Species Richness and Exotic Species Invasion in Middle Tennessee Cedar Glades in Relation to Abiotic and Biotic Factors 1

Species richness and exotic species invasion in middle Tennesseecedar glades in relation to abiotic and biotic factors1

M. Shea Cofer2, Jeffrey L. Walck3, and Siti N. HidayatiDepartment of Biology, Middle Tennessee State University, Murfreesboro, TN 37132

COFER, M. S., J. L. WALCK, AND S. N. HIDAYATI (Department of Biology, Middle Tennessee StateUniversity, Murfreesboro, TN 37132). Species richness and exotic species invasion in middle Tennessee cedarglades in relation to abiotic and biotic factors. J. Torrey Bot. Soc. 135: 540–553. 2008.—Abiotic factors,particularly area, and biotic factors play important roles in determining species richness of continentalislands such as cedar glades. We examined the relationship between environmental parameters and speciesrichness on glades and the influence of native species richness on exotic invasion. Field surveys of vascularplants on 40 cedar glades in Rutherford County, Tennessee were conducted during the 2001–2003 growingseasons. Glades were geo-referenced to obtain area, perimeter, distance from autotour road, and degree ofisolation. Amount of disturbance also was recorded. Two-hundred thirty two taxa were found withAndropogon virginicus, Croton monanthogynus, Juniperus virginiana, Panicum flexile, and Ulmus alata presenton all glades. The exotics Ligustrum sinense, Leucanthemum vulgare, and Taraxacum officinale occurred onthe majority of glades. Lobelia appendiculata var. gattingeri, Leavenworthia stylosa, and Pediomelumsubacaule were the most frequent endemics. Richness of native, exotic and endemic species increased withincreasing area and perimeter and decreased with increasing isolation (P # 0.03); richness was unrelated todistance to road (P $ 0.20). Perimeter explained a greater amount of variation than area for native andexotic species, whereas area accounted for greater variation for endemic species. Slope of the relationshipbetween area and total richness (0.17) was within the range reported for continental islands. Disturbed gladescontained a higher number of exotic and native species than nondisturbed ones, but they were larger (P #

0.03). Invasion of exotic species was unrelated to native species richness when glade size was statisticallycontrolled (P 5 0.88). Absence of a relationship is probably due to a lack of substantial competitiveinteractions. Most endemics occurred over a broad range of glade sizes emphasizing the point that glades ofall sizes are worthy of protection.

Key words: competition, disturbance, diversity-resistance hypothesis, endemic species, stress.

Area is one of the most influential aspects in

species richness of continental and oceanic

islands (MacArthur and Wilson 1967). Other

factors, such as habitat heterogeneity, eleva-

tion, isolation, or disturbance, are important

as well but are often closely correlated with

area (Fox and Fox 2000). Some studies have

partitioned species into categories to examine

the role abiotic factors play in determining

richness of endemic vs. nonendemic, specialist

vs. generalist, or exotic vs. native species

(Krauss et al. 2004). For example, Moody

(2000) found that native, exotic, and endemic

species richness increased with island size.

However, the variance that remained after

consideration of island size was explained

largely by island isolation for native species

but weakly so for endemic and exotic ones.

On the other hand, species richness of

habitats also can be influenced by the invasion

of species, especially exotic ones. The number

of exotic species establishing in a community

often is dependent on native species richness

(Herben et al. 2004). Ecological models predict

that stressful (nonproductive) environments

harboring high endemism, such as cedar

glades, might easily be invaded by exotics

(Huston 1994, Bruno et al. 2003), and

empirical evidence on non-rock outcrop com-

munities supports this contention (Borges et

al. 2006, Seabloom et al. 2006). Community

invasibility is commonly assessed by examin-

ing the relationship of native vs. exotic species

richness (Herben et al. 2004).

Cedar glades are natural openings in the

eastern deciduous forest biome, where calcar-

1 Although the research described in this articlehas been funded in part by the United StatesEnvironmental Protection Agency through grant/cooperative agreement 832331 to JLW, it has notbeen subjected to the Agency’s required peer andpolicy review and therefore does not necessarilyreflect the views of the Agency and no officialendorsement should be inferred. Additional fundingwas provided by the Faculty Research and CreativeActivity Grant Program at Middle Tennessee StateUniversity.

3 Author for correspondence. Email: [email protected]

2 We appreciate T. Sutarso for statistical advice.The kind assistance of T. Hogan and K. Hurt(Stones River National Battlefield) is gratefullyacknowledged.

Received for publication June 7, 2007, and inrevised form April 1, 2008.

Journal of the Torrey Botanical Society 135(4), 2008, pp. 540–553

540

eous bedrock is at or near the soil surface. The

vegetation of glades consists of the following

assemblages: 1) C4 summer annual grasses, 2)

C3 winter annual, summer annual, and/or

perennial herbaceous dicots, 3) cryptogams, or

4) some combination of these groups. Twenty-

one plant taxa are endemic or nearly so to

them (Quarterman 1989, Baskin and Baskin

1999, 2003). The glade environment is charac-

terized by full sunlight, high soil moisture in

winter, summer drought, soil-depth controlled

vegetation zonation, and substrate-controlled

soil chemistry (Ware 2002). Due to shallow

soil, woody plant establishment is limited

except for deep soil pockets in crevices of the

bedrock. As such, cedar glades are continental

islands containing long-persisting (edaphic

climax) plant communities.

