Changes in vegetation following site preparation and understory restoration with the forest herbicide hexazinone

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Changes in vegetation following site preparation and understory restoration with the forest herbicide hexazinone
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Wilkins, R. Neal, 1962-
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Hexazinone   ( lcsh )
Forest dynamics   ( lcsh )
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Thesis:
Thesis (Ph. D.)--University of Florida, 1992.
Bibliography:
Includes bibliographical references (leaves 138-146).
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by R. Neal Wilkins.
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Typescript.
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Vita.

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CHANGES IN VEGETATION FOLLOWING SITE PREPARATION
AND UNDERSTORY RESTORATION WITH THE FOREST
HERBICIDE HEXAZINONE











By

R. NEAL WILKINS


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

UNIVERSITY OF FLORIDA


1992



























To Sandra, Ashley and Matt
















ACKNOWLEDGMENTS

Funding for this project was provided by the National

Agricultural Pesticides Impact Assessment Program (NAPIAP)

through a grant to the Intensive Management Practices

Assessment Center (IMPAC) of the USDA Forest Service

Southeastern Forest Experiment Station. This financial

support is appreciated. Study areas were provided by ITT

Rayonier, Container Corporation of America, and Georgia-

Pacific Corporation. Du Pont and Pro-serve corporations

provided chemicals and technical assistance.

Thanks go to Wayne Marion who was my committee chairman

for the first two years of this work. He had enough will-

power to allow me to conduct this work as I wished. Wayne

has since moved to the Pacific Northwest, but I am quite

positive that we will remain friends and I will continue to

learn from him. Thanks also go to George Tanner who moved

from committee member to committee chairman last year. His

perspectives and scepticisms on my sometimes wacky concepts

of plant ecology kept me from heading further out into the

twilight zone. Dan Neary provided moral support, and

arranged for financial support. The good fortune of having

worked for D.G.N. allowed me to concentrate my efforts on

research and my studies. Committee members George Blakeslee


iii







and Eric Jokela were invaluable in that they kept me from

developing tunnel-vision. David Hall and Mitch Flinchum

provided substantial comments.

Special thanks go to my friends John Wood and Joel

Smith for their tireless advice, field assistance, plant

identifications, and too much other stuff to list here.

Scott Berish worked in the field with me through most of the

project. It would have been difficult to find another

worker that could endure the field conditions that Scott

did--and like it! Forest Service Technician, Mike Allen

always made himself fragrant and available to help--and I

was glad to have the help.

Pat Outcalt and Sandra Coleman with the U.S. Forest

Service IMPAC unit were of great help in preparing

presentations, and organizing and editing manuscripts.

Susan Landreth provided cheerful computer and laboratory

assistance.

There are so many other people that helped me

throughout this project that it would be unwise to attempt

to name them all. They know who they are, and so do I. I

thank them also.

My parents have provided constant and unconditional

support throughout my entire schooling. I believe that the

only way that I could ever repay them would be to provide

the same support for their grandchildren. It's a load to

live up to.









The woman to whom this work is dedicated has played a

major role in my life -- as a wife, a friend, and mother to

my children. Sandra's salary as a registered nurse kept us

alive during graduate school. She deserves credit for

adding a big dose of reality and perspective to my graduate

studies. Some say that it must be more difficult to be a

graduate student when you have a family. Actually, it is

easier. An important thing that my wife and two children

(Ashley and Matthew) have provided me is a sense of what is

really important.

















TABLE OF CONTENTS
page

ACKNOWLEDGMENTS................................... iii

ABSTRACT........................................... xiii

CHAPTERS

1 GENERAL INTRODUCTION.................... 1

Characteristics of Hexazinone........... 3
Dissertation Format ..................... 5

2 VASCULAR PLANT COMMUNITY DYNAMICS FOLLOWING
HEXAZINONE SITE PREPARATION IN THE
LOWER COASTAL PLAIN................ 7

Introduction............... ............ 7
Methods ................................... 10
Study Sites........................ 10
Treatments........................ 12
Measurements....................... 13
Data Analyses.. ................... 13
Results..................... ............ 15
Xeric Sandhill ..................... 15
Mesic Flatwoods................... 25
Hydric Hammock..................... 34
Discussion.... ......................... 42
Species Response.................... 42
Community Response Across
a Gradient...................... 48

3 A COMPARISON OF THE EFFECTS OF CHEMICAL
AND MECHANICAL SITE PREPARATION ON
VASCULAR PLANT DIVERSITY AND
COMPOSITION OF HYDRIC HAMMOCK SITES 53

Introduction............................ 53
Hydric Hammocks and
Site Preparation................. 53
Plant Diversity................... 55
Goals and Objectives ............... 57
Methods................................ 57
Study Area Description and History. 57
Study Sites......................... 58
Field Sampling. ..................... 59










Measures of Plant
Community Attributes............. 60
Statistical Analyses............... 62

Results.............. ...... .. ............ 64
Species Response to Treatment....... 64
Diversity Response to Treatment.... 67
Windrow Influences................. 74
Discussion.............................. 76
Species Compositions............... 76
Diversity.................... ... .. 79
Summary and Recommendations............. 83


4 USE OF HEXAZINONE FOR UNDERSTORY
RESTORATION OF A SUCCESSIONALLY
ADVANCED XERIC SANDHILL............ 86

Introduction............................ 86
Methods ................................. 89
Study Area....... ........ ......... 89
Treatments.......................... 90
Plant Measurements................ 91
Data Analyses................ ..... 92
Results.................................. 93
Oak Mortality...................... 93
Wiregrass Response................. 94
Understory Response................ 99
Discussion......... ..................... 103

5 SYNTHESIS AND SUMMARY................... 107

APPENDIX.......................................... 112

LITERATURE CITED.................................. 138

BIOGRAPHICAL SKETCH............................... 147















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

CHANGES IN VEGETATION FOLLOWING SITE PREPARATION
AND UNDERSTORY RESTORATION WITH THE FOREST
HERBICIDE HEXAZINONE

By

R. Neal Wilkins

August 1992

Chairman: George W. Tanner
Major Department: Wildlife and Range Sciences
(Forest Resources and Conservation)

Changes in forest plant communities following

applications of hexazinone were evaluated with three

studies: i) rate-response experiments on 3 one-year-old

clearcuts, each representing a point along a generalized

edaphic gradient xericc sandhill, mesic flatwoods, and

hydric hammock); ii) comparisons of hexazinone and

mechanically site prepared (shear and window) loblolly pine

(Pinus taeda) plantations established 7 years earlier on

hydric hammock sites; and iii) rate-response experiments for

understory restoration of a successionally advanced, xeric

sandhill site.

Cover by woody species decreased with increasing

hexazinone rates (0.0, 1.7, 3.4, and 6.8 kg/ha) on all sites

along the edaphic gradient. Herbaceous vegetation recovered

from first-season reductions to levels that did not vary


viii









with treatment xericc sandhills and mesic flatwoods) or

increased with increasing hexazinone rates hydricc hammock).

Hexazinone tolerance by Gelsemium sempervirens and Vaccinium

spp. on the xeric sandhill, and Ilex qlabra and G.

sempervirens on the mesic flatwoods influenced diversity

response by woody and herbaceous vegetation. With

increasing rates, herbaceous diversity decreased on the

xeric sandhill, did not vary on the mesic flatwoods, and

increased on the hydric hammock.

When compared with hexazinone applications (3.4 kg/ha),

mechanical site preparations resulted in greater numbers of

woody species. While mechanically-treated stands had higher

alpha diversity, hexazinone treated stands exhibited greater

beta diversity. On mechanically-treated stands, beta

diversity and distributions of some species were related to

window proximity.

Hexazinone (0.42, 0.84, and 1.68 kg/ha) selectively

controlled oaks (Quercus spp.) <14 cm dbh on the

successionally-advanced xeric sandhill, and prompted release

of wiregrass (Aristida stricta). This suggests that

hexazinone may have use for restoration of longleaf pine

(Pinus palustris) wiregrass ecosystems.

Plant community responses to hexazinone are functions

of application rate, edaphic factors, adaptive strategies of

resident species, and the presence-absence of hexazinone

tolerant species.















CHAPTER 1
GENERAL INTRODUCTION

Prediction of forest productivity in response to

vegetation management strategies, e.g., herbicide

applications, is restricted by a lack of knowledge

concerning plant response to changing environmental

conditions (Radosevich and Osteryoung 1987). Adding to

these uncertainties are mandates for the maintenance of

biological diversity [National Forest Management Protection

Act; Federal Register 44(181), 219.13(6)], and restoration

and management of critical wildlife habitats [Endangered

Species Act of 1973 (16 U.S.C. 1531 et seq.)]. Furthermore,

public involvement in vegetation management policy formation

has necessitated consideration of practically all non-timber

impacts of herbicide use (USDA 1989). These additional

management constraints increase the uncertainty of herbicide

use and requires assessment of the impacts and uses of these

compounds from different perspectives.

Hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl-

1,3,5-triazine-2,4(lH,3H)-dione) is a selective herbicide

registered for site preparation, release, and herbaceous

weed control for pine (Pinus spp.) production in the

southern United States (Nelson and Cantrell 1991). Research

on hexazinone for forestry uses has been largely








concentrated on efficacy trials for control of woody or

herbaceous weeds that compete with pines, and thereby affect

their survival and growth. Little emphasis has been placed

upon vegetation dynamics other than from a weed control

perspective. On a few sites, basic plant community

responses to hexazinone have been documented. These have

tended to be for low-rate applications on old-field pine

plantations (Blake et al. 1987), or only for woody

vegetation following release treatments (approximately 50%

lower than rates used for site preparation) (Zutter and

Zedaker 1988).

At present, generalizations concerning the influence of

hexazinone site preparation on plant community dynamics of

sites in the lower Coastal Plain are based on incomplete

data. Documentation of plant community responses to

hexazinone on a variety of sites in the lower Coastal Plain

would allow for improved predictive abilities. Furthermore,

comparisons with alternative treatments would allow

decisions to be made concerning the relative merits of

different site preparation techniques. Finally, weed

control strategies developed for pine plantations might be

adapted to control vegetation in natural pine systems that

are not managed for fiber production.

Two basic strategies for detecting response patterns in

multi-species communities are: i) experiments where

treatment parameters are varied in a factorial fashion and

individual plant responses are measured under a variety of








species mixes; and ii) descriptions of the observed patterns

in the abundance of species (i.e., richness, diversity,

dominance, and spatial distribution) (May 1981). This

dissertation reports on a set of studies that incorporates

elements of both of May's strategies. Existing plant

communities were used, so complete sensitivity analyses for

all plant species mixes under a variety of conditions (May's

first strategy) were not practical. The explicit purpose of

conducting these studies was to enhance predictability

associated with hexazinone use. The geographical region of

interest was the lower Coastal Plain of north-central

Florida.

Characteristics of Hexazinone

An understanding of the impacts of hexazinone on forest

plant communities would be incomplete without a basic

knowledge of the herbicide. Questions posed about forest

herbicide characteristics routinely pertain to environmental

fate and mode of action. Hexazinone is a triazine herbicide

that is highly soluble in water (33000 mg/L at 25 C), and is

potentially very mobile in subsurface solution (Neary et al.

1983, Bouchard et al. 1985). Following applications of 1.68

kg a.i./ha, off-site movement of hexazinone has been

observed to be minimal and of low toxicity risk to adjacent

aquatic ecosystems (Neary et al. 1983). Following the same

treatment, invertebrates experienced no major changes in

community composition (Mayack et al. 1982). Following

applications of 2.00 kg a.i./ha on sandy loam sites in








Arkansas, off-site movement was 2.0-3.0%, and <0.10% of that

applied was returned to the forest floor upon oak

defoliation (Bouchard et al. 1985).

Persistence of hexazinone in forest soils is relatively

short-lived. The half-life of hexazinone in silt loam soils

in Delaware, Illinois and Mississippi has been reported as

1, 2 and 6 months, respectively (Rhodes 1980). In Alabama,

half-lives were 4-6 weeks in clay soil, and <4 weeks in

loamy sand (Sung et al. 1981).

Although foliar penetration of triazine herbicides does

occur (and this can be enhanced by adjuvents) (Esser et al.

1975), primary uptake of hexazinone is from soil solution by

roots (Ashton and Crafts 1973). Soil adsorption of

hexazinone is amplified by increasing clay and organic

matter content (Nelson et al. 1981), such that root uptake

and subsequent herbicidal activity are decreased (Minogue et

al. 1988).

Following absorption, hexazinone is distributed through

the transpirational stream to its site of action in the

chloroplasts (Ashton and Crafts 1973). There, the compound

binds to a specific protein and inhibits its ability to

mediate electron transport -- the Hill reaction (Van Rensen

1989). This results in a build-up of triplet state

chlorophyll that generates singlet oxygen (Dodge 1982).

Singlet oxygen peroxidizes cell membrane lipids and the

affected plant dies from oxidative stress (Balke 1987,

Bartels 1987).








Some woody species gain tolerance to hexazinone by

having greater abilities to metabolize the compound before

it reaches the site of action [e.g., loblolly pine (Pinus

taeda) when compared with Quercus spp. (McNeil et al. 1984),

and Pvrus melanocarpa when compared to Rubus hispidus

(Jensen and Kimball 1990)], while others gain tolerance with

reduced translocation [e.g., Vaccinium spp. when compared

with Solidago fistulosa (Baron and Manaco 1986) and

Juniperus virginiana when compared with loblolly pine

(McNeil et al. 1984)].

Dissertation Format

This dissertation incorporates results from three

separate studies, each concentrating on a different aspect

of plant community response to hexazinone. The opening

chapter (Chapter 2) focuses on responses of individual

species and plant communities to hexazinone site preparation

at different application rates on three contrasting sites

xericc, mesic, and hydric). Response modelling was

conducted for dominant plant taxa, and an overall null

hypothesis was tested that short-term rate-responses of

plant species diversity and species-abundance distributions

are qualitatively the same for all sites in the region.

In Chapter 3, relative impacts of two site preparation

alternatives on hydric hammock vegetation were compared.

Hexazinone site preparation was contrasted with a mechanical

treatment (shearing and windrowing). Two replicates of each

treatment were evaluated for testing the null hypothesis








that abundance of individual species as measured by foliar

cover, numbers of species, and spatial organization of those

species were the same following both treatments. Results of

these comparisons were used to evaluate the relative impacts

of these alternatives on the composition of plant

communities and wildlife habitats within the hydric hammock

ecosystem.

Chapter 4 explores the potential uses of hexazinone as

a management tool for understory restoration of longleaf

pine (P. palustris) wiregrass (Aristida stricta)

communities on xeric sandhill sites. This endangered

ecosystem (Means and Grow 1985) occupies approximately 15%

of its original 28 million ha in the southeastern U.S.

(Croker 1979). On a large proportion of those sites where

it currently exists, fire suppression has allowed for

encroachment of scrub oak midstories, resulting in

suppressed herbaceous ground cover. Development of methods

for facilitating the reintroduction of summer ground-fires

to these sites has been identified as a restoration priority

(Noss 1988). The object of the study was to test hexazinone

rates that would release relictual wiregrass from scrub oak

competition with a minimum of impacts on other plant

species.

The concluding chapter is a synthesis and summary.

Results of all three studies are used to predict some long-

term impacts of hexazinone use.















CHAPTER 2
VASCULAR PLANT COMMUNITY DYNAMICS FOLLOWING HEXAZINONE SITE
PREPARATION IN THE LOWER COASTAL PLAIN

Introduction

Establishment of pine (Pinus spp.) plantations in the

lower Coastal Plain has traditionally been accomplished with

intensive mechanical disturbances (Worst 1964, McMinn 1969)

involving methods that often result in long-term degradation

of site productivity (Morris et al. 1983, Swindel et al.

1986). Herbicide use offers an alternative that may be more

economical and cause less soil disturbance and nutrient

displacement than mechanical means. Chemical site

preparation has been shown to provide effective competition

control and is, therefore, replacing intensive mechanical

methods in many forest types in the southeastern U.S.

