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Influence of Microstegium vimineum Presence on Insect Abundance in Hardwood Forests
Jordan M. Marshall and David S. Buckley

Southeastern Naturalist, Volume 8, Number 3 (2009): 515–526

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2009 SOUTHEASTERN NATURALIST 8(3):515–526 Infl uence of Microstegium vimineum Presence on Insect Abundance in Hardwood Forests Jordan M. Marshall1,2,* and David S. Buckley3 Abstract - Microstegium vimineum (Japanese Stiltgrass) is an exotic, annual grass that has invaded hardwood forests throughout the Southeastern United States. Four forests, in four Tennessee counties, were selected, and insects and plant communities were sampled along transects with and without M. vimineum. Insects were sampled using a terrestrial vacuum sampler. Percent plant cover was measured using a pointintercept technique, and plant species richness and diversity were calculated. Of the 60 families collected, significantly more individuals in the families Acrididae, Cicadellidae, and Gryllidae, were collected in areas with M. vimineum, whereas areas without M. vimineum resulted in significantly more individuals in the families Blattellidae and Chrysomelidae. Herbaceous plant richness and diversity did not significantly differ between areas with and without M. vimineum. Areas without M. vimineum had significantly lower percent vegetation cover (30.41%) than areas with M. vimineum (91.48%). The overall herbaceous plant community diversity and structure may be more infl uential factors in the abundances of insect families in central hardwood forests than the invasion of M. vimineum. Introduction The addition of exotic plant species to landscapes can change the composition of local plant communities, beyond a simple additive impact (e.g., Maskell et al. 2006, Meiners et al. 2002). In some cases, alterations to the surrounding landscape as a result of an exotic plant species introduction and eventual dominance can substantially alter the composition of native plant communities (Mack et al. 2000, Siemann and Rogers 2006). However, changes to plant communities as a result of exotic species introduction are often more subtle, and do not necessarily result in restructuring of native communities (Mandryk and Wein 2006). The addition of an exotic plant species to the overall community is often associated with some other anthropogenic factor, which, in turn, may be the overarching cause of alterations in plant or insect communities (Maskell et al. 2006, Palmer et al. 2004). One exotic species of concern within eastern hardwood forests is Microstegium vimineum (Trin.) A. Camus (Japanese Stiltgrass; Poaceae). This C4 annual grass species is native to Southeastern Asia and was first collected in North America near Knoxville, TN, in 1919 (Fairbrothers and 1Department of Forestry, Wildlife, and Fisheries, 274 Ellington Plant Sciences Building, University of Tennessee, Knoxville, TN 37996. 2Current address - Michigan Technological University, Cooperative Emerald Ash Borer Project, 5936 Ford Court, Suite 200, Brighton, MI 48116. 3University of Tennessee Agricultural Experiment Station, Department of Forestry, Wildlife, and Fisheries, 274 Ellington Plant Sciences Building, Knoxville, TN 37996. *Corresponding author - 516 Southeastern Naturalist Vol. 8, No. 3 Gray 1972). It is found throughout the Eastern United States from Florida to Massachusetts and as far west as Texas (Fairbrothers and Gray 1972, Hunt and Zaremba 1992, Redman 1995, USDA NRCS 2007). M. vimineum has become a common target in the control of exotic species throughout its introduced range on both private and public lands (Johnson 1997, Steele et al. 2006). Although anecdotal information on this species has been sufficient to generate considerable concern, relatively few formal scientific studies have been implemented to establish the ecological impacts of M. vimineum on different groups of native taxa. However, based on the impact studies that have been conducted, it has been found that: 1) hardwood tree seedlings might not be recruited into larger size classes due to suppression by M. vimineum (however, Quercus spp. (oak) and Acer spp. (maple) seedlings, two genera which are considered important in central hardwood forests, did not differ between plots with and without M. vimineum [Cole 2006]); 2) increases in M. vimineum biomass may reduce height