With regards to cedar glades, no study has

examined species richness in relation to area or

other abiotic factors. The few studies conduct-

ed on other rock outcrop communities have

shown that 1) area is an important factor in

species richness, and 2) the slope of the

species-area curve may differ between total

vs. specialist species (Wyatt and Fowler 1977,

Redei et al. 2003, also see Harrison et al.

2006). Moreover, no study heretofore has

examined the interplay between exotic inva-

sion and native species richness on a rock

outcrop community. Cedar glades are defined

by edaphic characteristics, and sources of

variability that may influence species richness

are relatively limited: size (area and perimeter),

isolation, and disturbance (both on-site and

distance from a road). On the other hand,

properties about the edge vs. center of cedar

glades also may be important with respect to

what kind of plants can invade them and to

the persistence of characteristics that maintain

them as cedar glades.

To this end, we compiled lists of species

from a set of cedar glades in the Central Basin

of Tennessee. This region is ideal for a study

on species richness since cedar glades are very

numerous and it is one of the six hotspots of

endemism in southeastern United States (Ba-

skin and Baskin 1999, Estill and Cruzan 2001).

Species were partitioned for analyses based on

nativity, endemism, growth habit and herba-

ceous life cycle. Environmental parameters

(size, degree of isolation, amount of distur-

bance, distance from a road) also were

recorded. From this data set, four aspects of

species richness on cedar glades were ad-

dressed. First, we examined the frequency of

species found among the glades, paying

specific attention to exotic ones. Second, we

analyzed the relationship between species

richness and environmental parameters. Third,

the influence of native species richness on

exotic invasion on glades was assessed. And

fourth, we determined the dependence of

endemic species for certain sizes of glades.

Materials and Methods. STUDY SITE. The

main area of the Stones River National

Battlefield, hereafter referred to as the Battle-

field, in Rutherford County, Tennessee con-

sists of oak-hickory and red cedar forests with

many cedar glades. The Battlefield was chosen

for the present study due to the consistency of

bedrock and soil types and to the amount of

size variation among glades. Forty glades over

approximately 25 ha were included in the

present study (Fig. 1). All of the glades were

located on Ordovician-aged Ridley Limestone

(Wilson 1964) and primarily on the Gladeville-

Rock outcrop-Talbott soil association (True et

al. 1977). The glades form a circular pattern

around a slightly elevated hill (Fig. 1). They

had ca. 127 m of wooded buffer to the nearest

fields, 413 m to major roads and 153 m to

urbanized/industrialized areas.

This section of the park has a history of

disturbance. The Battle of Stones River was

fought during winter 1862–1863, and mention

of cedar glades is made in personal letters by

solders and in official correspondence (Hogan et

al. 1995). Before and after the Battle, until the

establishment of the park (late 1920s–early

1930s), the land was used for farming, as wood

lots, grazing lands, or home sites. Aerial and

ground photographs during the 1920s and 1930s

show large cleared tracts; fence rows, unpaved

roads and glades are apparent. Since the 1930s,

much of the land surrounding the glades has

become densely forested but signs of distur-

bances still remain, e.g., a network of trails.

PLANT SURVEYS. All vascular plants on each

glade were tallied monthly during the 2001–

2003 growing seasons (February to Novem-

ber). Surveys were done by walking along the

edge of the glade (i.e., at the boundary of the

forest and glade opening) and then crisscross-

ing the glade at 0.5–1 m intervals. Nomencla-

ture followed USDA, NRCS (2004). Taxa were

classified based on nativity (native vs. exotic;

sensu USDA, NRCS 2004), endemism (endem-

2008] COFER ET AL.: SPECIES RICHNESS ON CEDAR GLADES 541

ic vs. nonendemic; Baskin and Baskin 1999),

and growth habit (forb including ferns and

herbaceous vines, graminoid, shrub, tree, and

woody vine; USDA, NRCS 2004). Forbs and

graminoids were further divided into annual or

perennial (including biennial) following Glea-

son and Cronquist (1991), Baskin et al. (1995),

and USDA, NRCS (2004). From the species

compilation for each glade, percentage fre-

quency was calculated. Voucher specimens of

plants new to the Battlefield or cedar glade

flora were deposited in the Battlefield’s her-

barium. Species authorities for taxa recorded at

the Battlefield are listed in the Appendix.

During 2001–2003, mean daily maximum and

minimum temperatures were 20.5 and 8.1uC,

respectively, and monthly precipitation totaled

121.9 mm at a weather station in Murfreesboro,

Tennessee, ca. 8 km from the study site. The 30-

yr normal temperatures are 20.5 and 9.3uC,

respectively, in Nashville, Tennessee, approxi-

mately 35 km from the Battlefield, and precip-

itation 100.1 mm (National Climatic Data

Center 2002). Mean annual total snowfall

FIG. 1. Aerial photograph of the cedar glades (numbered in white) at Stones River National Battlefield,Rutherford County, Tennessee used in the present study. The autotour road is seen in the middle of theimage. Photograph was taken in 1997.

542 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 135

during the study period (2.5 cm) was much

lower than the 57-year normal (25.4 cm).

