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Exotic Species Occurrence in Remnant and Restored Eastern Prairie Ecosystems and their Relation to Native Species Richness, Evenness, and Functional Group Abundance.

INTRODUCTION

As a result of severe fragmentation and periodic disturbance events, remnant prairies are likely susceptible to high levels of biological invasion from exotics with the potential for loss of native diversity (D'Antonio and Vitousek, 1992; Van Auken, 2000). Although small in comparison to Midwestern prairie sites, eastern remnant prairies are thought to have once been more widespread in distribution throughout the eastern U.S. (Barden, 1997; Davis et al, 2002; Schmidt and Barnwell, 2002; Tompkins et al., 2010a; 2010b; Noss, 2013). Because of the unique plant associations in these remnant prairies, and the number of rare and threatened species that inhabit them, a better understanding of the extent of exotic species abundance is of special concern to plant ecologists and land managers. Although many studies on exotic species have been reported from Midwestern prairie communities (Stohlgren et al., 1999; Larson et al., 2001; Cully et al, 2003), limited research has been done on exotics in eastern prairies. Recently, however, Tarasi and Peet (2017) included grassland sites from the Carolinas in a large-scale study on the relationship of native-exotic richness. Other recent studies from remnant and restored prairies in the Carolinas have indicated the prevalence of exotics, including the highly invasive Lespedeza cuneata (Dumont-Cours.) G. Don (sericea lespedeza) (Tompkins and Bridges, 2013a; Tompkins et al., 2013b).

Many factors have been identified that may support exotic invasion and maintenance in plant communities (Smith and Knapp, 1999); and numerous studies to date have focused on both biotic and physical factors that influence exotic species recruitment, establishment, and competitive interactions in ecosystems. Biotic factors that have been shown to affect exotic abundance include native species richness and evenness, functional group associations, variations in native dominant/sub-dominant species community structure, and the pool of potential nearby exotics (Williamson, 1999; Smith and Knapp, 1999; Levine and D'Antonio, 1999; Prieur-Richard et al., 2000; Symstad, 2000; Diaz and Cabido, 2001; Davis and Pelsor, 2001; Dukes, 2001; Kennedy et al., 2002; Farigone et al., 2003; Zavaleta and Hulvey, 2004; Pokorny et al., 2005; Souza et al., 2011; Tarasi and Peet, 2017). Findings from numerous studies have suggested that at smaller spatial scales at which native plant diversity is greater, abundance and invasion of exotics is often reduced, when compared to sites with less diversity. Inversely, at larger spatial scales native-exotic species abundance has often been shown to be positively correlated (Elton, 1958; Stohlgren et al, 1999; Naeem et al., 2000; Kennedy et al., 2002; Daehler, 2003; Von Holle, 2005; Davies et al., 2005; Pokorny et al., 2005; Fridley et al., 2007; Maron and Mailer, 2007; Tarasi and Peet, 2017). This pattern of variation among native-exotic species abundance is often referred to as the "invasion paradox," and is thought to be the result of multiple environmental processes co-varying within plant communities at different spatial or "grain" scales (Fridley et al., 2007; Tarasi and Peet, 2017). The recent study by Tarasi and Peet (2017) from the Carolinas also reported that at spatial scales of >1[m.sup.2] native-exotic species richness was positively correlated, while negatively correlated at smaller spatial scales.

In addition to diversity, the physical factors associated with a given site have also been shown to affect exotic species. Such factors include soil parameters, moisture, and light. These factors may also synergistically enhance or suppress the effects of native diversity to limit or increase exotic abundance (Hobbes and Huenneke, 1992; McIntyre and Lavorel, 1994; Kotanen et al., 1998; Stohlgren et al., 1999; Davis et al., 2000; Larson et al., 2001; Daehler, 2003; Cully et al., 2003). Although many studies have investigated covarying physical factors with plant diversity and their influence on exotics, a unifying theory of the interaction of both biotic and physical factors affecting the competitive interaction of exotics with native species has proven elusive.

