Biomass of inner S. validus plots decreased to that of
the neighbor control plots at the mesohaline site. Biomass
of S. validus was greater at the oligohaline (t = 3.41; N =
24; P < .01) and freshwater sites (t = 4.26; N = 26; P <
.001) when compared with neighbor control plots, but did not
differ between these two sites. Differences in S. validus
density between inner plots and neighbor controls were not
significant for freshwater or oligohaline transplants.
Inner S. validus vs. control S. validus plots.
Initially monospecific inner plots of S. validus exhibited
the greatest decrease in S. validus at the mesohaline site,
while similarly monospecific plots at the mildly oligohaline
site showed relatively little, if any, change (Fig. 5-5).
The effects of clipping and removal of neighbors were not
apparent in a comparison of mildly oligohaline gnd
mesohaline transplants relative to control S. validus plots

Fig. 5-5. Means and standard errors for differences in
relative density and biomass between (A) inner S. validus
and inner control plots, (B) inner S. validus (with neighbor
species removed) and neighbor control plots, (C) outer
neighbor (with S. validus removed) and outer control plots,
and (D) inner S. validus and outer neighbor plots for
freshwater, oligohaline and mesohaline sites. Separate
solid lines at right of graph indicate significant
differences between sites.

o N> ^ k»
. I
O K> -O- k)
Stems / M2
o o
s
o u>
Grams dry weight / M¿
VO
*vl

98
(although effects of removal can be seen when inner plots
are compared with neighbor controls for these 2 sites in
Fig. 5-5).
Effects on S. validus biomass following transplanting
into neighbor plots were significant at all 3 sites (Fig. 5-
5). Scirpus validus biomass in inner plots increased at the
freshwater site (t = 4.21; df = 26; P < 0.01), but decreased
at oligohaline (t = -3.06; df = 24; P < 0.01) and mesohaline
(t = -10.01; df = 25; P < 0.01) sites, where inner S.
validus transplants at both sites initially contained only
S. validus. Changes in S. validus biomass were of a
significantly smaller magnitude at the oligohaline site when
compared with freshwater and mesohaline sites (F = 55.38; df
= 2; P < 0.05). Differences in density between inner S.
validus and control plots were significant only at the
mesohaline site (t = -7.94; df = 25; P < 0.05). Differences
in Scirpus validus density between inner S. validus and
control S. validus plots were not significant between
oligohaline and mesohaline sites, although freshwater plots
had significantly greater densities when compared with
oligohaline and mesohaline plots (F = 9.32; df = 2; P <
.05) .
Inner S. validus at the freshwater site were the only
plots to show an increase in S. validus. due to the fact
that, unlike control plots at other sites, freshwater inner
control plots were not almost exclusively S. validus (Fig.

99
5-5). Inner plots at the freshwater site were not
completely filled by S. validus in the space made available
by removal of other species, although inner plot densities
were approximately 10% greater and biomass 20% greater for
S. validus control plots. The species most common in the
inner freshwater plots at harvest was E. montevidensis.
Inner S. validus plots at the oligohaline site
decreased significantly in density, but still had near-zero
differences when compared to control plots of pure S.
validus. and there were no differences in biomass between
inner and control plots of S. validus (Fig. 5-5). Some
invasion of inner plots by species other than S. validus
occurred at the oligohaline site, predominantly by E.
montevidensis.
Decreases in density and biomass of S. validus were
greatest at the mesohaline site (Fig. 5-5). Mesohaline
plots had significant net losses for both density and
biomass of S. validus. and changes were of a significantly
greater magnitude than for freshwater or oligohaline
transplants. Removal of neighbor species at the mesohaline
site resulted in the invasion of available space by species
other than S. validus, predominantly S. alterniflora.
Outer neighbor vs. neighbor control plots. Only the
mesohaline site exhibited a significant change in S. validus
biomass in the outer neighbor plots when compared with
control plots (t = -3.19; df = 25; P < .01) (Fig. 5-5),

100
resulting in less S. validus (and more S. alterniflora) in
outer neighbor plots. The removal of S. validus from
neighbor plots also resulted in a significant decrease in S.
validus densities at only the mesohaline site at the end of
the experiment.
Removal of S. validus did not result in decreases in S.
validus biomass relative to control plots at the freshwater
site (Fig. 5-5). Scirpus validus maintained densities
similar to control plots for oligohaline transplants as
well, which were initially monospecific S. validus. The
difference in S. validus biomass between outer neighbor and
neighbor control plots did not vary different among sites.
Removal of S. validus from outer neighbor plots had no
significant effect on S. validus densities at the
oligohaline site and S. validus in outer neighbor plots
recovered and reached densities similar to controls (Fig. 5-
5). Decreases in S. validus densities in the outer neighbor
plots were significantly less at freshwater and oligohaline
sites when compared with the mesohaline site (F = 6.52; df =
2; P < .05).
Inner S. validus vs. outer neighbor plots. Combined
removal of S. validus from outer neighbor plots and removal
of neighbor species from inner plots resulted in significant
differences in S. validus biomass and densities at all sites
(Fig. 5-5). These results were due primarily to a net gain
of S. validus in inner plots, combined with relatively no