ENVIRONMENTAL PARAMETERS. Glades were

delineated during the 2005 growing season by

walking around their edge (i.e., along the tree

line bordering the glade) as well as around tree

islands in the glade using a TrimbleH GPS

PathfinderH Pro XR unit with a TSCe data

collector, ensuring an accuracy of , 1 m. Geo-

referenced data were imported into ArcMap

(ESRI 2004) to obtain area and perimeter (as a

measure of the amount of edge) for each glade,

distance of each glade from the autotour road,

and distance from edge of each glade to the

nearest edge of an adjacent glade (as a

measure of isolation). Area of tree islands

was removed for calculation of total glade

area. The glades were spatially corrected when

necessary and overlayed onto a 0.5 m resolu-

tion digital orthophoto quarter-quadrangle

map of the Battlefield. In addition, ground

truthing was conducted by tracing a 1.5 3

1.5 m plot in the field with the GPS unit and

determining its area with the software.

Glades were classified as disturbed if they

contained a hiking trail, logs to prevent

trespassing from the autotour road or from

trails, outplanted Astragalus bibullatus for a

restoration project, ‘‘graffiti’’ (i.e., flagstone

moved to make words), gravel or dirt piles,

and/or an eradication project for Ligustrum

sinense. Nondisturbed glades did not have

these features.

STATISTICAL ANALYSES. Means were com-

pared by t-tests or one-way analyses of

variances (ANOVAs) followed by protected

least significant difference tests (PLSDs, P 5

0.05). Pearson correlations were calculated

among the four parameters (area, perimeter,

distance from road, isolation), and simple

regressions were done between them and

species richness based on total inventory,

nativity, endemism, growth habit, and herba-

ceous life cycle (SAS 2003). Regression lines

were compared by analyses of covariances

(ANCOVAs). We chose R2 selection proce-

dure for multiple regressions, but glade area

and perimeter were combined by averaging

them into one variable termed island due to

multicollinearity. Standardized residuals from

the regression of native species richness vs.

perimeter were obtained and then regressed

against exotic species richness. The use of

residuals controlled for the most important

explanatory variable, so that comparing glades

of different sizes could be nearly equivalent

(Lonsdale 1999). Independent and dependent

variables were log10 transformed for all

analyses so as to calculate scale-independent

slopes (z-values) for comparison with other

studies.

Results. PLANT SURVEYS. Two-hundred thir-

ty two taxa occurred on the 40 glades (mean 6

SE: 75 6 4) with Andropogon virginicus,

Croton monanthogynus, Juniperus virginiana,

Panicum flexile, and Ulmus alata being present

on all of them (Appendix). The number of

native species (67 6 3) was greater than that of

exotics (8 6 1) (t-test, P , 0.0001); Ligustrum

sinense, Leucanthemum vulgare, and Taraxa-

cum officinale occurred on the majority of

glades. Nonendemic species (72 6 3) outnum-

bered endemic ones (3 6 0) (P , 0.0001), and

Lobelia appendiculata var. gattingeri, Leaven-

worthia stylosa, and Pediomelum subacaule

were the most frequent endemics. Number of

species varied among growth habits: forbs (45

6 2) . graminoids (13 6 1) . trees (9 6 0) 5

shrubs (7 6 0) . vines (2 6 0) (PLSD, P 5

0.05). Among herbaceous plants, perennials

(34 6 2) were more abundant than annuals (24

6 1) (t-test, P , 0.0001).

ENVIRONMENTAL PARAMETERS. Area (range:

17.13–2837.10 m2) and perimeter (17.22–

1205.24 m) of glades were positively correlated

(r 5 0.977, P , 0.0001), whereas they were

negatively correlated with isolation (5.66–

82.79 m) (r 5 20.451 and 20.472, respective-

ly; P # 0.0035). Distance to road (0–187 m)

was not correlated with area, perimeter, or

isolation (P $ 0.0947).

Richness of total species, native and exotic

species, and nonendemic and endemic species

increased significantly with increasing glade

area and perimeter and decreased significantly

with increasing isolation (Table 1). Except for

vines with glade area and isolation and trees

with isolation, all types of growth habits and

herbaceous life cycles also increased with area

and perimeter and decreased with isolation.

Glade perimeter explained a slightly higher

amount of variation among total and nonen-

demic species and types of nativity, growth

habits, and herbaceous life cycles than area. In

contrast, glade area accounted for slightly

greater variation than perimeter for endemic


species. Species richness was not related to

distance to road for total species or types of

nativity, endemism, growth habit, and herba-

ceous life cycles (P $ 0.2023). Comparisons of

regression lines between levels of nativity,

endemism, and herbaceous life cycle or among

growth habits were not significantly different

for glade area, perimeter, or isolation (AN-

COVA, P $ 0.4809). In multiple regressions,

island explained the most variation in the one-

variable model for all variables (Table 2).

Including other parameters in a 2- or 3-

variable model (isolation and/or distance to

road) only explained an additional 0–6%

variation. Exotic species richness was unrelat-

ed to native species richness when size

(perimeter) of glades was statistically con-

trolled (P 5 0.8818) (Fig. 2).

Disturbed glades were larger in area (692 6

212 m2) and perimeter (240 6 74 m) than non-

disturbed ones (137 6 26 m2, 66 6 7 m) (t-test,

P # 0.0330), but they did not differ in distance

from road or isolation (P $ 0.0550). Richness

of exotic (11 6 2 vs. 6 6 1) and native (78 6 6

vs. 60 6 3) species was higher on disturbed

glades than on nondisturbed ones (P # 0.0150),

but that of endemic species was equal (3 6 0).