(Walstad and Kuch 1987).

Hexazinone is one of nine herbicides currently

registered for forest site preparation in the southeastern

U.S. Hexazinone is available commercially as Velpar ULWTM,

Velpar LT (E.I. Du Pont De Nemours & Co.), and Pronone 10GTM

(Pro-Serve, Inc.). Manufacturer recommended hexazinone

rates for forest site preparation range from 2.0 to 6.7 kg

a.i./ha (E.I. Du Pont De Nemours & Co., Specimen Label).

Metabolic break-down and detoxification of hexazinone

varies among woody species (McNeil et al. 1984), resulting

7








in selective activity. Blueberries (Vaccinium spp.), for

example, are known to be quite tolerant (Zutter and Zedaker

1988), while oaks (Quercus spp.), sweetgum (Liquidambar

styraciflua) and sumacs (Rhus spp.) are known to be

especially susceptible to hexazinone (Neary et al. 1981,

Griswold et al. 1984, Miller 1984, Zutter and Zedaker 1988).

Questions concerning the impacts of chemical site

preparation treatments on dynamics of understory plant

communities remain largely unanswered. Swindel et al.

(1989) and Neary (1991) demonstrated that forest herbicides,

when repeatedly applied, alter species-abundance

relationships and substantially diminish the plant diversity

of young pine plantations. Neary (1991) suggested that

single herbicide treatments, such as those used for site

preparation, result in initial suppression of plant

diversity followed by a recovery along a trajectory similar

to that of an untreated site. Studies of plant community

changes following hexazinone applications support this

postulate by demonstrating that numbers of species in plant

communities are unchanged, or slightly higher, during the

second growing season following herbicide applications

(Blake et al. 1987). In contrast, Zutter and Zedaker (1988)

found that woody plant diversities of loblolly pine (Pinus

taeda) plantations decreased with increasing hexazinone

rates for at least two years following release applications.

Although long-term studies are most desirable in

determining likely influences of chemical site preparation








on biological diversity at a variety of scales, preliminary

questions must be asked, and their corresponding hypotheses

tested. Prior to posing questions concerning impacts of

hexazinone on plant species richness and diversity, it must

be determined whether there are consistent patterns of

responses among forest sites -- at least within a confined

geographical area.

Plant species richness and diversity are largely

determined by interspecific competition interacting with

site productivity, micro-site heterogeneity, and disturbance

regimes (Tilman 1982). The goal of a herbicide application

is to selectively kill plants and alter the competitive

relationships among plants. So, one could anticipate

qualitatively different impacts of chemical site preparation

on species diversity and richness among sites with

dissimilar plant communities and site characteristics. This

overall hypothesis was tested by examining hexazinone rate-

response relationships of early successional plant

communities along a generalized edaphic gradient.

Primary goals of this study were to determine initial

plant community response to a series of hexazinone

applications and to determine if overall plant community

response in the lower Coastal Plain is similar among xeric,

mesic, and hydric sites. The objectives were 1) estimate

hexazinone rate response models for individual plant

species, 2) determine how plant species richness, diversity,

and dominance varies as a function of hexazinone rate on








three sites; and 3) determine if plant community responses

to hexazinone site preparation are qualitatively the same

across a gradient of forest sites.

Underlying null hypotheses were: (i) species

composition is not affected by hexazinone, (ii) species

richness, diversity, and dominance are not affected by

increasing hexazinone rate, and (iii) responses of plant

communities to hexazinone site preparation are qualitatively

similar across a range of forest sites in the lower Coastal

Plain.

Methods

Study Sites

Areas chosen for experimental applications were within

three clearcut stands on privately-owned, commercial

forestlands in north-central Florida. The sites were

classified as xeric sandhill, mesic flatwoods and hydric

hammock. All three sites were scheduled by land managers

for aerial applications of hexazinone (Velpar ULWTM) in the

late spring of 1990.

The xeric sandhill site was in Gilchrist County,

approximately 10 km west of the town of High Springs (290

50'N, 820 45'W). The site was clearcut harvested in 1988

following the first rotation of a slash pine (Pinus

elliottii) plantation (approximately 20 years). Soils are

well-drained, deep, acidic sands of the Penney series

(thermic, uncoated Typic Quartzipsamments).








The mesic flatwoods site was in Alachua County,

approximately 4 km south and 3 km east of Waldo (290 45'N,

820 10'W). Most of the surrounding landscape was

intensively managed slash pine plantation, typical of the

flatwoods of north-central Florida. Prior to clearcut

harvest in early 1989, the site was occupied by a mixed-aged

(18-25 years-old) slash pine plantation that had developed a

mixed pine-hardwood canopy during the previous rotation.

Soils of the mesic flatwoods site are of the Sparr

series (loamy, siliceous, hyperthermic Grossarenic

Paleudults). Although this soil is somewhat poorly drained,

the site is characterized as being a mesic flatwoods when

compared with the poorly drained Spodosols that are

generally found in this area (Brown et al. 1990).

The hydric hammock site was located in the Gulf Hammock

area of Levy County, Florida, approximately 15 km west and 8

km south of the town of Otter Creek (290 15'N, 820 50'W).

Hydric hammock is a distinctive type of forested, freshwater

wetland. The vegetation is most similar to southern

bottomland hardwoods, except that it is not as closely

associated with a riverine system -- and the dominant

natural vegetation is evergreen (Vince et al. 1989). The

experimental site was located within what had been a natural

stand of hydric hammock until clearcut harvest in the winter

of 1988.

Soils of the hydric hammock site are poorly drained,

shallow, loamy textured marine sediments, typically with a








black organic muck extending to a sandy clay loam at 9 cm.

Limestone bedrock is within 30 cm of the surface, frequently

extending up to the surface. These soils are classified as

a Waccasassa (loamy, siliceous, Thermic Lithic Haplaquepts)

-Demory (loamy, siliceous, Thermic Lithic Haplaquolls)

complex. These areas are subjected to annual flooding and

remain inundated for 1 to 3 months of the year, usually in

late summer and fall.

Treatments

On 29 May, 30 May, and 16 June 1990, hexazinone was

applied at four rates [0.0 (control), 1.7, 3.4, and 6.8 kg

a.i./ha] within 900-m2 plots (30 X 30 m) at the xeric

sandhill, mesic flatwoods, and hydric hammock study sites,

respectively. Twelve plots had been previously designated

at each site and arranged in three blocks of four, with a

minimum 5 m buffer between each plot. The four herbicide

rates were randomly assigned to the remaining plots within

each block. The experimental design was a randomized

complete block.

On the mesic flatwoods and xeric sandhill, appropriate

amounts of hexazinone were applied as Pronone 10GTn from a

modified, hand-held, fertilizer spreader. The spreader was

calibrated for a precise application at the highest

treatment rate (64 kg/ha product) and was diluted with

manufacturer-supplied "blank" carrier granules to the

appropriate effective concentrations for the two lower

application rates. Vegetation was too tall for this method








of application on the hydric hammock site, so applications

were made using Velpar ULWTM from a modified SoloTM power

blower.

Measurements

Prior to herbicide application, four 20-m line

transects were permanently established within each plot.

Lines were parallel, spaced 5 m apart, and were on a 5 m

offset-stagger across a 25 X 25 m measurement plot centered

within each treatment plot. Foliar cover by all woody

species, and by those forb species that commonly reached

heights >0.5 m, were measured along the entire length of

each line. Cover by all other herbaceous vegetation was

measured along three 2-m transects equally spaced along each

20-m line. Foliar cover was remeasured after the first (Oct

90) and second (Sep 91) growing seasons following treatment.

Unless otherwise noted, taxonomic treatment of plant species

followed Wunderlin (1982).

Data Analyses

For all three sampling periods, three diversity

measures were computed separately for woody and herbaceous

vegetation for each treatment replicate per site. These

were (Hill 1973):

No = total number of species,

N1 = Exp[-E, piln(pi)], and

N2 = 1/Ei pi2

where pi is proportional cover of the ith species. These

diversity measures represent actual units of species numbers








(Hill 1973, Ludwig and Reynolds 1988). Diversity measures

N1 and N2 are increasingly influenced by equitable

distribution of cover among the component species of a

community, such that they become, respectively, more

insensitive to relatively rare species.

For each study site, herbaceous cover, woody cover, No,

N1, N2, and cover by individual species were tested for

treatment response by analysis of covariance (ANCOVA) for a

randomized complete block design. Coefficients of variation

for foliar cover tended to remain constant among treatment

means, so those data were log transformed to stabilize

variance. Model sums of squares were partitioned into

linear, quadratic, and lack-of-fit components. To avoid

redundancy in reporting, species of the same genus (family

in the case of Cyperaceae) were grouped if they displayed

similar response trends. Taxonomic groups presented are

only those for which significant (P<0.05) rate responses

were found, or those that were non-responsive to treatment

(P>0.90). A detailed listing of all species and scientific

authorities are presented in the Appendix.

Log-normal frequency distributions (Preston 1948) and

dominance-diversity curves (Whittaker 1965) were used to

graphically represent changes in species-abundance

relationships associated with treatment rates. These were

contrasted across study sites.








Results

Xeric Sandhill

Species response. Among the eight taxonomic groups

that accounted for 89% of pretreatment woody plant cover

(K=30%) on the xeric sandhill, only oaks, winged sumac (Rhus

copallina), and sand blackberry (Rubus cuneifolius) were

susceptible to hexazinone rates > 1.7 kg/ha (Table 2-1).

Greenbriers (Smilax spp.) were susceptible at rates >3.4

kg/ha. Carolina jessamine (Gelsemium sempervirens) was

immediately released at intermediate rates, and became the

dominant woody species by the end of the second growing

season at 1.7 and 3.4 kg/ha (59 and 62% of woody cover,

respectively). Pawpaw (Asimina spp.) showed a decreasing

trend over time regardless of treatment, but ANCOVA

indicated a linear increase with increasing hexazinone rates

in relation to its pretreatment cover. Deerberry (Vaccinium

staminium) and sparkleberry (V. arboreum) did not respond to

hexazinone at any rate tested (P>0.90). At 6.8 kg/ha,

Vaccinium spp. accounted for 71% of second season woody

plant cover, while only accounting for 27% at pretreatment.

Vaccinium spp. did not increase, however, in absolute cover

through the study period. Sand blackberry recovered by the

end of the second growing season to levels at which

significant treatment effects could not be demonstrated --

but this species did not recover at 6.8 kg/ha. Oaks did not

recover by the end of the second growing season, while










Table 2-1. Mean cover and herbicide rate response for plant taxa before
(PT), one (Y1), and two (Y2) growing seasons after hexazinone
application on a xeric sandhill site in Gilchrist County, Florida.



Life form Hexazinone rate (kg/ha)
Rate
Taxon Time 0.0 1.7 3.4 6.8 response


Woody


------ Foliar


Quercus spp.



Rhus copallina



Vaccinium
stamineum


Gelsemium
sempervirens


Rubus
cuneifolius


Vaccinium
arboreum


Smilax spp.



Asimina spp.



Grasses

Dichanthelium spp.



Paspalum
setaceum


15.03
16.27
27.23

1.30
2.17
6.10

4.06
3.04
2.88

0.50
0.65
1.21

0.77
1.48
1.02

0.61
0.83
0.71

0.30
0.69
0.63

0.96
0.23
0.23



9.96
17.15
14.78

2.08
4.15
2.96


13.72
0.54
0.96

1.36
0.46
4.04

3.50
2.92
2.38

2.95
14.25
26.35

0.73
0.29
0.38

2.40
2.90
2.69

0.82
0.88
2.54

1.56
1.23
1.15



15.73
0.90
4.99

1.01
0.14
2.82


cover (Z)


15.22
0.00
0.08

2.74
0.00
0.90

1.90
1.27
1.69

3.26
3.92
12.25

1.59
0.00
0.44

1.28
1.44
1.77

0.38
0.04
0.58

1.05
1.27
0.50



11.52
0.00
9.00

2.56
0.00
8.85


16.81
0.00
0.00

2.33
0.00
0.08

5.70
4.40
5.10

0.55
0.13
1.08

0.07
0.00
0.00

2.65
2.50
1.79

0.22
0.02
0.10

0.48
0.71
0.33



12.50
0.00
4.29

0.00
0.00
6.01


LOF ***
-LIN ***


-LIN **
-LIN *


NS
NS


LOF **
LOF **


-LIN *
ns


NS
NS


-LIN
ns


+LIN **
+LIN *


-LIN **
-LIN *


-LIN *
LOF *










Table 2-1--continued.


Life form Hexazinone rate (kg/ha)
Rate
Taxon Time 0.0 1.7 3.4 6.8 response


Grasses ------- Foliar cover (%) ------

Andropogon spp. PT 2.34 1.01 3.37 2.69
Y1 3.19 0.03 0.00 0.00 -LIN *
Y2 1.33 1.64 1.14 1.61 ns

Aristida PT 0.25 0.28 1.64 2.64
stricta Y1 0.69 0.56 0.42 0.69 -LIN *
Y2 1.11 1.14 3.42 0.29 ns
Forbs

Galactia spp. PT 0.00 0.19 1.15 0.80
Y1 0.75 0.03 0.00 0.00 ns
Y2 2.78 1.07 0.06 0.03 -LIN *

Eupatorium PT 0.48 0.76 0.00 1.30
compositifolium Y1 1.98 0.00 0.00 0.00 -LIN *
Y2 2.00 1.65 1.04 4.75 ns

Crotonopsis PT 1.12 1.17 1.27 1.17
linearis Y1 5.80 4.57 0.00 0.00 -LIN **
Y2 1.97 9.40 15.93 24.81 +LIN **

Cassia PT 0.27 0.08 0.36 0.16
fasciculata Y1 0.00 0.00 0.00 0.00
Y2 1.58 7.98 10.58 8.40 QUAD *


a LIN, QUAD, and LOF indicate linear, quadratic and lack-of-fit response
models, respectively. Response model designations followed by asterisks
were significant at P<0.05, P<0.01, and P<0.001 for *, **, and ***,
respectively. Sign (- or +) indicates direction of response with
increasing hexazinone rate. Comparisons for which there was a failure
to demonstrate a significant treatment response (0.05> P <0.90) were
signified by "ns", while comparisons for which cover was significantly
non-responsive (P > 0.90) were signified by "NS".








winged sumac seemed to recover only at 1.7 kg/ha of the

herbicide.

Among the eight taxonomic groups that accounted for 67%

of pretreatment herbaceous cover (X=30%) on the xeric

sandhill, all were decreased with increasing hexazinone

rates during the first season (Table 2-1). When compared to

other species, wiregrass (Aristida stricta) exhibited

hexazinone tolerance such that it accounted for 1, 3, 54 and

62% at 0.0, 1.7, 3.4 and 6.8 kg/ha, respectively, even

though it exhibited a decrease in absolute cover in the

first season.

Decreasing cover by Dichanthelium spp. and milk peas

(Galactia spp.) was significantly associated with increases

in hexazinone rate at the end of the second growing season.

The lack-of-fit term was significant for thin paspalum

(Paspalum setaceum) during the second season, indicating

that neither linear nor quadratic terms adequately explained

increases in cover at > 3.4 kg/ha. Partridge pea (Cassia

fasciculata) was released following all treatment rates, the

significant quadratic term reflecting an apex at 3.4 kg/ha.

Crotonopsis linearis responded to increased hexazinone rates

with a linear increase in the second growing season.

Dominance and diversity. Total woody cover decreased

in response to increasing hexazinone rates, with significant

linear and lack-of-fit terms during the first two post-

treatment growing seasons (Table 2-2). Release of Carolina

jessamine at 3.4 kg/ha masked decreases by other species,









Table 2-2. Means and ANOVA tables for woody species
diversity and foliar cover one (Year 1) and two (Year 2)
growing seasons after hexazinone application to a 1 year-old
clearcut on a xeric sandhill site in Gilchrist County,
Florida. All means were adjusted for pretreatment
measurements.