To better understand the relationship between exotic species abundance with native plant diversity and physical factors in eastern prairie communities, an analysis of sampled quadrat data was performed to address the following: (1) At what abundance is exotic species cover present within the prairie communities; and (2) Does the amount of native plant diversity, including species richness and evenness, functional group richness, and functional group similarity, potentially influence the abundance (percent cover) of exotic species in these communities at the alpha (quadrat) and gamma (site) spatial scales? Consistent with other studies, the analysis for this study included the effect of native diversity on exotic cover rather than exotic richness. The amount of exotic cover is expected to have a greater impact on resources within a given site (Meiners et al., 2002; Harrison et al., 2006). Therefore, the potential effect of varying levels of native diversity on exotic cover is likely more important in assessing any potential inhibitory effect of greater native diversity. Furthermore, this provides a more useful assessment as to the extent of exotic species abundance in these sites versus a measure of exotic species richness alone.

METHODS

STUDY SITES/VEGETATION SAMPLING

Data from 508 quadrats (1[m.sup.2]) from eight eastern prairies, including sites in both North and South Carolina, were used in this study (Fig. 1; Table 1). For all sites sampling was done primarily during the late-growing season when dominant forb and graminoid species were most prevalent. A stratified design was used to subdivide the sampled areas into transects located across both the long and short axes of each site. Quadrats were established at either 5 or 10 m intervals along each transect based on the size of the total area sampled for each site. All vascular species were identified (Weakley, 2015) and a percent cover value was assigned by visually estimating the proportional area within each quadrat utilized by a given species. Therefore, the total percent cover value for each quadrat could sum to >100%. In addition each native species was placed into a functional group category based on the following functional categories: [C.sub.3] grasses, [C.sub.4] grasses, summer (late-season) forbs, spring (early-season) forbs, legumes, and woody species (trees, shrubs, vines). The percent cover for all exotic species within each quadrat was also estimated. For the gamma scale, total native richness for each site was calculated as all native species encountered within the sampled quadrats.

VEGETATION ANALYSIS

Alpha scale.--A Welch's ANOVA was performed to provide summary statistics of alpha means for percent exotic cover, native richness, and functional richness among the sites with a P < 0.05. Initial regression analyses of diversity data with exotic cover indicated deviations from normality (nonparametric). As a result Spearman's rank co- efficient tests were used to correlate both species and functional richness with exotic cover at the alpha (quadrat) scale (n = 508) for all sites combined with a P < 0.05. All statistical procedures were performed usingJMP (2014).

Gamma scale.--Due to unequal number of quadrats sampled per site, rarefaction of the data set was utilized. Spearman's tests (P < 0.05) were then performed for both total native richness (rarefacted) and species evenness with mean percent exotic cover at the gamma (site) scale for the study sites. To determine species evenness, species diversity was first calculated using Simpson's reciprocal index (D), where D = [1.sup.s][[SIGMA].sub.i=1] [p.sup.2.sub.i] and [p.sub.i] is the proportion of cover contributed by species i to total cover, and Sis the total number of species present. Species evenness is then the ratio of D to S (Polly et at, 2005).

SOIL SAMPLING

In order to detect variation in soil parameters among the sites that might potentially confound the relationship of diversity and exotic species abundance, three soil samples were taken (n = 24) from each site at random and then submitted to the North Carolina Department of Agriculture Soil Testing Lab (www.ncagr.gov/agronomi/sthome) for analysis. The following soil parameters were analyzed: cation exchange capacity (CEC; meq/100 [cm.sup.3]); % Mg, % Ca (of CEC); pH; and levels of P, K, and Mn (kg/ha). An additional three samples from each site were submitted to the South Carolina Agricultural Laboratory, Clemson, South Carolina (wvw.clemson.edu/public/regulatory/ag_svc_lab/ soil_testing) to determine the percent organic N content. An ANOVA was performed on the soil data (P < 0.05) with a post-hoc Tukey's HSD to detect any differences in soil parameters. Soil data were also included in a Principal Components Analysis (PCA) along with means for native and functional species richness, and for percent exotic cover from the study sites.