The endemic species Leavenworthia stylosa,

Lobelia appendiculata var. gattingeri, and

Pediomelum subacaule occurred over the full

range of glade areas (17.13–2837.10 m2) and

Talinum calcaricum (22.11–2837.10 m2) and

Phacelia dubia var. interior (210.29–2837.10

m2) over a slightly narrower range. In contrast,

Astragalus tennesseensis was not found on

glades . 686.45 m2.

Discussion. The total number of taxa (232)

on the 40 glades in the present study was less

than that recorded for 22 glades in seven

counties of northern Alabama (269 taxa in

Baskin et al. 1995) but more than that for 10

glades in four counties (132 taxa in Somers et

al. 1986) and 10 glades in five counties of

middle Tennessee (152 taxa in Rollins 1997).

Thirty four taxa were recorded as new to the

cedar glade flora of southeastern United

States, but 44% of them were exotic. Baskin

and Baskin (1999) listed 36 taxa as character-

istic species of southeastern United States

cedar glades based on presence and abun-

dance. Of these, Eleocharis compressa, Oxalis

priceae subsp. priceae, Solidago nemoralis, and

Trichostema brachiatum were not recorded in

the present study. On the other hand, Andro-

pogon virginicus, Carex blanda, Euphorbia

dentata, Galium virgatum, Panicum malaco-

phyllum, Ligustrum sinense, Quercus imbri-

caria, and Panicum laxiflorum had high

($ 90%) frequency in our study and were

not on Baskin and Baskin’s (1999) list.

The percentage of the total glade flora at the

Battlefield that consisted of exotic plants

(18.8) is higher than that for glades in northern

Alabama (13.4 in Baskin et al. 1995) but

comparable to the glade flora of southeastern

Table 1. Simple regressions for dependent variables with three independent parameters for cedar glades:log10Y 5 b0 + b1(log10X). *0.05 $ P . 0.01; **0.01 $ P . 0.001; ***P # 0.001.

Dependent variables

Area Perimeter Isolation

R2 b0 b1 R2 b0 b1 R2 b0 b1

Total species 0.629 1.50 0.17*** 0.683 1.39 0.24*** 0.246 2.11 20.21**Nativity

Exotic 0.169 0.48 0.17** 0.223 0.33 0.27** 0.126 1.19 20.29*Native 0.608 1.44 0.17*** 0.646 1.34 0.24*** 0.218 2.05 20.20**

Endemism

Endemic 0.232 0.29 0.18** 0.215 20.04 0.24** 0.118 0.73 20.25*Nonendemic 0.635 1.48 0.17*** 0.688 1.38 0.24*** 0.241 2.09 20.20**

Growth habit

Forbs 0.623 1.23 0.18*** 0.664 1.13 0.26*** 0.260 1.91 20.23***Graminoids 0.462 0.66 0.19*** 0.482 0.56 0.27*** 0.155 1.33 20.22*Vines 0.057 —a — 0.120 20.15 0.21* 0.019 — —Shrubs 0.131 0.59 0.11* 0.160 0.51 0.16* 0.098 1.04 20.18*Trees 0.274 0.65 0.13*** 0.280 0.58 0.18*** 0.049 — —

Herbaceous life cycle

Annuals 0.413 0.94 0.18*** 0.433 0.84 0.26*** 0.159 1.62 20.22*Perennials 0.574 1.10 0.19*** 0.607 0.99 0.27*** 0.215 1.78 20.23**

a Dependent factor was not significant in regression.


United States (17.6 in Baskin and Baskin

2003) and the Tennessee flora (18.6 in

Wofford and Kral 1993). However, much

local variation occurs in terms of the compo-

sition of the exotic flora of glades. The most

frequent (. 60%) exotic species in the present

study were Allium vineale, Leucanthemum

vulgare, Lonicera japonica, L. maackii, and

Ligustrum sinense. On the northern Alabama

glades, in contrast, A. vineale had a moderate

presence, L. japonica and L. sinense (as L.

vulgare) had low presence, and L. vulgare and

L. maackii were absent (Baskin et al. 1995). In

southeastern United States, L. japonica, L.

maackii, and L. sinense are considered severe

threats and A. vineale and L. vulgare lesser

threats (Tennessee Exotic Pest Plant Council

2002). Compared to the herbaceous exotics,

survival and growth of woody exotics are

relatively low and thus do not pose a major

threat, except along the edges of glades, where

they can form a dense thicket (Cofer et al.,

pers. obs.). To this end, several herbaceous

exotics occurred in low frequency in the present

study but are listed as severe or significant

threats in southeastern United States and

should be monitored closely: Bromus commu-

tatus, Carduus nutans, Daucus carota, Lespede-

za cuneata, and Melilotus officinalis (Tennessee

Exotic Pest Plant Council 2002).

Perimeter explained a slightly higher amount

of variation than area for total species richness

and richness of native and exotic species and

nonendemic species. In contrast, area was the

best predictor for endemic species richness.

The endemic species occur over a broad range

of soil depths and can be found throughout a

glade (ca. 1–12 cm; Somers et al. 1986, Rollins

1997). On the other hand, the nonendemic

species occur predominantly on the relatively

deeper soils near glade edges (Cofer et al., pers.

obs.). Some of this nonendemic component

consists of typical native forest species (e.g.,

Botrychium virginianum, Geum canadense, Sa-

nicula canadensis) and exotic species (e.g.,

Lespedeza cuneata, Lonicera maackii, Ligus-

trum sinense). Other studies on rock outcrop

communities only have included area as a

factor and have not partitioned endemic vs.

nonendemic (or other groups of species). At a

broad scale, Wyatt and Fowler (1977) found

that area was positively related to total species

FIG. 2. Relationship between exotic speciesrichness and standardized residuals of native speciesrichness vs. perimeter of glades. The relationshipwas not significant (P 5 0.8818).