101
loss of S. validus in outer neighbor plots at freshwater and
oligohaline sites (Fig. 5-5). Mesohaline differences, on
the other hand, reflected a decrease in S. validus from
inner plots and an increase in S. alterniflora in outer
neighbor plots.
Inner S. validus plots had significantly greater (P <
.01) S. validus biomass than outer neight r plots (from
which S. validus had been removed during the previous spring
and summer) at freshwater (t = 5.86; df = 26), oligohaline
(t = 7.41; df = 24), and mesohaline (t = 5.40; df = 25)
sites. Increases in S. validus densities were significant
at oligohaline (t = 2.13; df = 24; P < .05) and mesohaline
(t = 7.50; df = 25; P < .01) sites. Increases in S. validus
biomass in inner plots relative to outer neighbor plots
differed significantly between oligohaline and mesohaline
sites (F = 3.31; df = 2; P < .05). The magnitude of
differences in S. validus biomass between inner and outer
plots did not differ significantly between oligohaline and
freshwater sites or between mesohaline and freshwater sites.
Relative increases in densities of S. validus were
significantly greater (P < .01) at oligohaline (t = 2.13; df
= 24) and mesohaline (t = 7.50; df = 25) sites. While S.
validus biomass increased in inner plots at the freshwater
site, S. validus densities did not change significantly.
Increases in S. validus densities in inner .plots
relative to outer plots were greatest at the mesohaline

102
site, where there was 30% more S. validus in the inner plot.
Inner S. validus plots at freshwater and oligohaline sites
had S. validus densities from 8 - 13% higher than outer
plots, significantly less than plots at the mesohaline site
(F = 5.97; df = 2; P < .05). Scirpus validus was replaced
by predominantly E. montevidensis at freshwater and
oligohaline sites, and by S. alterniflora at the mesohaline
site.
Greenhouse Experiments
Plots containing only Scirpus validus
Relative yield of S. validus biomass and density in
experimental plots was significantly affected by salinity
treatment and initial proportion of S. validus (Table 5-1).
Interaction effects between salinity and initial S. validus
proportion were not significant in determining relative
yields of S. validus density or biomass. Relative yields of
S. validus densities were significantly greater at the
mesohaline treatment when compared with the freshwater
treatment, but did not differ between oligohaline and
freshwater treatments or between oligohaline and mesohaline
treatments (MSE = 0.09; DF = 63; P = 0.05). Density of S.
validus was significantly greater at the mesohaline
treatment when compared with oligohaline and freshwater
treatments, but not between freshwater and oligohaline
treatments (MSE = 0.18; DF = 63; P = 0.05). Biomass and

103
Table 5-1. Two-way ANOVA results of relative yield for A)
biomass and B) density of S. validus in greenhouse
experiments for the effects of salinity treatment (0, 5, 10
ppt) and initial proportion of S. validus (0, 10, 30, 50,
100 percent).
A.
Source of variation
df
MS
F
Salinity
2
0.43
4.66”
prop. S. validus
1
8.20
88.69"
Salinity X prop. S.
validus
2
0.03
0.28 NS
"P < 0.001, *P < 0.01,
B.
NS = not significant.
Source of variation
df
MS
F
Salinity
2
2.42
13.25"
prop. S. validus
1
9.05
49.54"
Salinity X prop. S.
validus
2
0.00
0.99 NS
**P < 0.001, *P < 0.01, NS = not significant.

104
density of S. validus increased with increasing initial
proportions of S. validus at all salinities (Fig. 5-6).
Plots containing all species except Scirpus validus
Initial proportions of S. validus had a significant
effect on relative yield of biomass and density for all
species combined (excluding S. validus). although salinity
treatment did not (Table 5-2). Biomass and density of all
species excluding S. validus decreased with increasing
initial proportions of S. validus (Fig. 5-7). Interaction
effects between salinity treatment and initial S. validus
proportions were not significant in determining relative
yields of total species excluding S. validus. Three
species, E. montevidensis. Leersia virainica. and Scirpus
robustus, were consistently present in pots at the
mesohaline treatment at the end of the experiment. Other
than S. validus. these 3 accounted for nearly all the
biomass and density at the mesohaline treatment.
Plots containing all species
Salinity treatment and initial proportions had
significant effects on total relative yield of biomass and
density for all species in experimental plots (Table 5-3).
Total relative yield of biomass and density were greater for
mesohaline treatments when compared with freshwater and
oligohaline treatments, while freshwater and oligohaline
treatments showed no differences (MSE =0.09; DF = 63; P =
0.05). Biomass and density of all species excluding S.