growth of out-planted tree seedlings as well as reduce woody stem densities of tree seedlings (Oswalt et al. 2004, 2007); and 3) M. vimineum was superior to a native grass in resource acquisition, leading to reduced biomass accumulation in the native species (Leicht et al. 2005). To our knowledge, no studies have been implemented to investigate impacts of M. vimineum on native bird or mammal species. In terms of impacts of M. vimineum on arthropods, survival of nymphal and larval Ixodes scapularis Say (Acari: Ixodidae) were not impacted by M. vimineum (Carroll 2003). Due to the importance of arthropods in trophic webs and as indicators of ecosystem function, condition, and integrity, investigation of potential impacts of M. vimineum on native insect communities is warranted. The objectives of this study were to 1) test the null hypothesis that insect communities do not differ between plant communities with and without M. vimineum, and 2) investigate relationships between insect family abundances and plant species richness and diversity in this forest type. Site Description Insect collection forest sites were located in Anderson, Blount, Knox, and Morgan counties, TN (Fig. 1). Study sites were within the Appalachian section of central hardwood forest characterized as being an Oak-Hickory forest type (Fralish 2003). For all four sites, mean annual temperature was 15 °C and mean annual total precipitation was 1400 mm (National Climatic Data Center 2007). Patches dominated by M. vimineum of sufficient size for multiple samples were selected at each study site and had defined edges, beyond which M. vimineum had not invaded. Patch area was not measured. M. vimineum dominance was estimated within each patch to be at least 75 percent cover through an ocular measurement. Patches were located at the University of Tennessee Oak Ridge Forest (Anderson County, TN; 36°0'4"N, 84°13'34"W), 2009 J.M. Marshall and D.S. Buckley 517 Springbrook Park (Blount County, TN; 35°48'4"N, 83°58'52"W), the Ijams Nature Center Quarry Restoration (Knox County, TN; 35°57'2"N, 85°52'54"W), and the University of Tennessee Cumberland Forest (Morgan County, TN; 36°3'43"N, 84°26'53"W). The Anderson County site was actively managed using silvicultural techniques, with recent harvesting activities occurring in 2005 (R.M. Evans, The University of Tennessee, Oak Ridge, TN, pers. comm.). A municipality managed the Blount County site with minimal activity within the forested sampling locations and there appeared to be little recent disturbance in this location (J.M. Marshall, pers. observ.). A restoration program was initiated in 2001 at the Knox County site by the Ijams Nature Center, which included trail construction and maintenance (James 2003). Grading of a road at the Morgan County site occurred in 1998, but the site has remained undisturbed since that time (M.R. Schubert, The University of Tennessee, Oliver Springs, TN, pers. comm.). Methods Sampling A portable terrestrial vacuum sampler, as described by Harper and Guynn (1998), was used to collect insects. Vacuum samples were collected within a Figure 1. Insect and plant sampling locations in Anderson, Blount, Knox, and Morgan counties, TN. 518 Southeastern Naturalist Vol. 8, No. 3 bottomless frame box (50 x 50 x 50 cm). Ten sampling locations, with 5-m intervals, were selected along a transect established on the long axis of each patch of M. vimineum. At the Anderson County and Morgan County sites, large patches of M. vimineum existed that accommodated all ten sampling locations within a single, contiguous patch. Patches of M. vimineum at the Blount County and Knox County sites were smaller, but were large enough for 3–5 samples per patch. Patches in close proximity (20–40 m apart) at these sites were sampled to approximate the area covered by the large patches at sites in Anderson and Morgan counties. Native forest understory areas without M. vimineum were selected along transects within the same forest stand as controls. Ten sampling locations were selected along the control transects at each site, with 5-m spacing. Three insect collections were made 6 June, 20 July, and 7 September 2006, between 10:00 am and 3:00 pm, allowing for six-week periods between collections. These non-sampling periods were long enough to ensure recolonization of plots by insects, and thus a reasonable level of independence existed between samples obtained on