Table 2. Percent variation (R2 value) explained with one, two, or three independent variables in themodel. Dependent variables were placed into two categories depending on the order of ranking based on R2

values of independent variables. The first independent variable is the only one included in the one-variablemodel; the first and second variables were included in the two-variable model and the first, second and thirdvariable in the three-variable model.

Order of independent variablesNumber of variables in model

Dependent variables One Two Three

Island–road–isolationTotal 65.83 67.70 68.95Exotic 19.26 22.20 23.30Endemic 22.84 24.75 24.75Forbs 64.73 67.14 67.37Annuals 42.62 43.81 43.96Perennials 59.41 60.87 61.90

Island–road–isolationNative 63.05 65.21 66.08Nonendemic 66.42 68.16 69.41Graminoids 47.47 48.89 49.36Shrubs 14.40 20.72 22.19Trees 27.90 33.77 34.13Vines 8.10 10.12 10.19


richness on 16 granite outcrops in southeastern

United States. At a fine scale, Uno and Collins

(1987) and Houle (1990) reported a positive

relationship between area and total species

richness in soil-filled depressions on granite in

Oklahoma and Georgia, respectively.

Other studies have shown that species

richness may be better explained by habitat

heterogeneity rather than area even though

they are difficult to discriminate from each

other (e.g., Triantis et al. 2003). Soil depth is

an important component in determining the

diversity of habitats for species and commu-

nities on rock outcrops. For example, soil

depth was positively related to total species

richness in depressions on granite but it also

was positively related to area (Uno and Collins

1987, Houle 1990). Soil depth explained 71.6%

of the variation in total species richness (vs.

68.3% for area) on a Georgia outcrop and

47% (vs. 45%) on an Oklahoma outcrop.

Although studies on cedar glades in middle

Tennessee have classified plant communities

based on soil depth (Somers et al. 1986, Drew

1991, also see Freeman 1933, Quarterman

1950), the scale of sampling greatly influences

the patterns in community composition and

vegetation-environment relationships (Rollins

1997). Rollins (1997) showed that classifica-

tion of communities at a 0.1-m2 scale reflected

microhabitat differences (i.e., soil depth),

whereas that at a 100-m2 scale reflected

differences between glades. This suggests that

microhabitat plays a minor role in determining

species richness at the whole glade level.

Although species richness decreased with

increasing isolation, the importance of this

factor for determining richness is difficult to

judge in the present study. First, isolation was

negatively correlated with glade size which

explained a larger amount of variation in

richness than isolation. Second, all glades were

within 6–83 m of each other, and many were

connected by a network of trails. On the other

hand, most glade species being heliophytes

were not observed along the shaded forested

trails. Thus, we doubt that the trails are

serving as major corridors for species migra-

tion between glades. Although Wyatt and

Fowler (1977) found that an isolated granite

outcrop in North Carolina had fewer total

species than other outcrops in southeastern

United States of comparable size, Krauss et al.

(2004) showed that habitat isolation for

calcareous grasslands in Germany did not

have a significant effect on species richness of

habitat specialists or generalists.

For continental islands, in general, slope

values of species richness vs. area relationship

are within the range 0.12–0.19 (Connor and

McCoy 1979). The slope of total species

richness vs. area in the present study (0.17)

was equivalent to that for granite outcrops in

southeastern United States (Wyatt and Fowler

1977). Therefore, present day extinction and

colonization processes for cedar glades might

be similar to other continental islands. On

Swedish alvars, which are structurally similar

to cedar glades, van der Maarel and Sykes

(1993) found that local immigration and

extinction were about equally important. They

concluded that species move around in the

community and sooner or later reach every

part of its area (sensu carousel model, van der

Maarel and Sykes 1993). On the other hand,

Margules et al. (1994) found a very high level

of species turnover but not an equilibrium

number of species over an 11-year period on

the Ingleborough limestone pavements.

The present study did not support the

diversity-resistance hypothesis since no rela-

tionship between native vs. exotic species

richness was found when glade size was

controlled statistically. However, Herben et

al. (2004) reported that this relationship is

highly scale dependent. Studies reporting

negative relationships had a mean area of plots

, 30 m2. Positive or nonsignificant relation-

ships were described in studies done in much

larger plots or areas, similar to our work (mean

area of glades 5 360 m2). Absence of a

relationship is a relatively uncommon phe-

nomena (Herben et al. 2004) and could be due

to a lack of substantial competitive interactions

(Huston 1994, Bruno et al. 2003). Competitive

displacement on glades, where biomass and

competition for light are relatively low, is

probably less important than tolerance to

(drought) stress for determining community

composition (Walck et al. 2001). Such was the

case among herbaceous plants in , 1 year old

burn sites of Mediterranean scrub communi-

ties, where no relationship was found between

richness of native vs. exotic species since they

are resource rich and biomass poor (Sax 2002).

On the other hand, a positive relationship was

shown in 4- and 9-year old burn sites.

Disturbance (e.g., Tierney and Cushman

2006) and closeness to roads or utility

corridors (e.g., Flory and Clay 2006) influence


exotic species invasion. In the present study,

disturbed glades supported a higher number of

exotic species than non-disturbed ones but on-

site disturbance was coupled with glade size.