105
0 20 40 60 80 100
Initial proportion of S. validus
Fig. 5-6. Means and standard errors of relative yields for
density and biomass of S. validus for different greenhouse
soil salinities, plotted against initial S. validus density.
Solid squares represent freshwater, empty squares represent
oligohaline, and solid triangles represent mesohaline soil
salinities.

106
Table 5-2. Two-way ANOVA results of relative yield for A)
biomass, and B) density, of all species, excluding S.
validus, in greenhouse experiments for the effects of
salinity treatment (0, 5, 10 ppt) and initial proportion of
S. validus (0, 10, 30, 50, 100 percent).
A.
Source of variation
df
MS
F
Salinity
2
0.01
0.28
prop. S. validus
1
0.33
76.93**
Salinity X prop. S.
validus
2
0.00
0.69
**P < 0.001, *P < 0.01,
NS = not significant.
B.
Source of variation
df
MS
F
Salinity
2
0.00
0.28
prop. S. validus
1
0.43
38.92**
Salinity X prop. S.
validus
2
0.00
0.17 NS
**P < 0.001, ‘P < 0.01, NS = not significant.

107
Initial proportion of S. validus
Fig. 5-7. Means and standard errors of relative yields for
density and biomass of all species excluding S. validus for
different greenhouse soil salinities, plotted against
initial density of these species. Solid squares represent
freshwater, empty squares represent oligohaline, and solid
triangles represent mesohaline soil salinities.

108
Table 5-3. Two-way ANOVA results of relative yield for A)
total biomass, and B) total density, of all species in
greenhouse experiments for the effects of salinity treatment
(0, 5, 10 ppt) and initial proportion of S. validus (0, 10,
30, 50, 100 percent).
A.
Source of variation
df
MS
F
Salinity
2
0.40
4.55*
prop. S. validus
1
5.23
58.77**
Salinity X prop. S.
validus
2
0.04
0.45 NS
**P < 0.001, *P < 0.01,
NS = not significant.
B.
Source of variation
df
MS
F
Salinity
2
2.59
12.97**
prop. S. validus
1
5.53
27.69**
Salinity X prop. S.
validus
2
0.00
0.01 NS
**P < 0.001, *P < 0.01, NS = not significant.

109
validus decreased with increasing initial proportions of S.
validus (Fig. 5-8).
Feral Hog Disturbance
Percent cover of cutgrass in disturbed plots was
significantly less than in undisturbed plots (t = 11.98; P =
0.0001; n = 6; disturbed mean = 5.83, SE = 6.11; undisturbed
mean = 39.16, SE = 3.27). Percent cover of cutgrass
decreased significantly as cutgrass was replaced by softstem
bulrush or softstem bulrush mixed with spikerush and
cattail.
Discussion
Previous experiments (Chapter 3) have demonstrated the
contradiction between field distributions of S. validus
along a salinity gradient and the responses to this gradient
established in greenhouse experiments. Results of these and
of present experiments demonstrated that competitive
interactions were important in determining the distribution
of S. validus. as well as associated plant species, and
indicated the changing role of competition in structuring
plant communities along an environmental gradient. Results
of disturbance experiments indicated feral hog disturbance,
combined with higher salinities, had a significant effect on
vegetation composition.
Several studies have demonstrated that differential
physiological tolerance of plant species along environmental
gradients does not alone explain plant species zonation.

110
Initial proportion of S. validus
Fig. 5-8. Means and standard errors of total relative
yields for density and biomass of all species for different
greenhouse soil salinities, plotted against initial density
of all species. Solid squares represent freshwater, empty
squares represent oligohaline, and solid triangles represent
mesohaline soil salinities.

Ill
Competitive exclusion across a water depth gradient has been
demonstrated for Typha latifolia and T. anaustifolia (Grace
and Wetzel 1981), in which T. latifolia was physiologically
restricted to shallower depths, while T. anaustifolia was
competitively excluded by T. latifolia to deeper water.
Both species, however, grew best at the shallower depth. In
addition, Goldberg (1985) found that a tree species
characteristic of more favorable soil conditions was
restricted from more acid, less fertile soil by
physiological intolerance, while the species from the less
favorable soil was restricted from the more fertile soil by
competition.
Results of the present study were consistent with these
findings. Freshwater plant species, predominantly E.
montevidensis. Z. miliaceae. Polygonum spp., and Saaittaria
spp. were inhibited by increased salinities in greenhouse
experiments, and restricted to the freshwater marsh due to
salinity intolerance. An ecological generalist, S. validus
occurred over a wider range of salinities, but was limited
in its competitive abilities. Scirpus validus did poorly
when grown with other species under freshwater conditions,
both in greenhouse and field experiments, where competitive
abilities of freshwater species were greater. Competitive
inhibition of S. validus. expressed as its increase when
neighbor species were removed, was greater under- freshwater
and mildly oligohaline conditions when compared with the