different dates (Tscharntke et al. 2005). Insects were identified to family using Daly et al. (1998) and Triplehorn and Johnson (2005). Taxonomic arrangement was based on the Integrated Taxonomic Information System (2007), with the inclusion of the order Collembola. Family richness and Shannon diversity index were calculated for each collection period for each site (n = 24). Richness (S) was the count of families that were encountered. Shannon diversity indices were calculated as H = -Σ pi ln (pi), where pi = the proportion of the ith family (Hayek and Buzas 1997). A plant survey, centered on the insect-collection points, was conducted at each sampling location in areas with and without M. vimineum on 7 and 9 June 2006. Plants were identified to species, except for individuals in the genera Carex, Rubus, Solidago, Vaccinium, and Viola. Percent cover for each species was measured using a point-intercept technique within a 0.34-m2 frame containing 49 points spaced 7 cm apart. Percent plant cover was calculated as the proportion of points intercepted x 100 (Floyd and Anderson 1987). Richness (count of species encountered [S]) and Shannon diversity indices for herbaceous plant species were calculated for each site in areas with and without M. vimineum, which was excluded from the diversity index calculations (n = 8). Percent canopy cover was measured 1 m above the center of each sampling location with a CI-110 Digital Canopy Imager (CID Inc., Camas, WA) on 7 and 9 June 2006 (n = 80). Air temperature and relative humidity were measured at the time of insect sampling at five sampling locations in each of the two treatment areas, with and without M. vimineum, using a Kestrel 3000 Pocket Weather Meter (Nielsen-Kellerman, Boothwyn, PA) (n = 120). Data analysis Sampling locations and dates were considered independent samples for statistical analysis. One-tailed t-tests were performed to identify directional 2009 J.M. Marshall and D.S. Buckley 519 differences in treatments with and without M. vimineum for canopy cover, air temperature, and relative humidity, as well as plant species richness and diversity and insect family richness and diversity. Families that accounted for at least 3 percent of the total abundance in each treatment area, with and without M. vimineum, were used to test for differences of individual family abundances between those treatment areas. Simple linear regression was used to test for relationships between insect family diversity and plant species diversity. All statistical analyses were performed using NCSS (v. 2004). Results The ocular estimate of 75% M. vimineum cover was similar to the actual mean cover (± SD) of 79.49% ± 8.82 for this species. Percent forest canopy cover was significantly greater in areas without M. vimineum than in areas with M. vimineum (mean ± SD: 79.43% ± 4.56 vs. 75.81% ± 5.54, respectively; t = 3.19, df = 78, P = 0.002). Air temperature did not differ between areas with and without M. vimineum (mean ± SD: 27.26 °C ± 3.48 vs. 27.32 °C ± 3.10, respectively; t = 0.11, df = 118, P = 0.913). Also, relative humidity did not differ between areas with and without M. vimineum (mean ± SD: 49.02% ± 9.34 vs. 49.82% ± 10.17, respectively; t = 0.45, df = 118, P = 0.654). A total of 2839 insects were captured over the three sampling dates representing 11 orders and 60 families. Total insect family richness (mean ± SD: 15.19 ± 3.55 vs. 15.08 ± 4.25, respectively; t = -0.52, df = 22, P = 0.304) and diversity (mean ± SD: 1.80 ± 0.44 vs. 1.74 ± 0.40, respectively; t = -0.32, df = 22, P = 0.752) did not differ significantly between areas with and without M. vimineum (Table 1). Also, insect abundance did not differ signifi- cantly between areas with and without M. vimineum (mean ± SD: 138.42 ± 86.94 vs. 98.08 ± 59.90, respectively; t = -1.323, df = 22, P = 0.199). Nine families each accounted for more than 3 percent of the total insects collected (Table 1). Formicidae and Entomobryidae were the dominant two families in both areas with and without M. vimineum, with Formicidae accounting Table 1. Mean (SD) insect family abundance count, family richness, and family diversity in areas with (n = 12) and without Microstegium vimineum (n = 12) in central hardwood forests. 