In the past, glades in middle Tennessee were

used for pastures (Baskin and Baskin 1996).

Rollins (1997) observed that glade sites with a

history of grazing were more likely to have

weedy grass and forb species than those

without such history. However, foraging was

probably confined to the red cedar woods

since glades lack sufficient vegetation to

sustain livestock (Hogan and Webber 1999).

On the other hand, distance to the Battlefield’s

autotour road was unrelated to exotic richness

in glades. This lack of relationship is probably

due to the relatively low use of the road and it

not being directly linked to a major highway.

The serious disturbance of our study site

prior to it becoming a National Battlefield

must have had a great impact on the distribu-

tion of the flora. Particularly, differences

among regional sets of cedar glades in terms

of the total species richness, and particularly

the exotic species richness, might be due in part

to the disturbance history of the Battlefield.

The impact may have ‘‘homogenized’’ the flora

when the area was relatively sparse of trees,

and when the surrounding forest developed the

flora became somewhat ‘‘dehomogenized’’ via

the creation of glades of different sizes.

Differential extinction may have taken place

within an initial flora, establishing species

richness on glades dependent on their size.

Although endemic species richness increased

with glade size, five of the six endemic species

occurred nearly over the entire range of glade

sizes. Apparently, the distribution among

glades for these five taxa is unrelated to

dispersal syndrome since none of them have

effective means of seed dispersal. Gravity is

the likely mode of dispersal for all the endemic

species in our study, except for wind/water in

the case of Lobelia appendiculata var. gattin-

geri (Mann et al. 1999). Only Astragalus

tennesseensis was not found over a broad

range of glade sizes. However, its seeds are

lighter than those of another family member

(Psoralea subacaule) that occurs over the full

range of glade sizes and has a similar dispersal

mode. Thus, the restriction of A. tennesseensis

to relatively small glades must be due to

factors other than dispersal.

Our study suggests that glades of all sizes

would be important in preservation efforts,

and small as well as large ones potentially may

serve as corridors (‘stepping stones’) on the

landscape. Mann et al. (1999) pointed out that

the spatial configuration of glades and wood-

lands may be critical for dispersal and estab-

lishment of endemic glade species, particularly

those dispersed by gravity. Moreover, Rollins

(1997) found that glades contained different

community types at a large scale of observa-

tion, whereas they showed repeatable patterns

of community types at a small scale. Her con-

clusions regarding conservation efforts based

on large scale observations of community types

adds support to our results that each glade,

regardless of size, is worthy of protection.

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Appendix

Taxa recorded on 40 cedar glades at Stones River National Battlefield, Rutherford County, Tennessee.Nat. 5 nativity (E, exotic; N, native), End. 5 endemism (E, Endemic; NE, nonendemic), GH 5 growthhabit (F, forb; G, graminoid; S, shrub; T, tree; V, vine), HLC 5 herbaceous life cycle (A, annual; P,perennial; —, not applicable), Freq. 5 frequency among the 40 glades. An asterisk (*) indicates that thetaxon is new to the flora of cedar glades (cf. Baskin and Baskin 2003).

Species Nat. End. GH HLC Freq.

Andropogon virginicus L. N NE G P 100Croton monanthogynus Michx. N NE F A 100Juniperus virginiana L. N NE T — 100Panicum flexile (Gattinger) Scribn. N NE G A 100Ulmus alata Michx. N NE T — 100Carex blanda Dewey N NE G P 98Euphorbia dentata Michx. N NE F A 98Galium virgatum Nutt. N NE F A 98Panicum malacophyllum (Nash.) Gould N NE G P 98Rhus aromatica Ait. N NE S — 98Ruellia humilis Nutt. N NE F P 98Senecio anonymus (Wood) W.A. Weber &

A. Love N NE F P 98Houstonia purpurea L. N NE F P 95Ligustrum sinense Lour. E NE S — 95Quercus imbricaria Michx.* N NE T — 95Scutellaria parvula Michx. N NE F P 95Sporobolus vaginiflorus (Torr. ex Gray)

Wood N NE G A 95Celtis laevigata Willd. N NE T — 90Dalea gattingeri (Heller) Barneby N NE F P 90Panicum laxiflorum Lam. N NE G P 90Acalypha virginica L. N NE F A 88Oxalis dillenii Jacq. N NE F P 88Galium pilosum Ait. N NE F P 85Hypericum frondosum Michx. N NE S — 85Leucanthemum vulgare Lam. E NE F P 85Lobelia appendiculata var. gattingeri (Gray)

McVaugh N E F A 85Verbena simplex Lehm. N NE F P 85Verbesina virginica L. N NE F P 85Delphinium carolinianum Walt. ssp.

calciphilum Warnock N NE F P 83Hypericum sphaerocarpum Michx. N NE F P 83Plantago virginica L. N NE F A 83Sanicula canadensis L. N NE F P 83Aristida longespica Poir. N NE G A 80Fraxinus americana L. N NE T — 80Heliotropium tenellum (Nutt.) Torr. N NE F A 78Leavenworthia stylosa Gray N E F A 78Prunus serotina Ehrh.* N NE T — 78Symphoricarpos orbiculatus Moench N NE S — 78Chamaesyce maculata (L.) Small N NE F A 75Forestiera ligustrina (Michx.) Poir. N NE S — 75Isanthus brachiatus (L.) B.S.P. N NE F A 75Erigeron strigosus Muhl. ex Willd. N NE F P 73Panicum capillare L. N NE G A 73Pediomelum subacaule (Torr. & Gray)