Woody diversity
Hexazinone Woody
Time rate (kg/ha) No N, N2 cover (%)


Year 1 0.0 14.5 8.9 7.0 32.2
1.7 10.9 *b 2.7 1.5 22.7 **
3.4 6.9 *** 3.8 2.9 8.1 ***
6.8 4.3 *** 3.9 3.3 7.1 ***

Year 2 0.0 16.3 8.7 6.1 47.7
1.7 14.6 3.3 2.0 42.0
3.4 9.9 4.5 3.1 19.4 **
6.8 6.5 ** 4.6 3.8 8.7 ***

Source P-values

Year 1 Pretreat 0.003 0.053 0.125 0.002
(Covariate)

Control vs. <0.001 0.004 0.008 <0.001
Hexazinone

Rate <0.001 0.023 0.037 <0.001
Linear <0.001 0.013 0.033 <0.001
Quadratic 0.345 0.059 0.056 0.702
Lack of fit 0.157 0.335 0.321 0.006


Year 2 Pretreat 0.255 0.269 0.408 0.042
(Covariate)

Control vs. 0.010 0.027 0.040 0.002
Hexazinone

Rate 0.009 0.121 0.151 <0.001
Linear 0.002 0.198 0.420 <0.001
Quadratic 0.592 0.091 0.080 0.221
Lack of fit 0.364 0.240 0.221 0.050


a Hill's (1973) diversity numbers.

b Means followed by asterisks were significantly different
from control (0.0 kg/ha hexazinone) by Least-Square Means
(*=P<0.05, **=P<0.01, ***=P<0.001).








thus accounting for lack-of-fit to a linear term. Total

herbaceous cover decreased in response to increasing

hexazinone rates during the first season (Table 2-3). A

significant quadratic term was reflective of the extreme

susceptibility of the herbaceous community to rates >3.4

kg/ha, and the slightly greater coverage by the somewhat

resistant wiregrass at 6.8 kg/ha. After the second season,

total herbaceous cover did not vary among treatments. By

the end of the second season, total woody cover slightly

exceeded (1.5%) total herbaceous cover at 0.0 kg/ha. But

total herbaceous cover exceeded woody cover by 9, 41, and

50% at 1.7, 3.4, and 6.8 kg/ha hexazinone, respectively.

All three woody diversity measures decreased with

increasing hexazinone rates after the first season (Table 2-

2). Diversity numbers for woody vegetation responded to

increased hexazinone rates with significant linear decreases

during the first season. Woody species richness (No)

followed a similar trend during the second season. Values

of N1 and N2 were lower on hexazinone treatments, especially

at 1.7 kg/ha, but only approached marginal significance for

a quadratic term. This was attributed to elimination of

three dominant oak species and the concurrent release of

Carolina jessamine at 1.7 kg/ha. These changes resulted in

decreased woody species numbers in cover classes above 4%

and a shift for one species (Carolina jessamine) into the

32% (upper-bound) class at 1.7 kg/ha (Figure 2-1). The

lesser release of Carolina jessamine at rates > 3.4 kg/ha









Table 2-3. Means and ANOVA tables for herbaceous species
diversity and foliar cover one (Year 1) and two (Year 2)
growing seasons after hexazinone application to a 1 year-old
clearcut on a xeric sandhill site in Gilchrist County,
Florida. All means were adjusted for pretreatment
measurements.


Herbaceous diversity
Hexazinone Herbaceous
Time rate (kg/ha) No N, N2 cover (%)


19.9 10.3
7.5 **b 3.6 **
1.7 *** 1.8 **
1.3 *** 0.7 **


25.2
22.3
18.3
13.9


12.8
12.4
8.3 *
5.6 **


6.8
2.6
1.7
0.5

7.8
8.5
6.1
3.9


51.3
15.4 **
0.8 ***
1.1 ***

46.1
51.3
60.1
58.4


Source

Year 1 Pretreat
(Covariate)

Control vs.
Hexazinone

Rate
Linear
Quadratic
Lack of fit


Year 2 Pretreat
(Covariate)

Control vs.
Hexazinone

Rate
Linear
Quadratic
Lack of fit


P-values


0.478


<0.001


0.002
<0.001
0.075
0.326


0.272


0.031


0.041
0.009
0.692
0.773


0.480


0.001


0.007
0.001
0.172
0.935


0.784


0.026


0.015
0.004
0.932
0.248


0.248


0.004


0.018
0.004
0.352
0.810


0.435


0.243


0.106
0.028
0.465
0.550


0.551


0.003


0.001
0.003
0.038
0.342


0.984


0.342


0.621
0.320
0.502
0.719


a Hill's (1973) diversity numbers.

b Means followed by asterisks were significantly different
from control (0.0 kg/ha hexazinone) by Least-Square Means
(*=P<0.05, **=P<0.01, ***=P<0.001).


Year 1




Year 2


0.0
1.7
3.4
6.8

0.0
1.7
3.4
6.8















IR,


0.0 kg/ha


0.02 0.06 0.25 1 2 4 8 163264
Cover cass (%)


U.Z 1 z 4 k
Cover dass (%)


16
14 3.4 kg/
o 12
0
o. 10
S 8
.r 6
E
Z 4
2
U.1^


'ha


0.02 0.06 0.25 1 2 4 8163264
Cover dass (%)


Cover dass (%)


Woody M Grass-(like) ( Forb


Figure 2-1. Frequency distributions of plant species across
log2 cover classes (upper boundary of class denoted) at the
end of the second growing season following hexazinone
applications on a xeric sandhill site in Gilchrist County,
Florida.


]






23

resulted in a more equitable distribution of cover among the

species of greatest abundance. This was illustrated in the

comparisons of dominance-diversity curves in Figure 2-2a.

Even though the 1.7 kg/ha treatment plots were more species

rich, N1 and N2 were influenced downward by increased

relative dominance. Distributions of woody species in the

0.1 to 1% cover ranges were quite similar on the control

(0.0 kg/ha) and 1.7 kg/ha rates, but not so at rates 23.4

kg/ha (Figure 2-2a).

All three herbaceous diversity measures decreased with

increasing hexazinone rates during the first season (Table

2-3). During the second season, diversity measures

indicated a recovery but they continued to exhibit

significant linear decreases with increasing hexazinone

rates. None of the herbaceous diversity measures at 1.7

kg/ha were significantly different from the untreated

control.

With increasing treatment rates, progressively more

species of grasses and forbs were represented in cover

classes >4% (Figure 2-1), resulting in values for N2 that

were not significantly different from control for 1.7 and

3.4 kg/ha at the end of the second growing season. Total

cover was more equitably distributed among the dominant

species at 1.7 kg/ha than at 0.0 kg/ha, and accounts for the

lack of a significant difference in N2 despite the

significantly lower numbers of species (No). This is







































0.014-
0


Woody species


Species rank


Herbaceous species


5 10 15 20 25 30 35 40 45
Species rank


--- 0.0 kg/ha --- 1.7 kg/ha -*- 3.4 kg/ha -- 6.8 kg/ha


Figure 2-2. Dominance-diversity curves for plant species at
the end of the second growing season following hexazinone
applications on a xeric sandhill site in Gilchrist County,
Florida. a) Woody species; b) Herbaceous species.








reflected in the divergence in the left tails of dominance-

diversity curves in Figure 2-2b.

By the end of the second season, plant communities on

hexazinone treated plots had fewer woody species of all

abundance classes and more herbaceous species had gained

dominance. This is reflected in Figure 2-1 by successive

replacement of woody species by grass, grass-like

(Cyperaceae), and forb species in cover classes >4%. Losses

of herbaceous species were from lower (rare) portions of the

abundance distribution. This is illustrated in Figure 2-2b

by successively higher points-of-departure in the right tail

of the dominance-diversity curves with increasing treatment

rates.

Mesic Flatwoods

Species response. Of the five taxonomic groups that

accounted for 92% of pretreatment woody plant cover (T=11%)

on the mesic flatwoods, three were susceptible to

hexazinone and decreased with increasing dosage (Table 2-4).

Cover response by oaks, sweetgum (Liquidambar styraciflua),

and wax-myrtle (Mvrica cerifera) were linear, significantly

decreasing with increasing hexazinone rates during the first

and second post-treatment growing seasons. During the first

season, cover by Carolina jessamine and gallberry (Ilex

qlabra) was significantly higher on hexazinone treated sites

when compared to the untreated control. Second-season

responses by gallberry and Carolina jessamine were

quadratically related to treatment. These two species were










Table 2-4. Mean foliar cover and herbicide rate responses for plant
taxa before (PT), one (Yl), and two (Y2) growing seasons after
hexazinone application on a mesic flatwoods site in Alachua County,
Florida.



Life form Hexazinone rate (kg/ha)
Rate
Taxon Time 0.0 1.7 3.4 6.8 response*


Woody


------ Foliar


Quercus spp.


Liquidambar
styraciflua


Myrica cerifera



Gelsemium
sempervirens


Ilex glabra



sses

Dichanthelium spp.



Andropogon spp.



Paspalum setaceum



Axonopus affinis



s-likes

Cyperus retrorsus


4.68
10.04
15.46

0.79
9.77
14.96

2.43
3.46
5.33

0.05
0.92
1.35

0.12
0.35
0.46



13.37
24.41
20.00

1.35
1.24
5.19

1.58
6.85
2.68

0.82
2.69
0.92


2.18
4.65
0.14


8.09
3.15
5.02

0.78
0.44
1.54

1.58
1.63
1.69

0.08
1.42
4.94

0.38
2.54
4.10



15.69
6.04
25.01

0.13
1.99
7.81

0.10
2.33
7.33

0.29
3.13
10.00



2.18
2.19
0.68


cover (%) -------


2.41
1.33
2.00

4.43
0.27
0.69

2.14
0.60
1.15

0.26
1.90
4.17

0.89
4.00
14.69



8.97
0.04
19.92

0.36
0.00
15.68

0.62
1.11
2.83

0.03
0.00
0.00



2.51
0.62
0.50


4.43
0.27
0.88

3.09
0.00
0.00

2.16
0.31
0.02

0.64
1.85
2.29

2.51
1.69
5.21



19.06
0.04
26.75

0.19
0.00
25.29

2.03
0.00
4.68

0.32
0.00
0.00


0.76
0.03
2.04


Gras


Grae


-LIN **
-LIN *


-LIN *
-LIN *


-LIN *
-LIN *


ns
QUAD *


ns
QUAD *


-LIN ***
ns


ns
+LIN *


-LIN *
ns


ns
LOF *


-LIN *
ns










Table 2-4--continued.


Life form Hexazinone rate (kg/ha
Rate
Taxon Time 0.0 1.7 3.4 6.8 response


Forbs ------ Foliar cover (Z)------

Eupatorium PT 0.07 0.33 1.57 0.19
compositifolium Y1 4.29 0.00 0.00 0.00 LOF **
Y2 7.31 2.52 3.46 1.33 -LIN *

Cassia nictitans PT 0.13 0.14 0.00 0.15
Y1 8.44 0.69 0.00 0.00 -LIN *
Y2 3.48 8.25 2.58 1.00 ns

Eupatorium PT 0.17 0.00 0.00 0.00
capillifolium Y1 1.90 0.00 0.00 0.00 ns
Y2 1.38 0.54 2.06 6.35 +LIN *


* LIN, QUAD, and LOF indicate linear, quadratic and lack-of-fit response
models, respectively. Response model designations followed by asterisks
were significant at P<0.05, P<0.01, and P<0.001 for *, **, and ***,
respectively. Sign (- or +) indicates direction of response with
increasing hexazinone rate. Comparisons for which there was a failure
to demonstrate a significant treatment response (0.05> P <0.90) were
signified by "ns".








released at intermediate treatment rates (1.7 and 3.4 kg/ha

hexazinone), and were slightly suppressed at 6.8 kg/ha.

All eight of the taxonomic groups that accounted for

85% of pretreatment herbaceous cover (R=22%) on the mesic

flatwoods were decreased with increasing hexazinone rates

(Table 2-4). Although a significant treatment response

could not be demonstrated for broomsedge (Andropogon spp.)

and carpetgrass (AxonoDus affinis) during the first year,

elimination of the small amounts of those species that did

occur at pretreatment on plots treated with rates >3.4

kg/ha, indicated that these species were susceptible.

Dichanthelium spp., thin paspalum, nutsedge (Cyperus

retrorsus), and wild sensitive plant (Cassia nictitans)

recovered during the second season to levels not

significantly related to treatment rate. Cover by

broomsedge increased with increasing rates during the second

growing season, becoming a dominant species on the two

highest treatment rates. Carpetgrass was released at 1.7

kg/ha but eliminated by the two higher rates, resulting in a

significant lack-of-fit term. Eupatorium compositifolium

linearly decreased with increasing hexazinone rates, while

E. capillifolium linearly increased with increasing rates.

Dominance and diversity. Total woody cover at the end

of the first season decreased with increasing hexazinone

rates (Table 2-5). Block by treatment interactions resulted

in a failure to demonstrate statistical significance in the

obvious downward trend in woody cover during the second








Table 2-5. Means and ANOVA tables for woody species
diversity and foliar cover one (Year 1) and two (Year 2)
growing seasons after hexazinone application to a 1 year-old
clearcut on a mesic flatwoods site in Alachua County,
Florida. All means were adjusted for pretreatment
measurements.


Woody diversity
Hexazinone Woody
Time rate (kg/ha) No NI N2 cover (%)


Year 1 0.0 10.3 5.7 4.5 26.2
1.7 7.2 4.1 3.0 11.8
3.4 7.2 4.0 3.0 8.3 *
6.8 2.2 ** 2.0 1.9 6.3 **

Year 2 0.0 11.2 6.1 4.9 42.8
1.7 10.1 5.2 4.0 22.4
3.4 9.6 3.8 2.5 24.5
6.8 5.5 *** 2.6 2.2 16.3

Source P-values

Year 1 Pretreat 0.070 0.303 0.473 0.775
(Covariate)

Control vs. 0.003 0.054 0.050 0.019
Hexazinone

Rate 0.004 0.040 0.053 0.040
Linear <0.001 0.008 0.011 0.008
Quadratic 0.762 0.942 0.706 0.528
Lack of fit 0.160 0.642 0.542 0.792


Year 2 Pretreat 0.049 0.218 0.188 0.456
(Covariate)

Control vs. <0.001 0.050 0.061 0.129
Hexazinone

Rate <0.001 0.040 0.065 0.427
Linear <0.001 0.008 0.016 0.187
Quadratic 0.035 0.805 0.400 0.426
Lack of fit 0.274 0.919 0.790 0.452


a Hill's (1973) diversity numbers.

b Means followed by asterisks were significantly different
from control (0.0 kg/ha hexazinone) by Least-Square Means
(*=P<0.05, **=P<0.01, ***=P<0.001).








season. This resulted because of disproportionate release

of gallberry in one 6.8 kg/ha replicate.

Total herbaceous cover decreased with increasing

hexazinone rates during the first season and was completely

eliminated by hexazinone rates of 6.8 kg/ha (Table 2-6).

Total herbaceous cover did not vary with treatment during

the second growing season. By the end of the second season,

total herbaceous cover exceeded total woody cover by 12, 55,

44, and 56% at hexazinone rates of 0.0, 1.7, 3.4, and 6.8

kg/ha, respectively

All three measures of woody diversity decreased with

increasing hexazinone rates at the ends of both post-

treatment growing seasons (Table 2-5). Values of Ni and N2

were decreased at 3.4 kg/ha from the first into the second

season due to dominance by gallberry. Mean numbers of woody

species per plot (No) at 1.7 kg/ha were not significantly

different from control. In addition, occurrence of some

quantitatively rare species on one of the 1.7 kg/ha plots,

resulted in comparatively more species in the smaller cover

classes (Figure 2-3). The 1.7 kg/ha plots had fewer woody

species in common among themselves than did 0.0 kg/ha plots,

resulting in a higher combined species richness (longer

right tail in Figure 2-4a).