112
mesohaline marsh. These results indicated a significant
reduction in biomass and densities of S. validus at the
freshwater site in response to the greater competitive
abilities of the more specialized freshwater species, even
though, experimentally, it exhibits significantly better
growth under freshwater conditions. Scirpus validus
persisted, however, by colonizing gaps in the vegetation due
to disturbance or die-back of other species.
As a generalist species, S. validus was able to take
advantage of any available resources, whereas the more
specialized, niche-differentiated species require more
specific resources. Removal of other species provided gaps
in freshwater vegetation that were colonized by the more
opportunistic S. validus. Removal of S. validus does not,
however, enhance the growth of its neighbors. The
neighboring freshwater species, because of their greater
specialization, may suffer increased interspecific
competition among each other and smaller increases in
biomass and density even when gaps are available by the
removal of S. validus. If distribution of S. validus was
limited solely by dispersal, then transplants of it into
freshwater marsh would have been unaffected by freshwater
species. Early successional characteristics of S. validus,
such as longer internode lengths, earlier seasonal
appearance, rapid gap invasion, and a general fugitive
behavior (personal observations) may allow it to colonize

113
open gaps quickly and maintain its presence in the
freshwater marsh. All freshwater individuals appeared to be
inhibited by the presence of neighbors, and gaps were
subsequently colonized by individuals in close proximity to
the gaps.
My results provided evidence in support of greater
competitive influence on a species' distribution at the more
benign end of its environmental tolerance range; the
distribution of S. validus in these tidal marshes was
determined more by competitive interactions under freshwater
conditions, and more by physiological tolerance along the
higher salinity portions of the gradient. Although S.
validus and S. alterniflora both have low competitive
abilities, when they occur together resource requirements
overlap and the competitive effect appears to be more
intense.
Experiments with early and late successional species
have found greater niche and resource overlap among
generalist species of early successional plants resulted in
relatively more aggressive competition, while late
successional species have more finely tuned resource
requirements and actually compete less among themselves
(Parrish and Bazzaz 1982, Pickett and Bazzaz 1978).
Relatively benign conditions of tidal freshwater marshes are
characteristic of late successional plant communities
comprised of species with high niche differentiation.

114
Brackish marshes, in contrast, impose a physiological
salinity stress on plants, creating a habitat analagous to
an early successional stage in which plants share resource
requirements and niche overlap is high. In a comparison of
tidal freshwater and salt marshes, Odum (1988) also
described a greater fundamental niche overlap among salt
marsh plants when compared with freshwater species. Greater
niche overlap among species of the salt-stressed mesohaline
marsh may explain the more conspicuous differences in inner
and outer plots following thinning and removal of S. validus
or neighbor species. Results from transplant experiments
suggest that along a gradient of salinity-imposed stress,
competition is more intense among less specialized species
of higher salinity marshes where demands for resources
overlap and species must compete for the same resources.
Several authors have contended that similar resource
requirements of plants for water, light, and carbon dioxide
make hypotheses regarding niche overlap and diversification
irrelevant to plant community structure (Goldberg and Werner
1983, Huston 1979). Miller and Werner (1987) have
established an inverse correlation between competitive
effects on species and competitive response of plant species
to other species, as well as an absence of differential
resource requirements in a first-year old-field community.
Miller and Werner found that some species had a large
inhibitory effect on inferior competitors, and this effect

115
was not related to the presence of additional neighboring
species. This results in the development of a competitive
hierarchy among species, in which species are competitively
superior or inferior relative to one another, regardless of
whether other species are present. While these studies may
be applicable for plant species assemblages within a similar
range of environmental conditions, a gradient of freshwater
and brackish marshes do not provide the same conditions for
all plants, nor do they support the same species
assemblages.
For example, competitive effects of a species on
neighbors and the response of that species to competitive
effects of neighbors were, in this study, inversely related
at the freshwater site. Competitive effects of neighbor
species on S. validus were significant, while the response
of neighbor species to removal of S. validus was
insignificant. Under mesohaline conditions, S. validus
exhibited no response to S. alterniflora removal, while S.
alterniflora did experience a significant increase when S.
validus was removed. Within a given set of environmental
conditions, then, the present study provides additional
evidence in support of the inverse relationship between
competitive effect and response suggested by Miller and
Werner. These results, by presenting comparisons between
low and high salinity marshes, also demonstrate that a
single species may have a range of competitive effects and