RESULTS

EXOTIC SPECIES OCCURRENCE AMONG THE SITES

A total of 155 plant taxa was identified from the sampled quadrats for all study sites. This included nine exotic species: Japanese honeysuckle [Lonicera japonica Thunberg], sericea lespedeza [Lespedeza cuneata], basket grass [Arthraxon hisipidus (Thunberg) Makino var. hispidus], Russian-olive [Elaeagnus umbellata Thunberg var. paivifolia (Royle) Schneider], multiflora rose [Rosa multiflora Thunberg ex Murray], cheat grass [Brornus secalinus L.], hairy chess grass [B. commutatus Schrader], Japanese chess grass [B. japonicas Thunberg ex Murray], and red fescue [Festuca rubra L. ssp. rubra]. Exotic species made up 11.1% of the total cover, with L. japonica and L. cuneata comprising 90% of the exotic cover. Lonicera japonica made up 7% of the total cover (native and exotic combined) and was present in 26% of the quadrats. Lespedeza cuneata made up 3% of the total cover and occurred in 23% of the quadrats.

Alpha scale.--At the alpha scale percent exotic cover was significantly different ([F.sub.7,175] = 10.3; P < 0.001). Also, both native richness ([F.sub.7.177] = 29.7; P < 0.001) and functional richness ([F.sub.7,175] = 16.8; P < 0.001) were significantly different among the sites (Table 2). Correlation analyses found a significant (P < 0.001), but weak ([r.sub.s] = -0.14), negative relationship of native richness with exotic cover; and a nonsignificant (P = 0.263; [r.sub.s] = -0.049) correlation of functional richness with exotic cover.

The nine exotic species identified from the sites belonged to four functional groups (Legume, [C.sub.3], [C.sub.4], Woody). Because most of the exotic cover among the sites was due to the presence of Lespedeza cuneata and Lonicera japonica, additional correlations were performed for each with total percent native cover for their respective functional categories. This was done to measure any potential negative correlation with these exotics and functional cover abundance within quadrats. Again, due to deviations from normality, Spearman's tests (P < 0.05) were performed. Both correlations were not significant (L. cuneata P = 0.07; [r.sub.s] = 0.07; L. japonica P = 0.33; [r.sub.s] = 0.04).

Gamma scale.--At the gamma scale, the relationship between mean percent exotic cover and total site richness was not significant (P = 0.95; [r.sub.s] = 0.023). There was also no relationship (P = 0.238; [r.sub.s] = -0.438) for species evenness with exotic cover for the study sites.

SOIL FEATURES

Soil parameter values indicated a broad range of soil conditions among the sites (Table 3). Soil values were generally higher for four of the sites (Latta Plantation, Blackjacks Heritage Preserve, McDowell Prairie, and Shuffletown Prairie), which occur on unique soil series with mafic conditions (higher nutrient and pH levels). The remaining sites have more acidic soils with generally lower nutrient/pH levels more typical of the Carolina Piedmont region (Tompkins, 2010a; 2010c). The results of the PCA indicated that Principal Component One contained 51.1% of the variance and Principal Component Two contained 14.1% of the variance (Fig. 2; Table 4), with the remaining percentage in the other components. The analysis indicated that both exotic cover and soil parameters (with the exception of % N) were clustered together, with a second cluster including both native and functional species richness and % N.