Herbaceous diversity numbers responded to increasing

hexazinone rates with significant linear decreases during

the first season post-treatment (Table 2-6). Herbaceous

diversity numbers No and N2 did not significantly respond to









Table 2-6. Means and ANOVA tables for herbaceous species
diversity and foliar cover one (Year 1) and two (Year 2)
growing seasons after hexazinone application to a 1 year-old
clearcut on a mesic flatwoods site in Alachua County,
Florida. All means were adjusted for pretreatment
measurements.


Herbaceous diversity
Hexazinone Herbaceous
Time rate (kg/ha) No N, N2 cover (%)


Year 1 0.0 9.0 6.8 4.4 61.0
1.7 9.5 4.2 4.0 20.7 *
3.4 5.6 2.3 ** 2.0 4.4*
6.8 0.2 0.6 ** 0.6 ** 0.0 **

Year 2 0.0 12.4 8.4 5.8 54.9
1.7 17.3 6.5 4.9 76.9
3.4 16.1 7.1 5.0 68.2
6.8 16.1 5.6 3.4 72.0

Source P-values

Year 1 Pretreat 0.06 0.114 0.452 0.564
(Covariate)

Control vs. 0.177 0.005 0.013 0.007
Hexazinone

Rate 0.027 0.003 0.004 0.026
Linear 0.008 0.001 0.001 0.006
Quadratic 0.515 0.682 0.768 0.415
Lack of fit 0.392 0.538 0.186 0.747


Year 2 Pretreat 0.214 0.223 0.883 0.136
(Covariate)

Control vs. 0.118 0.065 0.287 0.278
Hexazinone

Rate 0.372 0.106 0.380 0.655
Linear 0.248 0.026 0.115 0.531
Quadratic 0.244 0.597 0.858 0.541
Lack of fit 0.241 0.221 0.686 0.426


a Hill's (1973) diversity numbers.

b Means followed by asterisks were significantly different
from control (0.0 kg/ha hexazinone) by Least-Square Means
(*=P<0.05, **=P<0.01, ***=P<0.001).















1U

8 0.0 kg/ha
7
6
5
4
3
2
1

0.03 0.13 05 1 2 4 8 163264
Cover class (%)


0.03 0.13 0.51 2 4 8163264
Cover class (%)


1U

1.7 kg/ha
0
0 7
S6






0.03 0.13 0.51 24 8 163264
Cover class (%)
Cover class (%!


-
9 6.8 kg/h
8
" 7
0
' 5

4

z 2
1 RN
I -- *** U


n iI -


a


0.03 0.13 0.51 2 4 8163264
Cover cass (%)


SWoody Grass-(llke) Forb


Figure 2-3. Frequency distributions of plant species across
log2 cover classes (upper boundary of class denoted) at the
end of the second growing season following hexazinone
applications on a mesic flatwoods site in Alachua County,
Florida.


^ 1 ------ -- -


I









Woody species


100

10

1

0.1

0.01
0


5 10 15 20
Species rank


Herbaceous species
100

10

1

0.1


0.01


0 5 10 15 20 25 30
Species rank


- 0.0 kg/ha -*- 1.7 kg/ha -*- 3.4 kg/ha -- 6.8 kg/ha


Figure 2-4. Dominance-diversity curves for plant species at
the end of the second growing season following hexazinone
applications on a mesic flatwoods site in Alachua County,
Florida. a) Woody species; b) Herbaceous species.








treatment during the second season. However, N1 decreased

with increasing hexazinone rates. Figure 2-3 illustrates

the successive shifts of herbaceous species into larger

cover classes as hexazinone rate increased. At 6.8 kg/ha,

two species (Dichanthelium spp. and broomsedges) were in the

32% cover class, while 15 species covered l1% each.

Dominance-diversity curves illustrated only minor departures

(Figure 2-4b) and no treatment related responses could be

discerned.

Hydric Hammock

Species response. Cover by each of the nine taxonomic

groups that accounted for 80% of pretreatment woody cover

(X=47%) tended to decrease with increased rates of

hexazinone (Table 2-7). Sea-myrtle (Baccharis halimifolia),

sweetgum, grapes (Vitis spp.), and cabbage palm (Sabal

palmetto) responded to increased hexazinone rates with

linear decreases in cover during both the first and second

growing seasons following treatment. Hornbeam (Carpinus

caroliniana) cover during the second season was lowest on

all treatment plots even though the statistical response was

quadratic. The pretreatment covariate was significant

(P=0.05), thus impacting that trend. Greenbrier (Smilax

spp.) responded to increases in hexazinone rate by linearly

decreasing during the first season. Greenbrier cover

decreased from the first to the second season following all

treatments such that second-season means were the same. Red

bay (Persia borbonia) and persimmon (Diospyros virainiana),










Table 2-7. Mean foliar cover and herbicide rate response for plant taxa
before (PT), one (YL), and two (Y2) growing seasons after hexazinone
application on a hydric hammock site in Levy County, Florida.


Life form Hexazinone rate (kg/ha)
Rate
Taxon Time 0.0 1.7 3.4 6.8 response*


Woody


------ Foliar cover () ------


Baccharis
halimifolia


Liquidambar
styraciflua


Vitis spp.



Sabal
palmetto



Quercus spp.



Carpinus
caroliniana


Smilax spp.



Persia
borbonia


Diospyros
virginiana


Grass-like

Cyperaceae
(Rhynchospora-
Carex spp.)


14.53
43.17
47.23

8.71
15.93
23.23

11.85
12.58
15.50

3.85
6.22
6.49


5.00
3.85
4.31

2.44
1.76
4.06

3.68
4.74
1.88

0.24
0.56
1.37

0.30
0.55
1.09



71.38
62.89
39.93


9.73
23.52
35.95

6.29
5.08
7.69

4.36
1.63
4.02

1.22
2.54
2.10


4.10
2.50
2.23

2.25
0.75
1.44

3.16
2.80
1.50

0.39
0.50
1.95

0.73
0.00
1.42



51.69
35.23
29.88


9.63
8.23
20.06

7.39
1.65
1.83

15.32
0.46
1.90

1.94
1.94
2.27


1.86
0.96
0.69

4.38
0.10
1.73

3.67
2.16
1.15

0.79
0.19
0.81

0.33
0.00
0.08



50.02
22.77
29.54


10.73
2.25
5.70

11.09
0.52
0.23

2.13
0.00
0.23

1.45
1.67
1.83


1.50
0.29
0.06

2.72
0.83
2.33

2.57
1.10
1.00

0.56
0.00
0.33

1.74
0.23
0.10



68.89
13.77
35.88


-LIN ***
-LIN ***


-LIN ***
-LIN **


-LIN ***
-LIN **


-LIN *
-LIN *



ns
-LIN **


ns
QUAD *


-LIN *
NS


-LIN **
ns


QUAD **
ns


-LIN **
NS










Table 2-7--continued.


Life form Hexazinone rate (kg/ha
Rate
Taxon Time 0.0 1.7 3.4 6.8 response

Grasses ----- Foliar cover (Z) -------

Chasmanthium PT 12.10 3.78 18.20 17.86
laxum Y1 2.86 0.00 0.58 0.06 ns
Y2 8.57 5.26 8.82 1.60 -LIN *

Andropogon spp. PT 7.85 6.57 5.57 2.51
Y1 11.50 8.42 6.95 5.62 ns
Y2 7.01 21.88 19.31 17.93 QUAD **

Dichanthelium PT 11.68 18.90 18.23 6.18
commutatum Y1 7.44 8.26 11.37 4.89 ns
Y2 3.87 10.50 8.99 9.88 +LIN *
Forbs

Eupatorium PT 9.47 8.98 8.48 8.21
capillifolium Y1 37.23 4.29 0.00 0.00 LOF ***
Y2 1.18 1.79 9.92 23.12 +LIN ***

Mikania scandens PT 0.89 1.02 0.71 0.41
Y1 1.17 1.29 1.31 0.00 QUAD **
Y2 0.49 1.17 6.35 4.74 QUAD **


a LIN, QUAD, and LOF indicate linear, quadratic and lack-of-fit response
models, respectively. Response model designations followed by asterisks
were significant at P<0.05, P<0.01, and P<0.001 for *, **, and ***,
respectively. Sign (- or +) indicates direction of response with
increasing hexazinone rate. Comparisons for which there was a failure
to demonstrate a significant treatment response (0.05> P <0.90) were
signified by "ns", while comparisons for which cover was significantly
non-responsive (P > 0.90) were signified by "NS".








both of which decreased due to treatment during the first

season, recovered somewhat during the second season such

that a significant treatment response could not be detected.

Among the six taxonomic groups that accounted for 87%

of the pretreatment herbaceous cover (R=117%), only

Cvperaceae (Rhvnchospora spp. Carex spp. complex) showed a

linear decrease with increasing hexazinone rates during the

first season. Broomsedges were increased on hexazinone

treatments during the second season, but the response was

quadratic. Cover measurements of Dichanthelium commutatum

were highly variable during the first season resulting in a

non-significant response. This was followed, however, by a

significant linear increase with increasing herbicide rates

during the second season. During the first season, a lack-

of-fit term was highly significant for Eupatorium

capillifolium, indicating that neither the linear nor

quadratic terms adequately explained the obvious decreases

due to treatment. E. capillifolium recovered during the

second season, responding linearly to increased hexazinone

rates. Broomsedge was probably out-competed by E.

capillifolium at 3.4 and 6.8 kg/ha resulting in a

significant quadratic response during the second season.

Quadratic terms were significant for climbing hempweed

(Mikania scandens) during both post-treatment seasons.

Climbing hempweed cover was greatest at 3.4 kg/ha, slightly

decreasing at 6.8 kg/ha.








Dominance and diversity. Total woody cover responded

to increasing hexazinone rates with a highly significant

linear decrease during the first and second seasons (Table

2-8). Total herbaceous cover responded with a highly

significant linear decrease during the first season (Table

2-9). During the second season, total herbaceous cover

responded with a highly significant linear increase. By the

end of the second season, total woody cover exceeded total

herbaceous cover by 38% at 0.0 kg/ha, but total herbaceous

cover exceeded total woody cover by 28, 74, and 107% at 1.7,

3.4, and 6.8 kg/ha, respectively.

Species richness (No) of woody vegetation decreased

linearly with increased hexazinone rates during the first

and second seasons due to losses of species from all

abundance classes. N1 and N2 did not respond to treatment

during the first season, yet the quadratic term was

significant for during the second season. During the

second season, relative cover by sea-myrtle was 53% and 58%

at 1.7 and 3.4 kg/ha, respectively. All other woody species

were distributed < 8% at 1.7 kg/ha, and <4% at 3.4 kg/ha

(Figure 2-5). Diversity measure N2 was thus depressed at

those two treatment rates, but not at 6.8 kg/ha where

relative cover by sea-myrtle was at 38%.

Species diversity of herbaceous vegetation did not

significantly respond to hexazinone treatment during the

first season. During the second season, values of all three

diversity measures linearly increased with increasing









Table 2-8. Means and ANOVA tables for woody species
diversity and foliar cover one (Year 1) and two (Year 2)
growing seasons after hexazinone application to a 1 year-old
clearcut on a hydric hammock site in Levy County, Florida.
All means were adjusted for pretreatment measurements.


Woody diversity
Hexazinone Woody
Time rate (kg/ha) No N, N2 cover (%)


Year 1




Year 2


0.0
1.7
3.4
6.8

0.0
1.7
3.4
6.8


19.4
19.0
15.3
11.4 *

21.1
21.9
16.6
13.0 *


7.4
5.4
7.1
7.4


6.9
5.9
5.5
7.0


4.9
3.0
4.6
5.8

4.5
3.1
2.8
5.1


92.0
49.3
16.7
10.0

106.8
72.2
31.1
17.8


**



***


Source

Year 1 Pretreat
(Covariate)

Control vs.
Hexazinone

Rate
Linear
Quadratic
Lack of fit


Year 2 Pretreat
(Covariate)

Control vs.
Hexazinone


Rate
Linear
Quadratic
Lack of fit


P-values


0.457


0.097


0.081
0.020
0.849
0.532


0.478


0.124


0.058
0.017
0.765
0.251


0.011


0.476


0.380
0.577
0.395
0.157


0.253


0.659


0.746
0.758
0.338
0.904


0.006


0.590 <0.001


0.105
0.114
0.141
0.105


0.084


0.257 <0.001


0.087
0.228
0.026
0.904


a Hill's (1973) diversity numbers.

b Means followed by asterisks were significantly different
from control (0.0 kg/ha hexazinone) by Least-Square Means
(*=P<0.05, **=P<0.01, ***=P<0.001).


0.344


<0.001
<0.001
0.193
0.118


0.132


<0.001
<0.001
0.824
0.226









Table 2-9. Means and ANOVA tables for herbaceous species
diversity and foliar cover one (Year 1) and two (Year 2)
growing seasons after hexazinone application to a 1 year-old
clearcut on a hydric hammock site in Levy County, Florida.
All means were adjusted for pretreatment measurements.


Herbaceous diversitya
Hexazinone Herbaceous
Time rate (kg/ha) No Ni N2 cover (%)


13.3
15.8
12.8
10.6

15.1
21.6 *
17.3
22.1 *


6.5
6.4
6.5
5.9


7.9
8.7
10.5
11.9


5.1
4.6
4.5
4.5


5.8
5.7
8.3
* 8.8


128.4
76.9
52.5
27.6

69.0
100.4
104.8
124.7


**


***

**


P-values


Year 1 Pretreat
(Covariate)

Control vs.
Hexazinone

Rate
Linear
Quadratic
Lack of fit


Year 2 Pretreat
(Covariate)

Control vs.
Hexazinone


Rate
Linear
Quadratic
Lack of fit


0.358


0.936


0.455
0.218
0.507
0.373


0.872


0.019


0.027
0.027
0.647
0.019


0.219


0.874


0.969
0.678
0.875
0.947


0.232


0.110


0.099
0.024
0.818
0.657


0.210


0.440


0.617 <0.001


0.948
0.660
0.772
0.941


0.102


<0.001
<0.001
0.744
0.709


0.207


0.088 <0.001


0.041
0.012
0.653
0.142


0.002
<0.001
0.877
0.320


a Hill's (1973) diversity numbers.

b Means followed by asterisks were significantly different
from control (0.0 kg/ha hexazinone) by Least-Square Means
(*=P<0.05, **=P<0.01, ***=P<0.001).


Year 1




Year 2


0.0
1.7
3.4
6.8

0.0
1.7
3.4
6.8

























0.03 0.13 0.51 2 4 8
Cover class (%)


S1.7 kg/ha
2
0
8n


0.03 0.13 0.51 2 4 8163264
Cover class (%)


0.03 0.13 0.51 2 4 8163264
Cover dass (%)


6.8 kg/ha


.lFak


0.03 0.13 0.51 2 4 8163264
Cover dass (%)


SWoody W Grass-(like) Forb





Figure 2-5. Frequency distributions of plant species across
log2 cover classes (upper boundary of class denoted) at the
end of the second growing season following hexazinone
applications on a hydric hammock site in Levy County, Florida.


1


E
31
(A
'5


zr


1 --








hexazinone rates. The lack-of-fit term was also significant

for No indicating that variability in species richness was

not adequately explained by a linear response. Dominance-

diversity curves for the pooled data exhibited similar

relative distributions for those species with >0.5% cover

(Figure 2-6b). Quadratic responses by broomsedge and

climbing hempweed shifted those species into abundance

classes >8% at 3.4 kg/ha (Figure 2-5) allowing relative

abundance to be shared by more species, thereby compensating

for lower herbaceous species numbers in determinations of Ni

and N2.