116
responses depending on local conditions and co-occurring
species.
It might be argued that removal of S. validus results
in greater biomass being removed from the mesohaline plots
when compared with the freshwater or oligohaline sites since
fewer species are present at the mesohaline site and removal
of any one species results in the removal of more biomass
from a plot. Ostensibly, this could produce what appears to
be a strong competitive effect, when in fact the effect
would be small if measured on a per-mass basis (Goldberg and
Werner 1983). Any species, however, could take advantage of
these gaps, especially plants with a rhizome still intact
just below the surface. This would result in S. validus
recovery in plots where S. validus was removed and S.
alterniflora recovery where S. alterniflora was removed. In
the freshwater marsh, removal of neighbor species resulted
in greater biomass removal because all species except S.
validus were removed, and openings were larger than for
plots in which only S. validus was removed. Openings again
could be colonized by any of the surrounding species. In
fact, placement of donor plots of monospecific S. validus in
the freshwater marsh provided no available gaps;
nonetheless, these plots were immediately invaded and
dominated by other freshwater species other than S. validus.
The absence of S. alternif lora and S. robus.tus at the
strongly oligohaline marsh is difficult to explain in terms

117
of the present salinity regime. Intermediate disturbance
hypotheses (Tilman 1982, Huston 1979) proposed that species
richness would increase at an intermediate level of
disturbance that resulted in greatest spatial and temporal
heterogeneity. Recent studies along riverbanks have shown
plant species richness to increase at intermediate levels of
substrate heterogeneity (Nilsson, 1987). While neither
salinity variance nor the variance relative to mean salinity
(coefficient of variation) were higher or lower at the
strongly oligohaline site, the site is probably undergoing
the most recent alteration in salinity due to tidegate
operations (see Chapter One), that have resulted in the
present gradient of freshwater, oligohaline, and mesohaline
tidal marshes along the lower Savannah River. A study of an
impounded New England salt marsh showed the previously
dominant species, T. anaustifolia. was replaced by S.
alterniflora (Sinicrope et al. 1990) following the re-
introduction of tidal flushing. This appears to be what is
happening at the olighaline site; while soil salinities are
high enough to inhibit freshwater species other than S.
validus. and S. alterniflora does well once established,
there has not been time enough for S. alterniflora to invade
on its own.
Expansion of S. alterniflora at the oligohaline site
beyond the mesohaline donor transplants as well as into the
freshwater donor transplants at the oligohaline site

118
demonstrated that if salinities were high enough, S.
alterniflora. an even greater generalist than S. validus.
could successfully become established. S. alterniflora was
able to maintain or increase its density and biomass from
transplanted plots of S. alterniflora and also invaded
transplants of freshwater species at the strongly
oligohaline site. The relative abundance of S. alterniflora
has been shown to increase following short-term disturbance
(Bertness and Ellison 1987) and the disturbance effect of
transplanting freshwater plots to the oligohaline site could
explain its invasion. Spartina alterniflora did not invade
S. validus control plots at the oligohaline site, probably
due to the absence of S. alternfilora close enough to S.
validus to colonize the controls. My results suggest
establishment limits the distribution of S. alterniflora at
these increased salinities. The transitional salinities
inhibit most freshwater species, but S. validus. which was
established prior to the salinity increases, was able to
expand its distribution in the absence of competitors.
Another salt tolerant species, e.g. S. alterniflora. must
first undergo dispersal and establishment, and then compete
with and already established species which were able to
coexist successfully among highly competitive freshwater
species. Spartina alterniflora. like many salt-tolerant
species, grows best under freshwater or low salinity
conditions (see Chapter 4). Most salt-tolerant species were

119
out-competed under freshwater conditions, and so were
confined to brackish and salt marshes where freshwater
species cannot survive (Wainwright 1984, Barbour 1978).
The ability of S. alterniflora to survive at the
freshwater site may have been a result of the edging around
the transplants, which could have inhibited invasion by some
species. Scirpus validus, however, appeared to be the only
species substantially inhibited by the rubber edging around
transplant plots, most likely a conseguence of its rhizome
depth. Spartina alterniflora was able to send rhizomes down
deep enough to go under edging at mesohaline and oligohaline
sites, and E. montevidensis root mats occurred nearly on the
marsh surface, enabling it to expand over the edging.
Several freshwater species seeded into the plots. Rhizomes
of S. validus. however, are intermediate in their depth, and
tend to go around or turn in a different direction once they
come in contact with tree roots or other plant rhizomes
(personal observation).
Invasion of S. validus stands by S. alterniflora was
not confined to experimental transplants and may be
occurring throughout the mesohaline site. The boundary
between stands of S. validus and S. alterniflora. delineated
with wooden stakes in 1987, had moved more than a meter into
S. validus stands by the end of this study (personal
observation). This suggests that the monospecific stands of
S. validus in the mesohaline marsh will eventually be