Rydb. N E F P 73Allium vineale L. E NE F P 70Manfreda virginica (L.) Salisb. ex Rose N NE F P 65Sedum pulchellum Michx. N NE F A 65Taraxacum officinale G.H. Webber ex

Wiggers E NE F P 65Lonicera maackii (Rupr.) Herder* E NE S — 63



Croton capitatus Michx. N NE F A 60Quercus stellata Wangenh N NE T — 60Tridens flavus (L.) A.S. Hitchc. N NE G P 60Carex hirsutella Mackenzie* N NE G P 58Chamaesyce nutans (Lag.) Small N NE F A 58Geum canadense Jacq. N NE F P 58Sisyrinchium albidum Raf. N NE F P 58Conyza canadensis (L.) Cronq.* N NE F A 55Lonicera japonica Thunb. E NE V — 55Galactia volubilis (L.) Britt. N NE F P 53Minuartia patula (Michx.) Mattf. N NE F A 53Rosa carolina L. N NE S — 50Eragrostis frankii C.A. Mey. ex Steud. N NE G A 48Juglans nigra L. N NE T — 48Frangula caroliniana (Walt.) Gray N NE S — 45Glandularia canadensis (L.) Nutt. N NE F P 45Quercus muehlenbergii Engelm. N NE T — 45Rudbeckia triloba L. N NE F P 45Carya sp. N NE T — 43Cercis canadensis L. N NE T — 43Euphorbia spathulata Lam. N NE F A 43Nothoscordum bivalve (L.) Britt. N NE F P 43Smilax bona-nox L. N NE V — 43Bromus commutatus Schrad. E NE G A 40Carduus nutans L. E NE F P 40Diodia teres Walt. N NE F A 40Houstonia pusilla Schoepf N NE F A 40Lactuca serriola L. E NE F A 40Leucospora multifida (Michx.) Nutt. N NE F A 40Oxalis violacea L. N NE F P 40Quercus shumardii Buckl. N NE T — 40Rubus argutus Link* N NE S — 40Sabatia angularis (L.) Pursh N NE F A 40Valerianella radiata (L.) Dufr. N NE F A 40Viola egglestonii Brainerd N NE F P 38Ophioglossum engelmannii Prantl N NE F P 35Danthonia spicata L. Beauv. ex Roemer &

J.A. Schultes N NE G P 33Acer saccharum Marsh. N NE T — 33Opuntia humifusa (Raf.) Raf. N NE S — 33Salvia lyrata L. N NE F P 33Ambrosia artemisiifolia L. N NE F A 30Asclepias verticillata L. N NE F P 30Chaerophyllum tainturieri Hook. N NE F A 30Desmanthus illinoensis (Michx.) MacM. ex.

B.L. Robins. & Fern. N NE F P 30Galium circaezans Michx.* N NE F P 30Mecardonia acuminata (Walt.) Small N NE F P 30Parthenocissus quinquefolia (L.) Planch. N NE V — 30Carex umbellata Schkuhr ex Willd.* N NE G P 28Diospyros virginiana L. N NE T — 28Leavenworthia uniflora (Michx.) Britt. N NE F A 28Oenothera triloba Nutt. N NE F P 28Talinum calcaricum Ware N E F P 28Carex debilis Michx.* N NE G P 25Geranium carolinianum L. N NE F A 25Lespedeza cuneata (Dum.-Cours.) G. Don E NE F P 25Penstemon tenuiflorus Pennell N NE F P 25Polygala verticillata L. N NE F A 25Pyrrhopappus carolinianus (Walt.) DC. N NE F A 23Toxicodendron radicans (L.) Kuntze N NE V — 23Lespedeza procumbens Michx. N NE F P 20Paspalum setaceum Michx. N NE G P 20

AppendixContinued.



Vitis sp. N NE V — 20Asclepias sp. N NE F P 18Carex albicans Willd. ex Spreng. N NE G P 18Cyperus squarrosus L. N NE G A 18Daucus carota L. E NE F P 18Eragrostis spectabilis (Pursh) Steud. N NE G P 18Viburnum rufidulum Raf. N NE S — 18Aster pilosus Willd. N NE F P 15Astragalus tennesseensis Gray ex Chapman N E F P 15Carex oligocarpa Schkuhr ex Willd. N NE G P 15Cuphea viscosissima Jacq. N NE F A 15Draba verna L. E NE F A 15Juncus tenuis Willd. N NE G P 15Pseudognaphalium obtusifolium (L.)

Hilliard & Burtt. N NE F A 15Sphenopholis nitida (Biehler) Scribn.* N NE G P 15Berchemia scandens (Hill) K. Koch N NE V — 13Cerastium brachypetalum Desportes ex.

Pers.* E NE F A 13Juncus filipendulus Buckl. N NE G P 13Melica mutica Walt. N NE G P 13Melilotus officinalis (L.) Lam. E NE F P 13Phacelia dubia (L.) Trel. var. interior Fern. N E F A 13Solanum carolinense L. N NE F P 13Solidago altissima L. N NE F P 13Veronica arvensis L. E NE F A 13Viola bicolor Pursh N NE F A 13Ageratina altissima (L.) King & H.E.