Discussion

Species Response

Results reported here demonstrated that woody plant

compositions on xeric sandhill and mesic flatwoods sites

shifted largely as a result of different response models

among dominant species (i.e., hexazinone acted in a

selective manner on these sites). This contrasts with the

responses measured for dominant woody species on the hydric

hammock site, which all tended to decrease with increasing

hexazinone rates.

Oak species were prominent in the pretreatment

composition of all sites. Although oaks responded similarly

to hexazinone on all sites (i.e., decreasing dominance with

increasing hexazinone rates), the degree to which increasing

hexazinone rate played a factor in oak control was quite

different. For example, oaks on the xeric sandhill site









Woody species


100-

10

1


0.1


0.01
0


100

10

1

0.1


0.01


5 10 15 20 25 30 35 40
Species rank


Herbaceous species


0


5 10


15 20 25 30 35
Species rank


--- 0.0 kg/ha -+- 1.7 kg/ha -- 3.4 kg/ha -- 6.8 kg/ha


Figure 2-6. Dominance-diversity curves for plant species at
the end of the second growing season following hexazinone
applications on a hydric hammock site in Levy County, Florida.
a) Woody species; b) Herbaceous species.


A








were nearly eliminated (97% reduction) at 1.7 kg/ha, while

on the mesic flatwoods and hydric hammock, oak control at

1.7 kg/ha was 69% and 35%, respectively. This gradient in

oak control is probably a result of interactions with

hexazinone and the differing soil factors across the three

sites. Effects of pelletized hexazinone formulations on

oaks and other hardwood species are inhibited by fine soil

textures, increased organic matter contents, and increased

cation exchange capacities (Minogue et al. 1988). These

soil factors all would be expected to increase across the

gradient of acidic sands xericc sandhill), to sandy loam

mesicc flatwoods), to loamy organic muck hydricc hammock).

Three of the oak species that dominated on xeric

sandhills (Q. incana, Q. laevis, and Q. geminata) sprout as

a primary regenerative method. This is as an adaptation to

fire in xeric sandhills and scrub environments (Abrahamson

1984, Myers and White 1987). Reestablishment of these oaks

into areas from which their root stocks have been eliminated

is slow. On the xeric sandhills site, first season

eliminations of oaks from and all replicate plots at 23.4

kg/ha were followed by only trace recoveries at 3.4 kg/ha

and no recovery at 6.8 kg/ha during the second season.

Carolina jessamine was released from competition at 1.7

and 3.4 kg/ha and became dominant and co-dominant in woody

compositions on xeric sandhill and mesic flatwoods sites,

respectively. Similarly, gallberry was released on the

mesic flatwoods site, becoming dominant in the woody








composition at 3.4 kg/ha. Both Vaccinium species

experienced an increase in relative dominance on the xeric

sandhills site by virtue of their non-responsive nature to

hexazinone. While the rate-response of Carolina jessamine

has remained unreported in the reviewed literature, the

tolerance of Vaccinium spp. to hexazinone has been

documented (Zutter and Zedaker 1988, Zutter et al. 1988).

Second-year responses of gallberry reported here were

similar to those reported for yaupon (Ilex vomitoria) in the

Post Oak Savannah of Texas (Scifres 1982), indicating that

hexazinone tolerance may be a characteristic of the genus

Ilex.

Carolina jessamine became a dominant species when

released by hexazinone. This species is not generally

considered to be a sandhills denizen, yet it is sufficiently

shade tolerant and drought resistant (Godfrey 1988), and it

is present in small quantities in flatwoods pine plantations

adjacent to the xeric sandhills site. Sprouting from

extensive underground rhizomes, some vines of Carolina

jessamine were observed to extend greater than 2 m during 1

year. This species was able to grow prostrate, or climb and

tangle into structure of standing dead woody stems on those

plots where oaks were killed and application rates were

either 1.7 or 3.4 kg/ha.

On the mesic flatwoods site, Carolina jessamine did not

reach the same proportions as on the xeric sandhill

apparently due to the release of gallberry. Gallberry was








present only in small quantities at pretreatment; however,

its ability to aggressively invade a site by sprouting from

underground rhizomes (Hughes 1964) allowed it to dominate by

the second growing season at rates 3.4 kg/ha. More

moderate reduction of oak canopies at 1.7 kg/ha on the mesic

flatwoods site precluded the domination by gallberry at that

rate. At 3.4 kg/ha, gallberry cover reached its peak,

declining somewhat at 6.8 kg/ha. At this high rate,

however, gallberry response on one replicate was still high

enough to cause a treatment X block interaction, resulting

in the non-significant response of total foliar cover by

woody species to hexazinone rate.

Shifts in the relative composition of woody species

were not as apparent on the hydric hammock site. No

dominant woody taxon was completely eliminated, even at the

highest treatment rate. Furthermore, there were no apparent

releases of hexazinone-resistant species. The dominant

woody plant at all treatment rates was sea-myrtle -- a fast-

growing early-successional species that reproduces primarily

from wind-dispersed seeds. So, short-term invasion of sites

was not impaired as it probably was for the previously

dominant species on the other study sites. While being

dominant in young (1- to 3- year-old) clearcuts on hydric

hammock sites, sea-myrtle apparently yields dominance to

cabbage palm, Quercus nigra, and planted pines as early as 7

years later (Chapter 3).








Although Dichanthelium spp., broomsedge, and thin

paspalum were susceptible to the initial impacts of

hexazinone on both the xeric sandhill and mesic flatwoods

sites (first season response), second season responses

varied between sites. Dichanthelium spp. recovered on the

mesic flatwoods but remained negatively influenced by

treatment rate on the xeric sandhill. Broomsedge responded

to a release from woody competition with incremental

increases on the mesic flatwoods while exhibiting no

response to increased rates on the xeric sandhill. Thin

paspalum was apparently unable to compete with Dichanthelium

spp. and broomsedge on the mesic flatwoods site, but

experienced a substantial second season increase on the

xeric sandhill site at rates >3.4 kg/ha. Thin paspalum was

evidently better able to compete in the xeric sandhill

environment.

Cover by Crotonopsis linearis, partridge pea, and E.

compositifolium increased with increasing herbicide rate in

the second season on the xeric sandhill. These species

exhibit rapid growth, large seed crops, and relatively short

life-cycles -- all adaptations described by Grime (1979) as

being characteristic of a ruderal strategy. Although three

forbs with these ruderal strategies were prominent on the

mesic flatwoods site, only E. capillifolium responded with

an increase due to increasing hexazinone rates. Competition

from gallberry and broomsedge appeared to suppress








establishment of ruderal species on the mesic flatwoods

site.

E. caDillifolium responded in a similar manner on the

hydric hammock site. Commonly reaching heights >2 m, this

species became the most dominant plant above the ground-

layer (>0.5 m) at 6.8 kg/ha. Dominance of this shrub-like

herb at 6.8 kg/ha accounts for the suppression of broomsedge

and spike chasmanthium (Chasmanthium laxum). Occurrence of

climbing hempweed was associated with pretreatment

occurrence of grape species, occurring on microsites not

occupied by most other species (e.g., slash piles and tops

of wind-thrown snags). When the competition for this

structure was reduced by grape suppression following

hexazinone treatment, climbing hempweed, which was fairly

tolerant of hexazinone at <3.4 kg/ha, responded with

increases in cover.

Community Response Across a Gradient

All sites experienced a decrease in woody species

richness, and successive shifts towards domination by

herbaceous vegetation as a response to increasing hexazinone

rates. Effects of increasing hexazinone rates on woody

diversity measures N1 and N2 were different among sites due

to inconsistent concentrations of species-dominance. This

is in contrast to Zutter and Zedaker's (1988) results that

showed consistent linear decreases of Shannon-Weaver

[Log(N1)] and Simpson's (1/N2) indices during the second

growing season following hexazinone release of four loblolly








pine plantations in the southeastern United States.

Diversity measures from only the mesic flatwoods site of the

present study followed this same trend. At hexazinone rates

of 1.1 kg/ha, Blake et al. (1987) reported second-season

species richness increases for woody and herbaceous plants

on a site in Mississippi. The lowest rate used in the

present study was 50% greater than the single rate used in

the Mississippi study, so results are not directly

comparable.

Herbaceous diversity response to increasing hexazinone

rates shifted along the edaphic gradient. With increases in

hexazinone rate, trends during the second season were for

herbaceous diversity to: 1) decrease on the xeric sandhill

site; 2) be unchanged (No and N2) or slightly decreased (NJ)

on the mesic flatwoods; and 3) to increase on the hydric

hammock. Dominance-diversity curves also reflected

dissimilarities among sites (Figures 2-2a, 2-4a, 2-6a). On

the xeric sandhill, abundance distributions switched their

relative ordering from the left tail (abundant species) to

the right tail (rare species). This is a graphic

illustration of a shift from the log-normal distribution

toward a geometric distribution -- most often associated

with early successional communities (Bazzaz 1975, May 1981).

Abundance distributions were approximately the same on the

mesic flatwoods. For the hydric hammock species, however,

abundance distributions for 0.0 kg/ha rates diverged from

those of hexazinone treated plots on the left tail.








Hexazinone treatments (especially those at 1.7 and 6.8

kg/ha) sustained more species of intermediate abundance.

Differences in trends of total herbaceous cover across

sites were apparently functions of contrasting site

productivities as well as differences in levels of

competition from hexazinone-resistant woody species.

Herbaceous communities on the untreated plots at the xeric

sandhill site were rich in species of intermediate

abundance. Many of these species have xerophytic

adaptations that allow them to tolerate the drought

environment of the sandhills (Stalter 1984).

Characteristics shared by most of these stress-tolerators

are relatively slow growth rates and limited investment in

propagules (Grime 1979). Xerophytic herbs that were

initially suppressed by hexazinone were unlikely to recover

during this studies duration. Xerophytic species that were

not suppressed were unlikely to reach dominance because of

inherently low growth rates. Ruderal species such as

Crotonopsis linearis, partridge pea, and E. compositifolium

were thus able to capture increasing amounts of site

resources with increasing herbicide rates.

On the mesic flatwoods site, herbaceous vegetation was

unable to establish and grow within extensive areas covered

by gallberry clones. This suppressed herbaceous recovery

was apparent on plots treated with 3.4 kg/ha, and to a

lesser extent for the other rates. In absence of gallberry








competition, herbaceous recovery on the mesic flatwoods may

have approximated that on the hydric hammock.

Potential plant productivity of hydric hammock sites is

considered to be among the highest for forested wetlands in

Florida (Ewel 1990, Vince et al. 1989), indirectly

accounting for increases in total herbaceous cover with

increasing hexazinone rates. Since no competitive woody

species were released by hexazinone, the herbaceous

vegetation responded to a release from woody competition.

Each successive decrease in the woody canopy was met with a

concomitant increase in total herbaceous cover.

In addition to releases from woody competition,

hexazinone-related increases in herbaceous diversity were

likely related to initial (first season) suppression of

original herbaceous vegetation (mostly Carex spp. and

Rhynchospora spp.). Opportunities for invasion by new

herbaceous species were thus greater during the second

season following herbicide treatment. Herbaceous flora

common to disturbed hydric hammock sites are principally

hydrophytic, adapted for rapid site invasion, and have

extremely high growth rates. So, increased resource

availability as well as herbicide-related disturbance of the

herbaceous layer accounted for higher herbaceous species

richness and diversity.

These results indicate that species diversity responses

to hexazinone are not similar among early successional plant

communities in regenerating forests of the lower Coastal






52

Plain. Data presented here support the hypothesis that the

response of plant species diversity to a selective herbicide

interacts with a generalized environmental gradient.

Different diversity responses were not solely attributable

to a gradient in potential site productivity (i.e., edaphic

gradient). Presence or absence of woody species that were

hexazinone-tolerant and capable of rapid growth had major

impacts on plant diversity and species-abundance

distributions.















CHAPTER 3
A COMPARISON OF THE EFFECTS OF CHEMICAL AND MECHANICAL
SITE PREPARATION ON VASCULAR PLANT DIVERSITY AND COMPOSITION
OF HYDRIC HAMMOCK SITES


Introduction

Hydric Hammocks and Site Preparation

Hydric hammocks are a distinctive type of forested,

freshwater wetland found exclusively in temperate Florida

(Vince et al. 1989). Although this forest type has site

characteristics and common tree species that make it similar

to bottomland hardwoods (e.g., Liquidambar styraciflua,

Fraxinus spp., and Acer spp.) and cypress-tupelo (Taxodium

spp. Nvssa spp.) swamps, the basic evergreen character of

the dominant woody species (Quercus virainiana, Q.

laurifolia, Juniperus silicicola, Pinus taeda, and Sabal

palmetto) makes hydric hammocks unique. Hydric hammocks

intermingle with cypress swamps, pine flatwoods, mesic

hammocks, xeric sandhills, and salt marshes, providing

requisite habitat components to many vertebrate wildlife

species. Per unit area, hydric hammocks are considered to

be among the most critical wetlands for sustaining

vertebrate wildlife communities (Wharton et al. 1981, Vince

et al. 1989, Ewel 1990).

Simons et al. (1989) estimated that hydric hammocks

once covered approximately 202,500 ha. Forty percent of








this area has been converted to real estate development or

agricultural production, 20% has been converted to

commercial loblolly pine (Pinus taeda) and slash pine (P.

elliottii) plantations, 20% has been placed under public

ownership (presumably not to be converted to other uses).

The remaining 20% is owned by forest products companies.

Conversion to pine plantations has been by clearcut

harvesting, followed often by intensive mechanical site

preparation such as shearing and windrowing (Hudson 1983,

Simons et al. 1989, McEvoy 1990).

Conversions of natural stands to southern pine

plantations have been perceived as being destructive to

wildlife habitats, with negative consequences in proportion

to intensity of disturbance during site preparation (Schultz

and Wilhite 1974, Harris et al. 1975, White et al. 1976,

Harris et al. 1979). Objective determinations of relative

intensities of chemical versus mechanical techniques have

not been made.

Shearing residual stems and windrowing can result in

substantial transport and concentration of topsoil and

organic matter into linear piles of debris, resulting in

nutrient redistribution (Morris et al. 1983), long-term pine

productivity losses (Swindel et al. 1986), and delayed

recovery of native plant communities (Wilkins et al., in

review). Prior to the recent registration of several non-

phenoxy forest herbicides, windrowing was considered to be

the only practical method of controlling hardwood








competition for achieving satisfactory pine survival and

growth (Morris et al. 1981).

Since the early 1980's, the forest herbicide hexazinone

has been available as an alternative to mechanical site

preparation. Liquid hexazinone (Velpar-LTM) was first

tested as an aerial spray for site preparation of hydric

hammock sites in 1983 (G. Galpin, pers. comm.). Although

the preferred formulation of hexazinone has changed to a

granular product (Velpar-ULWTM), the use of both hexazinone

and windrowing continues for converting natural hydric

hammock stands to pine plantations on some private

ownerships.

Plant Diversity

Because conservation is partially based on the concept

that species-rich communities are better than species-poor

communities, ecological evaluations of species richness and

relative abundance (i.e., diversity studies) commonly place

importance upon maintaining maximal species numbers

(Magurran 1988). Reasons for this widespread conception are

not well defined. The notion that increasing species

diversity somehow increases ecosystem stability is

considered misleading (May 1981, Zaret 1982). More stable

environments, however, generally allow for the development

of more complex communities, which tend to be more diverse

(May 1981).

The National Forest Management Act (Federal Register

44(181), 219.13(6), 1979) requires that silvicultural








practices maintain the diversity of forest ecosystems.

Because the primary goal of forest vegetation management

(specifically site preparation) is to concentrate future

site productivity into a preferred tree species, there is a

direct conflict with the goal of maintaining plant species

diversity (Zedaker 1991). A tangible method for evaluating

site preparation treatments would be to determine which

treatments least impact plant diversity.

Studies of plant diversity involve careful examination

of species abundance relationships while disregarding

taxonomic status. Despite abundant literature and theory on

the subject (see Magurran 1988 for a review), definitions of

issues associated with diversity are not yet resolved.