120
replaced by S. alterniflora and smaller amounts of co¬
occurring S. validus and S. robustus. Greenhouse results
provide little information regarding the ability of S.
validus to compete with S. alterniflora at increased
salinities other than indicating that factors other than
salinity were responsible for increased S. validus at
mesohaline field sites when compared with freshwater sites.
The greater yield of freshwater species in greenhouse
experiments under freshwater conditions in the absence of S.
validus suggested reduced intraspecific interference of
freshwater species in species mixtures (Silvertown 1982).
Scirpus validus exhibited increases proportional to initial
greenhouse densities, indicating no inhibition due to the
presence of freshwater species under freshwater conditions.
Field experiments, however, indicated a competitive release
of S. validus when freshwater species were removed. This
suggests that the competitive release was diffuse rather
than species-specific, and a result of available gaps in the
field experiment once neighbors were removed. In the
greenhouse experiments, biomass and density of species other
than S. validus did not decrease significantly at higher
salinities. Few species, e.g., E. montevidensis. Scirpus
robustus. and Leersia virainica remained at the end of the
greenhouse experiment. This demonstrated that (1) most
species in the freshwater marsh were intolerant of increased
salinities, and (2) few species tolerant of increased

121
salinities were limited by something else in the mesohaline
marsh, possibly competition from S. alterniflora and S.
validus.
Although S. validus was out-competed in the freshwater
marsh by other local species, most of these species were
absent from the oligohaline marsh. Zizaniopsis miliaceae.
T. latifolia. and E. montevidensis remained common, but at
the higher salinities did not affect the distribution of S.
validus as they did at the freshwater site. Within-site
transplants at the mildly oligohaline marsh demonstrated
that although S. validus from inner nested plots decreased
relative to pure control stands, the change in S. validus
density and biomass was least at this site when compared
with freshwater and mesohaline sites. Within-site
transplants at the mildly oligohaline site exhibited little
or no change in S. validus density and biomass for inner S.
validus and outer neighbor plots. Very little invasion of
transplanted S. validus occurred at this site, suggesting
the low salinity marshes inhibit most species, allowing S.
validus to expand.
The greater yield of S. validus in freshwater species
mixes at oligohaline and mesohaline conditions in both field
and greenhouse experiments suggests S. validus had a
competitive advantage over surviving freshwater species at
higher salinities, although monocultures of S. validus do
more poorly at increased salinities.

122
Decreased re-establishment of neighbor species
following experimental removal may also explain the small
decrease in S. validus once it was removed from outer
neighbor plots at the oligohaline site. Seeds and seedling
of plants which generally tolerate low salinities as adults
often may be inhibited at the same low salinities (Beare and
Zedler 1987, Breen et al. 1977). In addition, disturbance
by animals may inhibit re-establishment of species such as
Z. miliaceae. T. latifolia. and E. montevidensis and allow a
more salt-tolerant species, e.g. S. validus to invade the
disturbed sites. It appeared that, once established at
these low salinities, S. validus suffered relatively low
competitive reduction in biomass and density. Oligohaline
marsh conditions may be the transition beyond which most
species were intolerant of the increased salinities.
The oligohaline marsh site was strongly affected by
disturbance events. In addition, seasonal variation in
plant growth, and increased salinities may be as important
as actual disturbance by feral hogs. During the growing
season feral hogs were observed throughout the area.
Rooting disturbances were small (approximately 25 X 25 cm in
size, personal observation), dispersed, and freguent
(approximately 3 per 10 m2, personal observation), and were
quickly recolonized by vegetative expansion of surrounding
plant species. During winter months hogs generally occurred
in larger groups, feeding more heavily on roots and rhizomes

123
than they do in spring when mast was more available (Wood et
al. 1979). Disturbed areas adjacent to levees in the study
area were trampled and uprooted, leaving the ground muddy
and void of standing vegetation. Plants able to sprout from
seeds and rhizomes during winter months, as S. validus does,
may recolonize the area at a much greater rate than Z.
miliaceae. The broad environmental tolerances of S. validus
(Beal 1977, Barko and Smart 1978, Langeland 1981, Joyce and
Thayer 1986) are characteristic of early successional
species, whereas Z. miliaceae appeared to be a later
successional species, exhibiting less tolerance to
disturbance and changing salinities.
Increased soil water salinities may exacerbate
disturbance effects on vegetation. Combined with bare areas
created by the hogs, increased soil salinities may inhibit
seedlings and new sprouts of giant cutgrass. Scirpus
validus. however, germinated and sprouted from rhizomes
under low and brackish interstitial salinities, which may
have inhibited colonization by giant cutgrass.
In summary, competitive interactions and their effect
on species distributions and associations in tidal marshes
were not exclusively related to the individual competitive
abilities of plant species, but instead reflected the
competitive balance or ratio of the species involved.
Species distributions may also be strongly affected by local
disturbance due to animals. The indicator species, S.