DISCUSSION

The results of this study were not consistent with the majority of other such studies for the effect of species and functional richness with exotic species abundance at differing spatial scales. Although native richness with exotic cover was negatively correlated at the alpha scale, the correlation was weak. Other studies have also reported that the abundance of exotics is not necessarily correlated with levels of diversity (Lavorel et al, 1999; Sax, 2002; Keeley et al., 2003; Von Holle, 2005; Sandel and Corbin, 2010). One consideration that may explain potential discrepancies in this study compared to others is that the percent exotic cover was estimated visually and not by removal and drying to determine biomass relative to native richness. In addition it is important to note that the majority of exotic cover among the study sites was the result of the presence of just two species, Lonicera japonica and Lespedeza cuneata. Both species are known to be especially aggressive and abundant throughout the southeastern U.S. region. As a result greater native diversity may be more inhibitory against certain exotics as has been suggested in the literature, while some exotics are potentially less affected (Von Holle, 2005). In fact, in a Midwestern study, L. cuneata was shown to reduce species richness in study plots (Brandon et al., 2004). That study suggested a shading-effect was the primary mechanism in reducing native richness and cover in plots where L. cuneata was present. Other studies have indicated the release of allelopathic compounds as an additional means by which L. cuneata may competitively exclude nearby species (Kalburtji and Mosjidis, 1992; 1993). Furthermore, L. cuneata has demonstrated a significant ability to reproduce successfully under various condition (Woods et al., 2009). Like L. cuneata, L. japonica has been shown to reduce native species richness, most likely through its impact on resource availability (Yurkonis and Meiners, 2004; Yurkonis et al., 2005). Control of both of these exotics is made especially problematic due to the periodic removal of mid-successional species from these prairie sites necessary to maintain conditions favorable to endemic species that likely also enhances the abundance of both exotics (Seglequest, 1971; Stransky, 1984; Brandon et al, 2004).

Results from the PCA suggested exotic abundance was primarily associated with higher soil parameter values (nutrients, pH, CEC, etc.), rather than from levels of native diversity. Therefore, exotic species abundance in these sites appears to be more a function of resource availability, even at smaller spatial scales. With the exception of Latta Plantation, all other sites in this study with more fertile soils had more relative exotic cover. It is unclear why exotic cover was lower for that site. One potential factor may be its management history that has included a long-term, periodic fire regime when compared to the other sites with similar soil conditions. Other studies have also indicated that resource availability may enhance exotic invasion (Aerts and Berendse, 1988; Huenneke et.al., 1990; Hobbs and Huenneke, 1992; Wedin and Tilman, 1996; Davis and Pelsor, 2001; Renee et. al., 2006; Thomsen et al., 2006; Maron and Marler, 2007; Sandel and Corbin, 2010). In contrast, studies by both Brandon et. al., (2004) and Houseman et al., (2014) have reported that L. cuneata abundance was instead reduced in plots with greater soil amendment. Despite this, although both resource availability and diversity may co-vary within a site to affect exotic success (Naeem et al., 2000; Huston, 2004; Renee et al., 2006), results from this study suggest more fertile soils contributed primarily to exotic abundance in the study sites and minimized any potential inhibitory effect due to native diversity abundance.

CONCLUSIONS

The results from this study indicate a large presence of aggressive exotics in many of the study sites and reflect the successful invasion of those species. These species may pose a longterm threat to these unique communities due to the large assortment of rare species that often occur in small populations in these communities. The results also provide information on the current status of exotic species and the potential interactions of exotics with both native species and soil conditions within these sites. However, further study is needed to better understand the potential negative effects exotic species occurrence may have on native diversity in these sites.

Acknowledgments.--The author would like to thank the staff of Crowders Mountain State Park, the Mecklenburg County Park and Recreation Division of Nature Preserves and Natural Resources, and the South Carolina Department of Natural Resources for permission to conduct research at their sites. The author is also most grateful to Dr. William C. Bridges, Jr. of Clemson University for his comments on the statistical analysis used in this study.

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SUBMITTED 19 DECEMBER 2018

ACCEPTED 11 APRIL 2019

ROBERT TOMPKINS (1)

Department of Biology, Belmont Abbey College, 100 Belmoni-Mt. Holly Rrl. Belmont, North Carolina 28012

(1) Corresponding author: Telephone: (704) 616-8461; E-mail: roberttompkins@bac.edu

Caption: FIG. 1 -Map of study sites.