Choosing criteria for evaluating diversity is not a

straight-forward procedure. Different diversity indices may

appear to provide conflicting answers to the question, which

community is more diverse?.

Comparisons of communities using single indices of

diversity might result in irrelevant conclusions (Hurlbert

1971). An enumeration of species numbers at one scale (Hill

1973), as well as a measurement of spatial patterning, is a

more useful measure of biotic heterogeneity (i.e.,

diversity). A thorough representation of forest stand

diversity can thus be obtained by considering overall

diversity (stand-scale) as a function of mean within-

sampling unit (alpha-scale) and among-sampling unit (beta-

scale) components (Wilson and Shmida 1984). Patil and








Taillie (1982) likened this concept to an analysis of

variance (ANOVA) decomposition of total sums of squares into

within- and among-class sources of variability. A more

complete assessment of overall stand-scale diversity may be

further made by considering complete abundance vectors as

represented by dominance-diversity curves (Whittaker 1965)

or proportional diversity profiles (Swindel et al. 1987).

Goals and Obiectives

The goal of the research described here was to evaluate

the impacts of chemical and mechanical site preparation to

determine if, and how, the resultant plant communities

differ in composition and diversity. The null hypothesis

was that the occurrence and abundance of individual plant

species, number of species, and spatial organization of

those species were approximately the same following both

treatments. The objectives were to: 1) quantify shifts in

species composition due to site preparation treatment, 2)

determine if differences in site preparation resulted in

different overall species-abundance distributions, and

alpha- and beta-scale diversities, and 3) within mechanical

treatments, examine the influence of window proximity on

species composition and diversity.

Methods

Study Area Description and History

Selected study sites were part of an industrial private

landholding in Levy County, Florida, approximately 15 km

west of the town of Otter Creek. The region is known








locally as Gulf Hammock (Vince et al. 1989). Being in

excess of 40,000 ha, Gulf Hammock is the largest single

expanse of the hydric hammock type (Simons et al. 1989).

Gulf Hammock merges into gulf salt marshes on the west,

flatwoods and mesic hammocks on the east, and wet flatwoods

that form a matrix for smaller hydric hammocks to the north.

Several bald cypress (Taxodium distichum) tupelo (Nyssa

aquatica) swamps run through the area, draining it into

Otter Creek, the Waccasassa River, and the salt-marsh

interface. Forest products of the Gulf Hammock area had

been extensively logged for southern redcedar (Juniperus

silicicola) in the late 1800's (Yearty 1959), for cabbage

palm (Sabal palmetto) from 1910 to 1945 (Burtchaell 1949),

and native loblolly pine (Pinus taeda) during most of this

century. Conversion to loblolly pine plantations had taken

place on approximately 80% of the hydric hammock in Gulf

Hammock between the early 1970s and the late 1980s (Simons

et al. 1989).

Study Sites

Four pine plantations that had been site prepared in

1984 were chosen for study. Two of the stands were aerially

treated with 3.4 kg/ha hexazinone. The other two stands

were cleared using a KG-blade and debris was piled into

windows. All four stands were hand-planted with loblolly

pine during the winter of 1984-85.








All available information indicated that these stands

were relatively similar in vegetation composition before

harvest, and that site preparation treatments were applied

without regard to site characteristics (G. Galpin, Georgia-

Pacific Corporation; pers. comm.). Judging from adjacent

stands, the overstory composition prior to harvest was

dominated by live oak (Quercus virginiana), diamond leaf oak

(Q, laurifolia), loblolly pine, and cabbage palm. Hornbeam

(Carpinus caroliniana) was a common midstory tree.

Soils are poorly drained, shallow, loamy-textured

marine sediments, typically with a black organic muck

extending to a sandy clay loam at 9 cm. Limestone bedrock

is within 30 cm of the surface, frequently extending up to

the surface. These soils are classified as a Waccasassa

(loamy, siliceous, non-acid, Thermic Lithic Haplaquepts)-

Demory (loamy, siliceous, Thermic Lithic Haplaquolls)

complex. Sites are subjected to annual flooding and remain

inundated for 1 to 3 months of the year, usually in late

summer and fall.

Field Sampling

The geometric center of each treated stand was located

and a 4 ha (200 X 200 m) sampling area established. Thirty

intersections of 10 x 10 m grid-lines were chosen at random

for sampling within each stand. A 5-m line transect was

established in a random direction at each sampling point.

Percent foliar cover for all woody plants along each

transect was recorded by species. An expandable rod with an








attached carpenter's level was used to vertically project

line interceptions up into the canopy. Occurrence of

herbaceous species was noted for each 1 m segment, as was

total herbaceous cover. The ratio of the number of 1-m

segments along which a herbaceous species occurred was used

to create an artificial abundance score (0, 0.2, 0.4, 0.6,

0.8, or 1.0) for each species on a transect. For transects

established within the mechanically treated stands, distance

was recorded from transect center to the approximate center

of the nearest window.

Measures of Plant Community Attributes

Diversity measures were calculated using a notation and

interpretation developed by Hill (1973). These indices were

calculated as:

NO = total number of species;

N1 = Exp[-Zi Piln(pi)]; [1]

N2 = 1/Zi pi2, [2]

where pi is the proportional abundance (cover) of the ith

species. N1 is the exponent of Shannon's index (Shannon and

Weaver 1949) and N2 is the inverse of Simpson's index

(Simpson 1949). These measures have been recommended

because they represent actual units of species numbers (Hill

1973, Ludwig and Reynolds 1988). The conceptual difference

in the three numbers NO, N1, and N2 is that they place

decreasing importance on species of lesser abundance such

that NO is number of species, N1 is number of "abundant"

species, and N2 is the number of "very abundant" species








(Hill 1973). Since percent cover for herbaceous species was

not estimated, calculations were restricted to the NO

measurements.

Estimates of overall species diversity (stand

diversity) were obtained using a jack-knifing technique to

create a series of normally distributed pseudovalues (Zahl

1977). The procedure required repeated calculation of a

standard estimate V (the diversity measure), deleting each

sample in turn, resulting in n=30 jack-knife estimates

(VJi). The pseudovalues (VPi) were then calculated as

VPi = (nV)-[(n-l) (VJi)]. [3]

The best estimate of the diversity measure was then the mean

of the pseudovalues (Magurran 1988). These means will

hereafter be referred to as standN, standN1, and standN2 for

the three Hill diversity measures NO, N1, and N2,

respectively.

Alpha diversity (the number of species within transect

samples) was estimated using the mean for each diversity

measure from the individual transects (Wilson and Shmida

1984). These means will hereafter be referred to as

alphagN, alphaN1, and alphaNZ.

Beta diversity (the amount of turnover in species

composition from one transect to another) was estimated

using the previously explained jack-knifing procedure [3]

for a modification of the beta measure proposed by Whittaker

(1960):

W-betaNK = (standNK/alphaNK)-1, [4]








where K denotes the specific diversity measure (i.e., K=0,

1, or 2). These means will hereafter be referred to as W-

betaNO, W-betaN, and W-betaN2.

Dissimilarity coefficients (Magurran 1988) were used as

another measure of beta diversity. These were calculated

for all pairwise comparisons of transect compositions within

each stand using modifications of Sorenson's similarity

coefficient (Bray and Curtis 1957), with the equation taking

the form:

dis-betaNK = 1 2jN/(aN + bN), [5]

Where aN = alphaNO for transect a when K = 0, aN =

LN(alpha,,) for transect a when K = 1, and N = 1/alphaN2 for

transect a when K = 2; bN was likewise for transect b; and

jN = the number of species found on both transects for

alphaNO; and jN = the sum of the lower of the two

transformations of species proportions prior to summation in

[1] and [2] (pLNp, and p,2 for alphal and alphaN2,

respectively). These will hereafter be referred to as dis-

betaNo, dis-beta1l, and dis-betaN2.

Statistical Analyses

Treatment means were compared using analysis of

variance (ANOVA) (general linear models procedure) for a

nested design, in which site was nested within treatment

(SAS Institute 1985). Two different model specifications

were used to test for significant treatment effect; a fixed-

effects model using the mean square error for the overall

model as the denominator of the F-ratio, and a random-








effects model in which the type III mean square error for

site nested within treatment was used.

It was impossible to divide the study area into

replicate portions, take pretreatment measurements, and

randomly impose manipulations (treatments). The study was,

therefore, not fully experimental (Goodall 1970). It was

necessary to consider two critical assumptions to make

statistical inference: 1) plant communities at pretreatment

were similar in composition across all four stands; and 2)

site preparation was applied to stands without regard to

site characteristics that were determinant of plant

community development.

If both assumptions were valid, then site could have

been treated as a fixed-effect. If the first assumption was

violated, then site would have been considered to be a

random-effect. If both assumptions were violated, neither

approach would be statistically defensible for testing

treatment effects. No evidence was available that would

indicate that these assumptions were violated. Furthermore,

stands assigned a specific site preparation treatment were

not chosen using site criteria considerations (G. Galpin,

Georgia-Pacific Corporation, Pers. Comm. and unpublished

files). Therefore, evidence indicated that the second

assumption was not violated. Without quantitative evidence

with which to base a decision concerning similarity of

pretreatment compositions, statistical significance under








both model specifications are presented, and discrepancies

between the two are noted.

Within the mechanically treated stands, sample

transects were arbitrarily divided into three zones

depending upon their spatial proximity to a window center-

line windoww = <5 m, adjacent = 5-10 m, and inter-windrow=

>10 m). ANOVA was used to test for responses in foliar

cover and alpha-scale diversity measures. Dis-beta

coefficients were compared within and among window

categories to determine if beta diversity was influenced by

window distance. Stand was used as a statistical block in

the model. Sample sizes were not equal, so means were

separated using the Waller-Duncan K-ratio procedure (Waller

and Duncan 1969, SAS Institute 1985).

Results

Species Response to Treatment

Under the random-effects model, pepper vine (Ampelopsis

arborea), Virginia creeper (Parthenocissus quinquefolia),

summer grape (Vitis aestivalis) and muscadine (V.

rotundifolia) had significantly higher foliar cover on

mechanically treated stands than on herbicide-treated stands

(Table 3-1). Foliar cover by cabbage palm and greenbrier

(Smilax bona-nox) was significantly greater on hexazinone

treatments. Because of variation between the stands within

a treatment category, the fixed-effects model indicated

significant treatment response for a greater number of

species, two of which were notable. Wax-myrtle (Myrica









Table 3-1. Mean foliar cover estimates for woody plant species in
loblolly pine (Pinus taeda) plantations established on hydric hammock
sites in Levy, County Florida. In 1984, 2 stands were aerially treated
with 3.4 kg a.i./ha liquid hexazinone (Velpar-LH); while another 2
stands were mechanically treated (sheared with KG-blade and windrowed).
Measurements were taken along 30 5-m transects within each stand during
the summer of 1991.


Foliar cover (%)

Hexazinone Mechanical

Species* Mean SE Mean SE P-valueb


Woody Species 181.80 207.33

Trees 94.39 100.01

Acer rubrum 0.06 0.04 1.44 0.77 0.27 **
A. saccharum floridanum 0.14 0.14 0.60 0.45 0.18
Carva aquatic 1.07 1.07 0.07 0.07 0.63
Carpinus caroliniana 6.91 2.28 14.21 2.28 0.48 ***
Carya glabra 0.07 0.07 0.42
Celtis laevigata .1.79 1.11 0.31 *
Fraxinus caroliniana 0.09 0.07 0.42
F. pennsylvanica 0.58 0.35 0.42
Juniperus silicicola 2.84 1.19 0.25 0.25 0.48 *
Liquidambar styraciflua 7.04 2.42 8.94 2.15 0.44
Magnolia virkinica 0.07 0.07 0.42
Morus rubra .0.27 0.19 0.42
Nvssa sylvatica 0.58 0.52 0.05 0.05 0.25
Ostrya virginiana 0.77 0.51 0.42
Persea borbonia 1.15 0.77 0.28 0.28 0.21
Pinus taeda 35.53 3.99 34.68 3.66 0.57
Quercus laurifolia 5.57 1.89 6.15 2.18 0.98
Q. michauxii 0.03 0.03 .. 0.42
Q. nigra 14.20 2.91 17.26 3.52 0.86
Q. virginiana 2.40 1.01 3.98 1.31 0.32
Sabal palmetto 15.78 2.65 3.57 1.37 0.08 ***
Tilia caroliniana 0.28 0.28 0.42
Ulmus alata 0.70 0.56 1.04 0.70 0.69
U. americana 2.15 1.42 0.42
U. crassifolia 0.37 0.29 1.42 0.53 0.38

Shrubs 51.44 46.04

Aster carolinianus 0.17 0.17 0.42
Baccharis halimifolia 12.30 2.12 18.32 2.04 0.23 **
Callicarpa americana 1.42 0.62 0.51 0.25 0.75
Cephalanthus occidentalis 0.37 0.28 0.42
Cornus foemina 2.59 1.11 5.66 1.23 0.15 **
Crataegus sp. 0.68 0.68 0.42
Cyrilla racemiflora 1.81 1.11 0.43 0.33 0.66
Diospyros virginiana 0.55 0.41 1.05 0.71 0.70
Euonymous americanus 0.05 0.05 0.42
Eupatorium capillifoliumc 1.89 0.68 0.25 0.10 0.52 *
Hypericum spp. 1.40 0.48 1.24 0.33 0.65
Ilex cassine 0.28 0.24 0.42
I. vomitoria 5.09 1.56 3.08 1.01 0.73
Leucothoe racemosa .0.07 0.07 0.42
Mvrica cerifera 19.88 3.11 5.66 1.35 0.61 ***









Table 3-1. Continued.


Foliar cover (Z)

Hexazinone Mechanical
Species Mean SE Mean SE P-valuea

Prunus americana 0.68 0.68 0.42
Rhus copallina 0.79 0.40 0.42 *
Rubus betulifolius 2.19 0.76 2.53 0.59 0.56
R. trivialis 0.07 0.05 0.42
Salix caroliniana 0.18 0.18 2.33 1.52 0.36
Vaccinium arboreum .0.01 0.01 0.42
Viburnum dentatum 1.29 0.75 0.31
V. obovatum 1.71 0.66 0.92 0.50 0.66

Vines 35.97 61.28

Ampelopsis arborea 2.20 0.97 13.61 2.99 <0.0001 ***
Berchemia scandens 0.78 0.40 0.28 0.20 0.55
Bignonia capreolata 0.55 0.26 0.03 0.02 0.01 *
Dioscorea floridana 0.05 0.05 0.07 0.07 0.93
Gelsemium sempervirens 2.39 0.84 0.03 0.03 0.44 ***
Lonicera sempervirens 0.20 0.14 0.10 0.07 0.19
Matelea caroliniensise 0.03 0.03 0.42
Mikania scandensc 0.81 0.27 0.41 0.14 0.80
Parthenocissus
quinouefolia 0.71 0.35 3.35 0.80 0.08 ***
Passiflora suberosac 0.53 0.53 0.42
Smilax bona-nox 13.62 1.66 2.42 0.44 0.03 ***
S. auriculata 0.93 0.41 0.49 0.30 0.57
S. walteri 0.35 0.35 0.42
S. tamnoides 0.78 0.22 0.07 0.05 0.003 **
Toxicodendron radicans 1.95 0.33 1.98 0.34 0.83
Trachelospermum difforme 0.36 0.21 0.22 0.16 0.48
Vitis aestivalis 4.08 1.62 18.26 2.63 0.03 ***
V. rotundifoliad 5.64 2.08 19.63 2.93 0.03 ***
V. vulpina .. 0.35 0.22 0.16


a Unless stated, taxonomic nomenclature follows Wunderlin (1982).
b P-values are for treatment response while site is considered a random-
effect, whereas asterisks denote a significant treatment response while
site is considered a fixed-effect (*-P<0.05, **-P<0.01, ***=P<0.001).
See text for discussion of model interpretation.

c Herbaceous perrennials that were considered to be a component of the
woody canopy as opposed to the herbaceous ground-layer.

d Identification and nomenclature follows Godfrey (1988) for this
species.








cerifera) was more prevalent in the hexazinone treatments,

while hornbeam was more common within mechanical treatments.