124
validus. was a generalist species and a poor competitor.
More specialized freshwater species have greater competitive
abilities and may actually compete less with each other,
although their effect on S. validus, a generalist, was
significant. As increasing salinities impose greater
physiological stress and species occurrence was limited by
physiological tolerance, co-occurring species with similar
competitive abilities experienced more intense competitive
interactions. As a result, the competitive balance may be
reversed: a competitive subordinate in the freshwater marsh
becomes a competitive dominant in the brackish marsh. Under
a given set of environmental conditions, the effects of
competitive interaction resulted in a competitive hierarchy
which strongly influenced plant species occurrence and
distribution. Over a wide range of environmental
conditions, however, a species with wide environmental
tolerances may cross over several distinct species
assemblages. While the competitive ability of a generalist
species itself was not altered, differences in co-occurring
species and environmental conditions resulted in altered
competitive hierarchies and variation in species
distributions along the environmental gradient.

CHAPTER 6
SUMMARY AND CONCLUSIONS
Summary
The 4 marsh study sites varied significantly in regards
to plant species composition, soil salinities, soil organic
content, and elevation and hydroperiod. Differences in
these environmental parameters were more subtle within each
site, although some sites exhibited steeper gradients than
others. Physical parameters which imposed a relatively
steep environmental gradient on plant species were more
strongly related to, and had greater effects on, plant
species composition and distribution than parameters which
exhibited relatively gradual gradients. Salinity changes
from tidal freshwater to mesohaline marshes corresponded
significantly to changes in marsh species composition, with
the freshwater marsh supporting a significantly higher
diversity of plant species than more saline sites.
Elevation and distance from tidal channels were significant
in differentiating among vegetation classes only within a
given salinity regime, and overlap between vegetation
classes was greater based on these parameters when compared
to salinity.
Steepness of environmental gradients, similarity in
resource requirements, and differences in scale of
125

126
measurement may all influence the extent of overlap among
vegetation classes. The extensive habitat overlap of
freshwater species was characteristic of a "finer grained"
or more homogeneous habitat in which habitat differences,
e.g. salinity or elevation, were not discrete at the level
measured. In contrast, greater differences in environmental
parameters over equal or small distances at oligohaline and
mesohaline sites resulted in steeper gradients and can be
considered "coarse grained" habitats.
Scirpus validus was the only species which occurred
throughout the study area, and exhibited changes in spatial
pattern and morphology, both of which were strongly
associated with salinity. Pattern differences suggested
that processes other than random chance were affecting the
distribution of S. validus. Significant differences in
morphological variation and associated salinities between
greenhouse and field experiments indicated that salinity
alone was not responsible for the the variation in field
morphology or spatial patterns. Light differences between
sites due to variability in vegetation composition and
structure suggested that light availablity may be an
important resource for which species were competing. This
resulted in pattern and morphology differences that
otherwise appeared related to a strong salinity gradient.
In addition to environmental gradients, feral hog
disturbance appeared to significantly affect species

127
composition at low salinity sites. Disturbed plots
underwent changes from Z. miliaceae-dominated vegetation to
predominantly S. validus. and indicated feral hog
disturbance can exacerbate the change from freshwater to
brackish marsh vegetation under the present salinity regime.
While much of the variation in plant species
associations and distributions can be attributed to the
strong salinity gradient, evidence from chapters two through
four showed factors other than physical gradients were also
responsible for observed differences in species
distributions. Results of competition experiments in
Chapter 5 indicated the occurrence of significant
interactions among plant species. Results also demonstrated
the significant variation in competitive interactions among
species and the effects of variation in competitive
interactions on plant community structure in freshwater and
more saline marshes.
Scirpus validuis. a generalist, exhibited a
characteristically poor competitive ability in both
freshwater and mesohaline marshes, as did Spartina
alterniflora, a mesohaline species. Species co-occurring
with S. validus in the freshwater marsh exhibited greater
competive abilities characteristic of more specialized,
relatively later successional species. As a result,
competitive interactions were more more balanced and
appeared more intense in the higher salinity marshes. The

128
overall effects on species composition and distributions
were, however, more strongly influenced by competition in
the freshwater marsh, where species' competitive abilities
were more varied.
Conclusions
Several authors have suggested that plant species
zonation and distributions are attributable to differential
competitive abilities along an environmental gradient, while
few have experimentally shown variations in competitive
abilities for different species along such a gradient (see
Chapters 1 and 5). Unlike the former, in which differences
in species distributions were noted and competition was
suggested as a possible mechanism, this study first
determined whether or not competition was actually
occurring. While the latter experiments have shown
differential competitive abilities for pair-wise
combinations of species, the relative aggressiveness of
species in mixtures and subsequent effects on dominance
hierarchies have not been addressed. Results of this study
not only confirm recent work by providing additional
evidence of differential competitive abilities along
environmental gradients, but add to the current
understanding by addressing the effects of competitive
interactions and environmental influences on plant community
structure.
Plant species composition and distributions across the