Caption: FIG. 2.--Principal Components Analysis (PCA) for soil parameters with diversity abundance (native richness, functional richness) and percent exotic cover for study sites (n = 8)
TABLE 1.-Study sites with size of sampled area
and number of sampled quadrats (l[m.sup.2])

Site                       # Quadrats   Sampled area (ha)

Crowders                   100          0.8
Prairie (CRP)

Crowders Roadside          100          0.1
Prairie I (CRI)

Crowders Roadside          50           0.1
Prairie II (CRII)

Crowders Roadside          50           0.1
Prairie III (CRIII)

Shuffletown Prairie (SH)   75           2.0

Latta Plantation (LP)      48           2.0

McDowell Prairie (McD)     55           2.0

Blackjacks Heritage        30           2.0
Preserve (BJHP)

Site                       Location

Crowders                   35[degrees]13'46"N, 81[degrees]17'39"W
Prairie (CRP)

Crowders Roadside          35[degrees]13'08"N, 81[degrees]17'08"W
Prairie I (CRI)

Crowders Roadside          35[degrees]12'49"N, 81[degrees]17'20"W
Prairie II (CRII)

Crowders Roadside          35[degrees]14'21"N, 81[degrees]16'06"W
Prairie III (CRIII)

Shuffletown Prairie (SH)   35[degrees]19'31"N, 80[degrees]57'31"W

Latta Plantation (LP)      35[degrees]21'44"N, 80[degrees]54'57"W

McDowell Prairie (McD)     35[degrees]07'01"N, 81[degrees]00'33"W

Blackjacks Heritage        35[degrees]54'04"N, 81[degrees]01'05"W
Preserve (BJHP)

TABLE 2.--Summary statistics for quadrats (1[m.sup.2]) for %
exotic cover, native richness, functional group richness, and
for total native richness with [+ or -] SE. Values followed by
the same letters are not significantly different at P < 0.01
by pair-wise comparisons (Bonferroni Correction)

Site    % Exotic cover            Tot. native
                                    species

CRP     8.1, C [+ or -] 1.48          46
CRI     6.08, C [+ or -] 2.08         58
CRM     2.1, C [+ or -] 1.48          57
CRIII   10.5, BC, [+ or -] 2.12       67
SH      23.0, A [+ or -] 1.71         76
LA      2.7, C [+ or -] 2.14          58
McD     16.3, AB [+ or -] 2.0         45
BJHP    12.3, B [+ or -] 2.71         45

Site    Native richness          Funct. richness

CRP     3.2, BC [+ or -] 0.22    3.43, A [+ or -] 0.09
CRI     6.1, AB [+ or -] 0.23    3.40, AB [+ or -] 0.09
CRM     7.2, A [+ or -] 0.32     3.50, AB [+ or -] 0.12
CRIII   5.4, BC [+ or -] 0.33    2.78, CD [+ or -] 0.13
SH      4.8, C [+ or -] 0.26     2.56, CD [+ or -] 0.10
LA      4.6, CD [+ or -] 0.33    2.97, BC [+ or -] 0.13
McD     3.3, D [+ or -] 0.31     2.36, D [+ or -] 0.12
BJHP    5.7, ABC [+ or -] 0.42   3.67, A [+ or -] 0.17

TABLE 3.--Mean soil parameter values for study sites
with [+ or -] SE. Values followed by the same letters
are not significant different at P < 0.05 by Tukey's HSD

Site              CEC                       pH

CRI     6.1, BC [+ or -] 0.41     4.8, D [+ or -] 0.03
CRII    4.5, C [+ or -] 0.84      4.9, CD [+ or -] 0.13
CRIII   7.5, BC [+ or -] 0.36     5.5, BC [+ or -] 0.0
CRP     5.1, C [+ or -] 0.37      5.3, CD [+ or -] 0.14
LA      13.5, AB [+ or -] 0.75    6.3, A [+ or -] 0.08
McD     10.8, ABC [+ or -]0.18    6.6, A [+ or -]0.14
BJHP    14.0, AB [+ or -] 1.26    6.5, A [+ or -] 0.03
SH      17.3, A [+ or -] 4.45     6.0, AB [+ or -] 0.32