Frequency scores were similar for most herbaceous

species (Table 3-2). Under the random-effects model, the

only species approaching significance in which differences

were substantial was wild petunia (Ruellia caroliniensis).

This was a common forb that was approximately twice as

abundant on hexazinone treatments.

Considering the fixed-effects model, spike chasmanthium

(Chasmanthium laxum) was substantially more frequent on

transects within hexazinone treatments. High mean frequency

scores for Juncus spp. on the mechanical treatments were due

to two transects with frequency scores of 100%. Statistical

significance under the fixed-effects model was demonstrated

for six other species for which actual differences in mean

frequency scores were quite small.

Diversity Response to Treatment

Nineteen woody species occurred in mechanical

treatments that did not occur on the hexazinone treatments.

Conversely. There were only three species on the hexazinone

treatments that did not occur on the mechanical treatments.

Thus, species richness at the stand scale (standN) was

greatest following mechanical treatments (Table 3-3). No

difference in stand diversity could be detected when

relative cover was considered (standN and stand N).

Dominance-diversity curves illustrate the similarities

in species-abundance relationships for all but the rarest









Table 3-2. Mean frequency scores (see text) for herbaceous plant
species in loblolly pine (Pinus taeda) plantations established on hydric
hammock sites in Levy County, Florida. In 1984, 2 stands were aerially
treated with 3.4 kg a.i./ha liquid hexazinone (Velpar LT); while
another 2 stands were mechanically treated (sheared with KG-blade and
windrowed). Measurements were taken along 30 5-m transects within each
stand during the summer of 1991.



Frequency score
Hexazinone Mechanical
Species Mean SE Mean SE P-valueb

Grasses

Andropogon spp. 0.67 0.47 1.33 0.80 0.70
A. capillipes 0.33 0.33 0.42
A. glomeratus 29.67 3.34 28.67 4.02 0.93
A. virginicus 1.00 0.74 0.42
Chasmanthium laxum 28.67 4.47 16.67 3.40 0.36 *
Dichanthelium spp. 4.67 2.04 3.00 1.15 0.67
D. commutatum 47.33 4.09 33.33 3.74 0.37
Digitaria sp. 0.33 0.33 0.42
Erianthus Riganteus 1.33 0.65 0.67 0.47 0.55
Oplismenus setarius 1.00 0.57 0.10
Panicum anceps 4.00 1.41 8.67 2.39 0.49
P. rigidulum 1.67 1.09 0.67 0.47 0.65
Paspalum sp. 0.33 0.33 0.42
Tripsacum dactyloides 0.67 0.67 0.42

Grass-like

Carex spp. 5.00 1.47 5.33 1.71 0.95
Chromolaena odorata 2.00 1.13 0.33 0.33 0.50
Cyperus spp. 1.00 0.57 3.33 1.18 0.02 *
Juncus spp. 2.67 1.21 11.67 2.48 0.40 ***
Rhynchospora spp. 67.00 3.68 55.67 3.59 0.40 *

Forbs

Asclepias perennis 1.00 0.57 0.10
Canna flacida 0.33 0.33 0.42
Centella asiatica 1.33 0.65 5.00 2.05 0.51
Cirsium sp. 3.67 1.61 0.33 0.33 0.02 *
Conoclinum coelestinum 0.33 0.33 1.33 0.65 0.54
Desmodium sp. 1.33 0.65 .0.42
Dichondra caroliniensis 5.33 1.64 9.33 2.20 0.42
Elytraria caroliniensis 4.00 1.04 4.00 1.14 0.99
Elephantopus nudatus 0.33 0.33 0.42
Erechtites hieracifolia 1.67 0.86 3.00 1.15 0.45
Eupatorium capillifolium 0.67 0.47 0.67 0.47 0.99
E. compositifolium 0.33 0.33 0.42
E. perfoliatum 3.33 1.79 0.67 0.47 0.07
Euthamia minor 0.33 0.33 0.42
Galactia mollis 0.33 0.33 0.42
Galium spp. 0.33 0.33 0.33 0.33 0.99
Hedvotis uniflora 0.33 0.33 0.42
Hydrocotvl umbellata 12.00 2.43 16.00 2.51 0.69
Hypericum mutilum 1.67 0.72 0.42 *
Hyptis alata 5.00 1.75 5.00 2.00 0.99
Lactuca spp. 0.67 0.47 0.33 0.33 0.42









Table 3-2. Continued.


Frequency score
Hexazinone Mechanical
Species Mean SE Mean SE P-value

Lippia nodiflora 4.00 1.83 1.67 1.37 0.28
Ludwigia maritima 5.00 1.47 7.33 2.07 0.40
Melothria pendula 4.00 1.04 0.10 ***
Mitreola petiolata 3.33 1.18 7.67 2.11 0.51
Oxalis app. 0.33 0.33 1.00 0.74 0.60
Pentodon pentandrus 1.00 0.57 0.42
Phvsalis arenicola 0.33 0.33 0.42
Polvyonum hydropiperoides 2.67 1.11 6.67 2.16 0.26
Proserpinaca pectinata 0.67 0.67 0.33 0.33 0.71
Rubus sp. 0.33 0.33 0.42
Ruellia caroliniensis 40.00 4.01 20.33 3.06 0.08 ***
Sagittaria latifolia 0.67 0.47 0.42
Scutellaria integrifolia 17.00 3.11 11.67 2.48 0.38
Scleria sp. 0.33 0.33 0.42
Solidago app. 2.33 0.84 8.33 2.14 0.05 *
Teucrium canadense 2.33 1.51 0.01
Viola affinis 9.00 1.81 10.33 2.10 0.83
V. lanceolata 2.00 1.13 0.33 0.33 0.50
Xyris sp. 0.33 0.33 0.42

Ferns

Asplenium sp. 0.67 0.47 0.42
Woodwardia virginica 1.00 0.57 0.42

* Unless stated, taxonomic nomenclature follows Wunderlin (1982).
b P-values are for treatment response while site is considered a random-
effect, whereas asterisks denote a significant treatment response while
site is considered a fixed-effect (*-P<0.05, **-P<0.01, ***-P<0.001).
See text for discussion of model interpretation.

c Synonymous with Andropogon virginicus L. var. glaucus Hackel.
(Wunderlin 1982).









Table 3-3. Plant diversity in loblolly pine (Pinus taeda) plantations
established on hydric hammock sites in Levy,-County Florida. In 1984, 2
stands were aerially treated with 3.4 kg a.i./ha liquid hexazinone
(Velpar-L$); while another 2 stands were mechanically treated (sheared
with KG-blade and windrowed). Measurements were taken along 30 5-m
transects within each stand during the summer of 1991.


Diversity measure

Hexazinone Mechanical
ScaleLuma Mean SE Mean SE P-valueb

Woody vegetation

Standso 48.17 2.40 64.95 3.13 0.05 ***
Stand 16.93 1.16 18.93 0.95 0.34
Stands2 11.16 0.98 12.89 0.89 0.60

alphaso 8.85 0.44 11.25 0.35 0.008 ***
alpha*, 5.73 0.23 6.99 0.26 0.003 ***
alphaN2 4.71 0.21 5.57 0.23 0.002 **

W-betao 4.43 0.25 4.78 0.25 0.50
W-betaa, 1.96 0.15 0.71 0.11 0.44
W-beta,2 1.37 0.17 1.31 0.14 0.93

Dis-betam 55.67 0.48 54.06 0.44 0.67
Dis-betan, 60.08 0.52 56.70 0.43 0.46 ***
Dis-betaN, 74.91 0.67 72.99 0.61 0.49 ***
Herbaceous vegetation
Stand, 46.10 2.77 49.16 2.67 0.68
Alphao 7.78 0.41 8.45 0.54 0.72
W-betao 5.37 0.39 5.19 0.33 0.84
Dis-betao 55.00 0.50 63.80 0.50 0.09 ***

a Diversity "scale" denotes consideration of overall species abundance
(Stand), mean within-transect species abundance (Alpha), cumulative
numbers of among-transect species turnovers (W-beta), and mean percent
dissimilarity of species compositions between all pairwise comparisons
of transects (Dis-beta). Diversity "level" denotes the level of
consideration given to the abundance of each species such that NO
considers all species regardless of abundance, whereas N1 and N2 place
progressively more weighting upon abundance (percent cover).
b P-values are for treatment response while site is considered a random-
effect, whereas asterisks denote a significant treatment response while
site is considered a fixed-effect (*-P<0.05, **-P<0.01, ***-P<0.001).
See text for discussion of model interpretation.








species (Figure 3-1). The point of departure between the

two treatments was for those species that had estimated

foliar coverages of <0.3% of the entire treatment -- beyond

which, more woody species occurred on the mechanical

treatments. When cumulative proportions were compared in a

diversity profile, the skewed right tail further illustrated

this pattern of decreased numbers of quantitatively rare

species (Figure 3-2).

The approximate locations of N1 and N2 are indicated in

Figure 3-2. At the points of intersection for these

measures, the diversity profile was at or near the point of

equivalent proportional abundances. This condition

indicated that there were no quantitative differences in

diversity at those depths of abundance consideration. The

points on the axes that corresponded to the stand,, and

standN2 positions indicated that these diversity measures

accounted for about 80 and 90 percent of the woody cover,

respectively.

Alpha-scale diversity was significantly higher on the

mechanical treatments at all three levels of abundance

consideration (Table 3-3). Not only were there more species

along each transect, but the relative abundance for each

species was more evenly distributed.

No treatment differences could be demonstrated for

beta-scale diversity using Whittaker's (1960) measure (W-

beta). Mean dis-betaN and dis-betaN2 values were, however,

greater on hexazinone treated sites. Statistical































0.1



0.01


Species rank


- Hexazinone -- Mechanical


Figure 3-1. Rank abundance plot for woody species found in
7 year-old loblolly pine (Pinus taeda) plantations on hydric
hammock sites that were site prepared with hexazinone (3.4
kg/ha) and sheared with a KG-blade and windrowed
(mechanical), Levy County, Florida.


















0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99


0.9------- --------- 0.9
0.8-..9 I
I I093
S07- 0.92
1t I ^I 0.9
0.61 0.I I91
0.5 0.9
S- 0 N(2)
L 0.41

3 0.31
O I N )

+ I
0.1 Ii
I I
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Cumulative proportion
HEXAZINONE








Figure 3-2. Diversity profile (Swindel et al. 1987)
comparing proportional abundance of woody species found in 7
year-old loblolly pine (Pinus taeda) plantations on hydric
hammock sites that were site prepared with hexazinone (3.4
kg/ha) and sheared with a KG-blade and windrowed
(mechanical), Levy County, Florida. N(1) and N(2) denote
the intersections of Hill's diversity numbers (Hill 1973)
with the respective cumulative proportion axes.








significance could only be detected when the assumptions of

the fixed-effects model were accepted.

For herbaceous vegetation, no differences could be

demonstrated between site preparation treatments for stand-

or alpha-scale diversities (Table 3-3), nor were there any

differences in W-beta. Dis-beta, however, was significantly

higher on mechanical treatments (fixed-effects model).

Windrow Influences

Differences in foliar cover, in relation to distance

from a window center, were statistically significant for

three woody species, and for total herbaceous cover (Figure

3-3). Muscadine reached its greatest foliar cover on (<5 m)

and adjacent (5-10 m) to windows, being only about one-

third as abundant in the inter-windrow area (>10 m)

(P=0.002). Loblolly pine cover on windows was

approximately one-half of that elsewhere (P=0.04). Yaupon

(Ilex vomitoria) cover was almost nonexistent on windows,

but was substantially greater in the inter-windrow zone (P

=0.05). Herbaceous cover in the adjacent and inter-windrow

zones was approximately double that of windows (P=0.002).

Although species compositions changed in relation to

window proximity, no windrow-related differences were

detected in the three measures of alpha-scale diversity for

woody species (P=0.40, 0.44, and 0.36 for alphaN, alphal,

and alpha,2, respectively). Alpha,N for herbaceous species

was 5.1 on windows, compared to 9.0 and 9.3 for adjacent

and inter-windrow zones, respectively (P=0.004).


























3

o
0 2

0
2
1


Windrow


Adjacent
Windrow proximity


Inter-windrow


EM herbs N loblolly pine M muscadine y yaupon


Figure 3-3. Mean herbaceous cover and woody species foliar
covers that showed significant response (P<0.05) by ANOVA to
window proximity. Zones were window (<5 m), adjacent (5-
10 m), and inter-windrow (>10 m). For each species,
multiple comparisons were significantly different (P<0.05)
by Waller-Duncan K-ratio if signified by different letters
among window distance categories.








Comparisons of beta-scale diversity measures within and

across window zones indicated that species composition

adjacent to windows was generally more homogenous than

elsewhere (Figure 3-4). The greatest contributions to

overall beta-scale diversity for woody species resulted from

compositional differences among window transects, and from

the across-zone comparisons of window and inter-windrow

transects.

For herbaceous species, the beta-scale measures were

similarly influenced by window proximity. Inter-windrow

transects were the most homogenous, followed by those

adjacent to windows. The window versus inter-windrow, and

window versus adjacent comparisons made major contributions

to overall beta-scale diversity for herbs.

Discussion

Species Compositions

Loblolly pine was the dominant woody plant in all

stands inventoried, and had attained essentially the same

mean canopy cover under both treatments. Hexazinone site

prepared stands had greater cover by cabbage palm,

greenbrier, wax-myrtle and southern redcedar than did

mechanically treated stands. When compared with mechanical

treatment, hexazinone treatment resulted in greater cover by

evergreen tree species. This feature is considered typical

of naturally occurring hydric hammock vegetation (Vince et

al. 1989, Ewel 1990). Regeneration of cabbage palms is of

importance for maintaining typical hydric hammock flora






















B
75- AB
AS
C AS AS c
70-


DDB
CC




55-
A
A
50 A

45-

in. r- iai a-


Wr Wr


Wr-Ad Ad -Ad Ad- Iw tw -w
Comparisons across window zones


Wr w


g WOODY NO E WOODY N1 = WOODY N2 M HERB NO

















Figure 3-4. Mean dis-beta measures (percent compositional
dissimilarity, see text for details) for hydric hammock
plant communities as influenced by comparisons within and
across window zones. Zones were window (Wr), adjacent
(Ad), and inter-windrow (Iw); at <5, 5-10, and >10 m from
window centerline, respectively. Mean dis-beta measures
differed across window zone comparisons (P<0.05) if
signified by different letters (Waller-Duncan K-ratio used
as multiple comparison procedure).








because of its classification as a dominant tree species in

>80% of mature hydric hammock stands inventoried by Vince et

al. (1989).

On mechanically site prepared stands, deciduous vines

of the Vitaceae family predominated and hornbeam cover was

higher. While hornbeam is considered to be an abundant

understory tree in these systems, summer grape is the only

Vitaceae species considered to be an "abundant" hydric

hammock species; and then only within the canopies of mature

trees (Vince et al. 1989). Woody plant communities in

mechanically treated stands were essentially deciduous with

the exception of planted loblolly pines.

The abundance of three woody species (loblolly pine,

muscadine, and yaupon) within the mechanical treatments

varied in relation to distance to the nearest window.

Loblolly pine was not as abundant directly on windows as it

was elsewhere, because trees were not planted on windows.

The concentration of muscadine on, and adjacent to, windows

very likely resulted from the combination of available

propagules and suitable climbing structure from tree tops

that were pushed into windows. Most grapevines were rooted

within the nutrient rich windows (Morris et al. 1983), and

extended >10 m to either side of the window (Figure 3-3).