Robins. N NE F P 10Aristida oligantha Michx. N NE G A 10Brickellia eupatorioides (L.) Shinners N NE F P 10Cardamine hirsuta L. E NE F A 10Carex crawei Dewey N NE G P 10Cirsium discolor (Muhl. ex Willd.) Spreng N NE F P 10Crepis pulchra L. E NE F A 10Digitaria ischaemum (Schreb.) Schreb. ex

Muhl.* E NE G A 10Galium aparine L. N NE F A 10Myosotis verna Nutt. N NE F A 10Plantago pusilla Nutt. N NE F A 10Potentilla simplex Michx. N NE F P 10Scleria pauciflora Muhl. ex Willd. N NE G P 10Senna marilandica (L.) Link N NE F P 10Setaria parviflora (Poir.) Kerguelen N NE G P 10Trifolium campestre Schreb. E NE F A 10Triodanis perfoliata (L.) Nieuwl. N NE F A 10Agrimonia pubescens Wallr.* N NE F P 8Evolvulus nuttallianus J.A. Schultes N NE F P 8Geranium dissectum L.* E NE F A 8Hypoxis hirsuta (L.) Coville N NE F P 8Isoetes butleri Engelm. N NE G P 8Krigia virginica (L.) Willd. N NE F A 8Matelea carolinensis (Jacq.) Woods N NE F P 8Melilotus alba Medikus E NE F P 8Passiflora incarnata L. N NE F P 8Physalis sp. N NE F P 8Polymnia canadensis L. N NE F P 8Rhus glabra L. N NE S — 8Spiranthes cernua (L.) L.C. Rich. N NE F P 8Spiranthes lacera (Raf.) Raf. N NE F P 8Sporobolus compositus (Poir.) Merr. N NE G P 8Thalictrum thalictroides (L.) Eames &

Boivin N NE F P 8

AppendixContinued.



Valerianella locusta (L.) Lat.* E NE F A 8Achillea millefolium L. E NE F P 5Ailanthus altissima (P. Mill.) Swingle E NE T — 5Andropogon gyrans Ashe N NE G P 5Belamcanda chinensis (L.) DC. E NE F P 5Bidens bipinnata L. N NE F A 5Bignonia capreolata L. N NE V — 5Cornus drummondii C.A. Mey.* N NE S — 5Echinacea tennesseensis (Beadle) Small N NE F P 5Euphorbia corollata L. N NE F P 5Gleditsia triacanthos L. N NE T — 5Helianthus hirsutus L. N NE F P 5Hypericum punctatum Lam. N NE F P 5Iris germanica L.* E NE F P 5Juncus dichotomus Ell. N NE G P 5Lonicera sempervirens L. N NE V — 5Maclura pomifera (Raf.) Schneid.* N NE T — 5Medicago lupulina L. E NE F A 5Ornithogalum umbellatum L. E NE F P 5Panicum sphaerocarpon Ell. N NE G P 5Passiflora lutea L. N NE V — 5Ranunculus bulbosus L. E NE F P 5Sonchus asper (L.) Hill E NE F A 5Sphenopholis obtusata (Michx.) Scribn. N NE G A 5Stellaria media (L.) Vill. E NE F A 5Verbascum blattaria L. E NE F P 5Veronica officinalis L. N NE F A 5Viola sororia Willd. N NE F P 5Acalypha gracilens Gray N NE F A 3Allium canadense L. N NE F P 3Arenaria serpyllifolia L. E NE F A 3Botrychium virginianum (L.) Sw.* N NE F P 3Bouteloua curtipendula (Michx.) Torr. N NE G P 3Calamintha nepeta (L.) Savi (Riquien) P.W.

Ball E NE S — 3Carex flaccosperma Dewey* N NE G P 3Clematis virginiana L. N NE V — 3Commelina erecta L. N NE F P 3Conoclinium coelestinum (L.) DC.* N NE F P 3Convolvulus arvensis L.* E NE F P 3Croton willdenowii G.L. Webster N NE F A 3Fimbristylis annua (All.) Roemer & J.A.

Schultes N NE G A 3Geranium molle L.* E NE F A 3Geum vernum (Raf.) Torr. & Gray* N NE F P 3Hedera helix L.* E NE V — 3Helenium amarum (Raf.) H. Rock N NE F A 3Hemerocallis fulva (L.) L.* E NE F P 3Kummerowia stipulacea (Maxim.) Makino E NE F A 3Lamium purpureum L.* E NE F A 3Lepidium virginicum L. N NE F A 3Liriodendron tulipifera L.* N NE T — 3Lobelia spicata Lam. N NE F P 3Muhlenbergia sobolifera (Muhl. ex Willd.)

Trin. N NE G P 3Muscari neglectum Guss. ex Ten.* E NE F P 3Pellaea atropurpurea (L.) Link N NE F P 3Poa annua L.* E NE G A 3Polanisia dodecandra (L.) DC.* N NE F A 3Potentilla recta L. E NE F P 3Prunella vulgaris L. N NE F P 3Rosa setigera Michx. var. tomentosa Torr.

& Gray N NE S — 3

AppendixContinued.



Scleria triglomerata Michx.* N NE G P 3Sida spinosa L. N NE F P 3Spiraea xvanhouttei (Briot) Carr.* E NE S — 3Valerianella umbilicata (Sullivant) Wood N NE F A 3Yucca filamentosa L. N NE F P 3

AppendixContinued.


Documents

Species Richness and Exotic Species Invasion in Middle Tennessee Cedar Glades in Relation to Abiotic and Biotic Factors 1