129
strong salinity gradient of tidal freshwater and brackish
marshes were most likely determined by species' salinity
tolerances. Within a given salinity regime, however,
competitive interactions had a significant influence upon
plant species distributions. Competitive interactions and
their effect on species distributions and associations were
not exclusively related to the individual competitive
abilities of plant species, but instead reflected the
competitive balance or ratio of the species involved.
As increasing salinities imposed greater physiological
stress and species occurrence was limited by physiological
tolerance, co-occurring species with similar competitive
abilities experienced more intense competitive interactions.
As a result, the competitive balance may be reversed: a
competitive subordinate of the freshwater marsh becomes a
competitive dominant in the brackish marsh. Under a given
set of environmental conditions, the effects of competitive
interaction resulted in a competitive hierarchy which
strongly influenced plant species occurrence and
distribution. Over a wide range of environmental
conditions, however, a species with wide environmental
tolerances may cross over several distinct species
assemblages. While the competitive ability of a generalist
species itself was not altered, its relative competitive
ability compared with co-occurring species under different
environmental conditions resulted in altered competitive

130
environmental conditions resulted in altered competitive
hierarchies and variation in species distributions along an
environmental gradient.
The role of competition in structuring plant species
assemblages across the salinity gradient on the lower
Savannah River was significant. Species competitive
interactions are not, however, more "important" in one
community than another. Competition is not a function of
absolute competitive ability or salinity stress, but does
reflect the competitive balance of competing individuals
under a given set of environmental conditions. Competition
among plant species in the environmentally benign freshwater
marsh reflected greater, more complex niche differentiation,
with distinct differences in competitive ability, and
resulted in less intense, more diffuse, competition among
several species. The salinity-stressed mesohaline marsh
supported fewer species with relatively similar resource
requirements, resulting in more intense competition for the
same resources.

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Biographical Sketch
Pamela Jean Latham was born 23 December, 1955, at Parris
Island Marine Corps Base, South Carolina, to Robert H. and Jean
B. Latham. After travelling extensively throughout the United
States, the family settled in Maitland, Florida in 1969. Pam
graduated from Lyman High School, Longwood, Florida in 1974 and
received her Bachelor of Science degree in Biology from the
University of Central Florida, Orlando, Florida, in 1979.
Pam taught high school chemistry and biology in the Seminole
County School system from 1979 through 1984 while working towards
a Master's Degree in Biological Science. Her Master's thesis was
titled "Structural Comparisons of Sand Pine Scrubs of East-
Central Florida", and was the first quantitative analysis of
Florida's endemic Sand Pine Scrubs. Pam received the degree of
Master of Science in 1985, also from the University of Central
Florida.
Later in 1985, Pam enrolled in the Ph.D. program in Systems
Ecology, in the College of Environmental Engineering Sciences, at
the University of Florida, Gainesville, Florida. She married
Clay Phillips, a fellow Lyman High School Alumnus in 1986 and
in June, 1990, Latham Kathleen Phillips was born. In August,
1990, Pam received her Ph.D., and hopes to continue to work in
ecosystems research and possibly begin teaching again.
142

I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and
is fully adequate, in scope and quality, as a dissertation for
the degree of Doctor of Philosophy.
YYi ■ }¿c3í/rJr\jUA^
Wiley Kitchens, Chairperson
Professor of Environmental
Enqineering Sciences
I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and
is fully adequate, in scope and quality, as a dissertation for
the degree of Doctor of Philosophy.
G. Ronnie Best, Co-chairperson
Research Scientist of
Environmental Engineering
Sciences
I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and
is fully adequate, in scope and quality, as a dissertation for
the degree of Doctor of Philosophy
Clay
Asso'
Environmental Engineering
Sciences
I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and
is fully adequate, in scope and quality, as a dissertation for
the degree of Doctor of Philosophy.
/í‘
Donald A. Graetz ^
Professor of Soil Science

I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and
is fully adequate, in scope and quality, as a dissertation for
the degree of Doctor of Philosophy.
I. Jac
Biologi
Sciences
University of Central
Florida
This dissertation was submitted to the Graduate Faculty of
the Collecte of Engineering and to the Graduate School and was
accepted as partial fulfillment of the requirements for the
degree of Doctor of Philosophy.
August 1990
-kVC.,
Winfred M. Phillips
L
Dean, College of Engineering
Madelyn M. Lockhart
Dean, Graduate School

UNIVERSITY OF FLORIDA
3 1262 08556 9654




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