The prevalence of muscadine on and adjacent to windows also

was noted by Swindel et al. (1986) on a flatwoods Spodosol

site. Yaupon might have been overtopped and out-competed in








the areas on and adjacent to windows, resulting in its

preference for areas >10 m from a window center.

The fact that occurrences of herbaceous species were so

similar suggests that treatments had analogous long-term

influences on herbaceous communities. There was, however,

a reduction in herbaceous cover associated with windows.

This probably resulted because of increased shading

associated with vine cover on the window structure.

All 19 species missing from hexazinone treated stands,

but were recorded on mechanically treated stands, have been

listed by either Vince et al. (1989), Godfrey (1988), or

Wunderlin (1982) as being part of the characteristic flora

of hydric hammocks. If the conservation of a full spectrum

of hydric hammock plants is of concern, then this requires

further management attention.

Diversity

Visual inspection and comparisons of dominance-

diversity curves (Figure 3-1) can provide insight into

community structure not attainable from single-number

diversity indices (Hughes 1986). The woody species

abundance distribution for both treatments were sigmoidal

and approximated a typical log-normal distribution

(Whittaker 1977). Some observations have indicated that

stressed, polluted, or otherwise disturbed ecosystems result

in a shift from the log-normal distribution to a geometric

distribution typified by increased dominance and decreased

richness (May 1981). The dominance-diversity curves of








these communities exhibited nearly identical dominance when

comparing treatments, but the hexazinone treated stands had

lower combined species richness. Even though the

distributions for dominant species were similar, there were

marked differences in species that made up the distribution.

Alpha-scale diversity numbers enumerated mean numbers

of species within assemblages, while beta-scale diversity

measures enumerated the number of complete assemblage

changes (W-beta) or mean compositional dissimilarity among

assemblages (dis-beta). The boundary of an assemblage (5-m

line) was, albeit, arbitrary. The results, however, did

reveal important patterns that probably resulted from

differences in site preparation treatment.

Alpha-scale diversity for woody species was highest in

the mechanically site prepared stands, regardless of the

importance relegated to abundant species. Although W-beta

measures did not demonstrate any differences in the

community turnover rates, dis-beta measures were higher for

N1 and N2 levels of diversity. This suggests that the

comparatively lower alpha-scale diversity was compensated

for by the uniqueness of individual species assemblages,

only when increased importance was placed on abundant

species. The lack of any difference in dis-betaN indicates

that the increased stand, in mechanical treatments was

principally a function of an increase in alpha-scale

diversity (or in this case, species richness).








Theoretically, alpha-scale diversity is a function of

the supply rate of controlling resources (i.e., nutrients,

light, water) at specific microsites transectss), while

beta-scale diversity is a function of the spatial

heterogeneity of resources (Tilman 1982, Shmida and Wilson

1985, Palmer 1991).

Both chemical and intensive mechanical site preparation

treatments can suppress or completely eliminate dominant

species from the community. So, treatment induced impacts

on alpha- and beta-scale diversity may not be strictly due

to manipulations of supply rates and spatial heterogeneity

of controlling environmental factors. Selective elimination

of competition or stimulation of some species may have

played an important role. For example, hexazinone probably

suppressed vines from the Vitaceae family (Chapter 2),

reducing competition for young cabbage palm seedlings and

wax-myrtle sprouts. Also, mechanical treatments probably

uprooted and eliminated understory cabbage palm, in much the

same way that the closely related saw-palmetto (Serenoa

reopens) is suppressed by mechanical treatment (Tanner et al.

1988). Similar impacts were likely encountered by species

of lesser abundance.

Although beta-scale diversity in the hexazinone

treatments could have been influenced by spatial variability

in herbicide rate, this was not quantified. In the

mechanically treated stands, however, dis-beta at all levels

of consideration for both woody and herbaceous vegetation








was influenced by distance from the windows. Thus, a

portion of the spatial heterogeneity that was reflected in

dis-beta was likely due to the redistribution of topsoil,

nutrients, organic matter, and plant propagules that occurs

with shearing and windrowing operations (Morris et al.

1981).

The pairwise transect comparisons that resulted in the

highest dis-beta measures resulted from those comparisons

involving window transects, indicating that the species

assemblages on windows were the most unique. Thus, for

woody vegetation, not only were mechanically prepared stands

apparently more homogenous at the N1 and N2 levels, but a

substantial portion of the heterogeneity that did exist at

all levels of consideration could be attributed to the

construction of windows. Similarly for herbaceous

vegetation, greater dis-beta values for mechanically

prepared stands is largely explained by the window effect.

Nutrient enrichment of microsites can result in

increased dominance and decreased species richness (Tilman

1982:108-114). Swindel et al. (1986) documented that

nitrogen, phosphorus, potassium, calcium, and magnesium

levels on windows were >10 times those levels on soils not

within windows. This likely contributed to a decrease in

alphaNO for herbaceous vegetation <5 m from window centers.

Intense growing season shade provided by the tangle of vines

associated with windows probably contributed further to








herbaceous diversity reductions by suppressing shade

intolerant species.

Summary and Recommendations

When compared with shearing and windrowing, use of

hexazinone for site preparation of hydric hammock sites in

the plantation conversion process results in a loss of some

woody species. The dominance distribution was not

influenced by treatment, although the plants that dominated

the composition (except for planted pines) were different in

species, form, and deciduous versus evergreen composition.

Hexazinone treatment resulted in a shift toward evergreen

trees, shrubs and vines, while deciduous vines dominated the

mechanically site prepared stands.

Hexazinone treatments, although resulting in decreased

species diversity at particular microsites, resulted in more

unique woody species assemblages (i.e., greater spatial

heterogeneity) than did mechanical treatments, especially

when the effects of windows were taken into account.

Numbers of species that were common were not influenced by

treatment.

The impacts of these two treatments on herbaceous

composition was not different, except that microsites within

5 m of a window center typically had fewer species numbers

and reduced foliar cover. There were no treatment

differences in overall species numbers (standN), mean

species numbers at microsites (alphaN), or species turnover

rate (W-betaNo). The dissimilarities of herbaceous species








assemblages among microsites (dis-betaN), however, was

higher on mechanically treated sites. Again, this was

largely a function of the uniqueness of microsites on or

near windows.

These data indicate that hexazinone application is the

more desirable site preparation technique when considering

the needs of a wide variety of vertebrate wildlife species.

One of the most important considerations is maintenance of

dominant evergreen species. These species are most

important for provision of winter food and cover for those

species that may use adjacent deciduous ecosystems during

the remainder of the year (Simons et al. 1989). Evergreen

hammock communities also supply an important winter foraging

base for a large and diverse community of over-wintering,

frugivorous birds (Skeate 1987). Hexazinone treated stands

were also more spatially heterogenous. This attribute has

been related to density and diversity of forest bird

communities in a variety of temperate ecosystems (Roth 1976,

Freemark and Merriam 1986).

Large live oaks, cabbage palms and other unmerchantable

trees remained standing on hexazinone treated stands,

providing potential snag habitat for cavity using birds

(Dickson and Conner 1985) for several years. In addition,

logging debris remains scattered across a hexazinone treated

site (as opposed to being piled into windowss, thus

favoring reptile and amphibian communities (Enge and Marion

1986).








Applications of hexazinone can probably be altered to

provide for the conservation of quantitatively rare woody

plant species. Important areas should be excluded from

application. For example, observations indicate that small

(<0.2 ha) circular depressions at Gulf Hammock contain high

densities of pop ash (Fraxinus caroliniana) and green ash

(F. pennsvlvanica). These were species that were absent

from herbicide-treated stands and rare on mechanically-

treated stands. Observations were that most other scarce

woody species also occurred in small aggregations as opposed

to scattered individuals.

It is recommended that hydric hammock sites destined

for conversion to pine plantation be inventoried prior to

harvest and again before site preparation. All micro-sites

that contain concentrations of relatively scarce woody

species should be noted, and avoided during subsequent

treatment. Attention should be given especially to those

species that did not occur on hexazinone treated stands in

the present study (Table 3-1). Furthermore, "skips" that

remain when helicopter application strips do not overlap

should be retained. In the past, these have been retreated.

These narrow (<15 m) untreated strips would allow for

reduced probabilities of extirpation of locally scarce woody

species. Finally, as other herbicides and mechanical

treatments are considered for use on hydric hammock sites,

their relative impacts on plant composition and diversity

should be compared to those of the present study.















CHAPTER 4
USE OF HEXAZINONE FOR UNDERSTORY RESTORATION
OF A SUCCESSIONALLY ADVANCED XERIC SANDHILL


Introduction

The undulating sand ridges of central peninsular

Florida were once dominated by a near continuous assemblage

of open longleaf pine (Pinus palustris) overstories, with a

ground cover dominated by pineland threeawn, otherwise known

as wiregrass (Aristida stricta) (Nash 1895). On many sites

there also was a thin midstory of turkey oaks (Quercus

laevis) and/or bluejack oaks (Q. incana) (Myers and White

1987). The characteristic understories of these pinelands

were maintained by recurrent summer ground fires at a

frequency of 3 to 4 years (Chapman 1932).

Longleaf pines exhibit a fire resistant grass-stage and

have thick, fire-insulating bark when mature (Wahlenberg

1946), as does the bark of large diameter turkey oaks.

Wiregrass quickly resprouts and sets an inflorescence

following a growing-season burn (Myers 1990). Furthermore,

both pine needles and wiregrass provide a constant fuel

source for frequent ground fires, thereby encouraging

continued fire (Clewell 1989). Although evergreen oaks and

other more mesic hardwoods will grow on these sites, they








are generally excluded when natural fire frequencies are

allowed.

Following exclusion from fire, xeric sandhills tend to

succeed to either a mesic or a xeric hardwood forest (Monk

1960, Veno 1976, Daubenmire 1990), depending upon the

character of the seed supply (Myers 1985). These stands are

characterized by increased midstory shading, heavy litter

accumulation, and a thinning of the herbaceous ground cover

(Veno 1976, Givens et al. 1981). When high-intensity fires

occur in these successionally-advanced stands, site

occupancy may change to a "scrub", dominated by sand pine

(Pinus clausa) in the overstory and a thick midstory of

xeric evergreen oaks (Ouercus myrtifolia, Q. geminata, and

Q_ chapmannii) (Myers 1985). Shrubs common to the sand pine

scrub communities have been shown to further inhibit growth

and reproduction of the former sandhill vegetation through

allelopathic leachates (Richardson and Williamson 1988).

Numerous vertebrate species are dependant upon the

perpetuation of longleaf pine communities characteristic of

the more frequently burned xeric sandhills. These include

the federally endangered red-cockaded woodpecker (Picoides

borealis), the state-listed (species of special concern)

Florida gopher tortoise (Gopherus polyphemus) and Sherman's

fox squirrel (Sciurus niger shermani), and the state-listed

(endangered) Florida mouse (Podomys floridanus) (Myers

1990). The entire longleaf pine ecosystem is considered to

be endangered due to fire exclusion and clearing for pine








plantations, agriculture and human development (Means and

Grow 1985).

Management efforts have recently concentrated on the

reintroduction of summer fires to xeric sandhills systems in

an effort to return the vegetation to a fire maintained

savanna. Prescribed fire has most commonly been used as the

restoration tool -- but only when fine ground fuels in the

form of wiregrass have remained abundant (Clewell 1989,

Myers 1990). Sites that have been excluded from fire for 40

to 50 years develop an oak midstory that results in such a

sparse cover of wiregrass that ground fires cannot be

readily ignited. Other management alternatives must

therefore be developed as part of the restoration process.

One method of facilitating an initial reclamation burn

is the use of a selective herbicide to suppress midstory

oaks while releasing wiregrass growth in the understory.

Hexazinone is one such herbicide that has been utilized,

with some success, for this purpose (Duever 1989).

Previous studies have shown that southern yellow pines

resist the effects of hexazinone while most oak species have

been noted to be quite susceptible (Gonzales 1983, Griswold

et al. 1984, Zutter et al., 1988). Differential

susceptibilities to hexazinone also have been noted for

several other hardwood species (Zutter and Zedaker 1988).

Operational trials have indicated that wiregrass also is a

hexazinone-resistant species (Duever 1989, Outcalt in

review, R. Mulholland, Pers. Comm.). These responses








suggest that hexazinone may play an important role in xeric

sandhill restoration efforts.

Several questions still remain regarding the use of

hexazinone for sandhill plant community restoration.

Adequate efficacy results for the selective control of small

diameter oaks are not available. Also, there is concern for

the survival of other understory plants of lesser abundance,

that are nevertheless important members of the sandhill

plant community. The overall goal of this research was to

quantify the initial hexazinone rate-response of trees and

understory vegetation in a successionally-advanced (fire

excluded) sandhill community. The specific objectives were

to: 1) determine oak stem mortalities and shrub canopy

reductions as a response to relatively low (1.68 kg/ha)

application rates of liquid hexazinone (VELPAR-LTm); and 2)

document the subsequent release and/or inhibition of plant

species in the understory, especially wiregrass.

Methods

Study Area

The experiment was installed on a sandhill site along

the western edge of Wekiwa Springs State Park in Orange

County, Florida, approximately 3 km east of the town of

Apopka (280 40'N, 810 30'W). Soils on the site are well-

drained, deep, acidic sands of the Lakeland-Eustis-Norfolk

association (hyperthermic, coated Typic Quartzipsamments).

Aerial photographs from the 1950's show the area as a

relatively open longleaf pine savanna, typical of the








sandhill vegetation once common to the sandy ridges of

central Florida. Fire control and exclusion was a park

policy until the 1980's. Park records indicate that this

site had not been burned for approximately 40 years prior to

the initiation of this study.

Although an overstory of scattered longleaf pines still

existed on the site, the ground cover was sparse and clumps

of wiregrass were isolated and suppressed by litter

accumulation. A dense layer of midstory vegetation had

developed, and was dominated by a mixture of sand-live oak

(Q. qeminata), turkey oak, laurel oak (Q. hemisphaerica),

myrtle oak (Q. myrtifolia), and sand post oak (0.

margaretta).

Treatments

Efficacy of hexazinone in forest soils across the

southern U.S. is negatively correlated with soil pH and

percent organic matter, and positively correlated with

percent sand (Minogue et al. 1988). Soils of the sandhills

typically contain very little organic matter (<1%), are

relatively low in pH, and contain >90% sand; low rates of

hexazinone can effectively control target species. It was

previously observed that the effects of application rates

>1.68 kg a.i./ha on a xeric sandhill invaded by evergreen

oaks (observed oak kill nearly 100%) was greater than that

desired. Because part of the management goal for

restoration was to retain larger oaks (14 cm dbh), the

maximum rate for testing was set at 1.68 kg a.i./ha. On 28








April 1990 a liquid formulation of hexazinone (Velpar-LTM)

was applied at three rates (0.42, 0.84, and 1.68 kg a.i./ha)

to 0.04-ha plots (20 X 20 m). Hexazinone was applied with a

single back-pack mounted SoloTM spotgun in a 1 X 1 m square

grid pattern. Appropriate concentrations of the compound

were used for each treatment rate, so that the volume of

liquid applied at each grid location was the same.

Treatments were replicated three times in a randomized

complete block design, resulting in a total of 12 plots,

including 1 control plot (no herbicide) per block. A 5-m

untreated buffer was maintained between each designated

treatment plot. The experiment was blocked according to

plant species composition and midstory woody plant

densities.

Plant Measurements

All measurements were conducted prior to treatment and

then again at one year post-treatment. Living oak trees (>2

cm diameter at 1.5 m) within a 0.02-ha subplot (14 X 14 m)

in each treatment plot were marked with paint and measured

for diameter. Basal area of each wiregrass clump within a

25 m2 subplot was determined by measuring right-angle

diameters at ground line using a caliper.

To correlate basal area with dry weight, a sample of

120 (40 per block) wiregrass clumps were harvested from the

5 m plot buffers and the 3 m boundary zones. Dead leaf

material was removed and each harvested plant was oven-dried

at 680C for 72 hrs and then weighed to the nearest 0.05 g.