Site               P                         K

CRI     0.8, C [+ or -] 0.8       144.2, B [+ or -] 5.97
CRII    4.9, BC [+ or -] 0.0      61.1, B [+ or -] 10.40
CRIII   36.7, A [+ or -] 3.48     106.5, B[+ or -] 16.25
CRP     10.4, BC [+ or -]2.11     111.7, B [+ or -] 9.37
LA      12.7, B [+ or -] 0.83     159.1, B [+ or -] 19.31
McD     12.8, B [+ or -] 4.23     150.1, B [+ or -] 6.87
BJHP    10.5, BC [+ or -] 2.21    139.1, B [+ or -] 18.79
SH      9.5, BC [+ or -] 2.36     314.7, A [+ or -] 72.15

Site              Mn                       %Ca *

CRI     21.1, D [+ or -] 9.75     32.0, DE [+ or -] 5.13
CRII    39.7, D [+ or -] 3.0      23.3, E [+ or -] 1.76
CRIII   66.5, CD [+ or -] 6.09    47.7, C [+ or -] 0.88
CRP     43.7, D [+ or -] 11.65    45.7, CD [+ or -]6.17
LA      368.1, A [+ or -] 26.23   68.3, AB [+ or -] 1.20
McD     349.2, A [+ or -] 6.06    76.7, A [+ or -] 1.45
BJHP    147.7, BC [+ or -] 3.44   70.1, A [+ or -] 0.88
SH      228.6, B [+ or -] 42.99   54.3, BC [+ or -] 2.33

Site             %Mg *                      %N

CRI     10.0, C [+ or -] 0.57     0.2, A [+ or -] 0.04
CRII    10.0, C [+ or -] 0.57     0.1, A [+ or -] 0.02
CRIII   26.0, AB [+ or -] 0.0     0.1, A [+ or -] 0.01
CRP     16.7, BC [+ or -] 1.45    0.2, A [+ or -] 0.13
LA      21.3, ABC [+ or -] 0.88   0.1, A [+ or -] 0.03
McD     15.7, BC [+ or -] 1.66    0.1, A [+ or -] 0.02
BJHP    19.6, BC [+ or -] 0.33    0.1, A [+ or -] 0.01
SH      33.3, A [+ or -] 6.93     0.1, A [+ or -] 0.03

* percent of CEC

TABLE 4.--Eigenvectors and eigenvalues for first
four principal component loadings

Component Loadings                    C1         C2

CEC                                   0.34881    0.27653
pH                                    0.37086    -0.19512
                                      0.12448    -0.24299
K                                     0.27419    0.54240
Ca%                                   0.34673    -0.33436
Mg%                                   0.30628    0.36066
Mil                                   0.33063    -0.31169
N%                                    -0.10443   0.09914
% Exotic Cover                        0.30382    0.31190
Native Richness                       -0.35589   0.28913
Functional Group Richness             -0.31129   0.04995
Eigen Values                          5.62       1.55
Percent of Total Variance Explained   51.1       14.1

Component Loadings                    C3         C4

CEC                                   0.21441    -0.21763
pH                                    0.23105    -0.04855
                                      -0.74963   0.04615
K                                     0.07999    -0.00148
Ca%                                   0.18623    -0.03538
Mg%                                   -0.27198   0.00552
Mil                                   0.29180    -0.00016
N%                                    0.20108    0.90721
% Exotic Cover                        -0.12479   0.14658
Native Richness                       0.05702    -0.27080
Functional Group Richness             0.28151    -0.17036
Eigen Values                          1.27       0.99
Percent of Total Variance Explained   11.5       9.08
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Author:Tompkins, Robert
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2019
Words:5416
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