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
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Latham, Pamela J., 1955-
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Salt marsh ecology   ( lcsh )
Salt marsh plants   ( lcsh )
Environmental Engineering Sciences thesis Ph. D
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Thesis:
Thesis (Ph. D.)--University of Florida, 1990.
Bibliography:
Includes bibliographical references (leaves 131-141).
Statement of Responsibility:
by Pamela J. Latham.
General Note:
Typescript.
General Note:
Vita.

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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.