120 samples were collected in each treatment. Mean abundance (count) Insect Family (Order) With M. vimineum Without M. vimineum Acrididae (Orthoptera) 7.08 (8.18) 1.42 (1.68) Blattellidae (Dictyoptera) 0.75 (0.97) 3.83 (3.21) Chrysomelidae (Coleoptera) 0.83 (0.72) 3.25 (3.33) Cicadellidae (Hemiptera) 6.42 (5.53) 1.17 (1.53) Curculionidae (Coleoptera) 1.33 (1.83) 4.25 (6.48) Entomobryidae (Collembola) 49.42 (76.01) 23.83 (18.86) Formicidae (Hymenoptera) 39.67 (39.37) 44.08 (46.09) Gryllidae (Orthoptera) 8.42 (6.29) 1.92 (1.83) Lygaeidae (Hemiptera) 5.92 (4.93) 3.17 (3.49) Family richness 15.19 (3.55) 15.08 (4.25) Family diversity 1.80 (0.44) 1.74 (0.40) 520 Southeastern Naturalist Vol. 8, No. 3 for 35.40% and Entomobryidae accounting for 30.96% of all insects collected. Of the nine families included in the analyses, only Blattellidae and Chrysomelidae exhibited reductions in abundance in plots with M. vimineum (Fig. 2). Families Acrididae, Cicadellidae, and Gryllidae were significantly more abundant in areas with M. vimineum (Fig. 2). Formicidae, Entomobryidae, Lygaeidae, and Curculionidae did not differ significantly between areas with and without M. vimineum. Forty-four herbaceous plant species were present in this study. Fifteen herbaceous plant species occurred only in areas with M. vimineum, whereas sixteen occurred only in areas without M. vimineum. Plant species richness (mean ± SD: 11.75 ± 4.27 vs. 15.50 ± 6.45, respectively; t = 0.97, df = 6, P = 0.185) and diversity (mean ± SD: 1.76 ± 0.28 vs. 1.99 ± 0.43, respectively; t = 0.92, df = 6, P = 0.197) were not significantly different between areas with and without M. vimineum (Table 2). In addition, herbaceous plant diversity was not related to M. vimineum percent cover. However, mean percent cover of herbaceous plant species other than M. vimineum was significantly higher in areas without M. vimineum than in areas with M. vimineum (mean ± SD: 30.41% ± 16.12 vs. 11.99% ± 4.33, respectively; t = 2.21, df = 6, P = 0.034). By adding M. vimineum to the herbaceous cover, percent herbaceous plant cover was significantly greater in plots with M. vimineum than without Figure 2. Mean (± SE) Acrididae (Orthoptera), Blattellidae (Dictyoptera), Chrysomelidae (Coleoptera), Cicadellidae (Hemiptera), and Gryllidae (Orthoptera) abundance (counts) with (n = 12) and without (n = 12) Microstegium vimineum in central hardwood forests. Note: One-tailed t-tests between treatments within families were significant (P < 0.05) for all families. 2009 J.M. Marshall and D.S. Buckley 521 (mean ± SD: 91.48% ± 7.27 vs. 30.41% ± 16.12, respectively; t = -6.91, df = 6, P < 0.001). The presence of tree seedlings in plots was independent on the presence of M. vimineum (X 2 = 2.58, df = 1, P = 0.108). Table 2. Mean (SD) herbaceous plant species percent cover, species richness, and species diversity in areas with (n = 4) and without Microstegium vimineum (n = 4) in central hardwood forests. Mean percent cover Plant species With M. vimineum Without M. vimineum Agrimonia pubescens Wallr. 0.00 1.53 (3.06) Ambrosia artemisiifolia L. 0.00 1.53 (3.06) Calystegia sepium (L.) R. Br. 0.51 (1.02) 0.00 Campsis radicans (L.) Seem. ex Bureau 1.53 (3.06) 0.00 Carex sp. L. 0.51 (1.02) 0.00 Daucus carota L. 0.51 (1.02) 0.00 Desmodium canescens (L.) DC. 0.51 (1.02) 0.51 (1.02) Dichanthelium clandestinum (L.) Gould 0.51 (1.02) 0.00 Duschesnea indica (Andr.) Focke 2.19 (3.16) 7.87 (15.74) Elaeagnus umbellata Thunb. 0.51 (1.02) 6.12 (12.24) Erechtites hieraciifolia (L.) Raf. ex DC. 0.00 0.51 (1.02) Euonymus americana L. 0.89 (1.79) 2.30 (1.74) Euonymus fortunei (Turcz.) Hand.-Maz. 0.00 9.01 (18.03) Galium concinnum Torr. & Gray 1.02 (2.04) 0.00 Geum canadense Jacq. 2.04 (2.89) 0.51 (1.02) Impatiens capensis Meerb. 2.24 (4.49) 0.00 Lespedeza repens (L.) W. Bartram 0.00 0.51 (1.02) Lonicera japonica Thunb. 8.79 (7.24) 2.37 (2.74) Maianthemum racemosum (L.) Link 0.51 (1.02) 2.30 (2.68) Microstegium vimineum (Trin.) A. Camus 79.49 (8.82) 0.00 Mitchella repens L. 0.00 8.06 (16.12) Ophioglossum vulgatum L. 1.02 (1.02) 0.00 Osmorhiza claytonii (Michx.) C.B. Clarke 0.00 1.22 (2.45) Oxalis stricta L. 0.00 0.51 (1.02) Parthenocissus quinquefolia (L.) Planch. 4.17 (3.73) 5.83 (5.15) Polygonum pensylvanicum L. 1.36 (2.72) 0.00 Polygonum persicaria L. 1.02 (2.04) 0.00 Polystichum acrostichoides (Michx.) Schott 0.00 7.65 (15.31) Potentilla recta L. 1.53 (3.06) 0.00 Ranunculus sp. L. 3.16 (4.09) 0.00 Rosa multifl ora Thunb. 0.00 1.53 (3.06) Rubus sp. L. 3.06 (3.53) 2.55 (5.10) Smilax rotundifolia L. 0.00 2.55 (3.06) Solanum carolinense L. 1.02 (1.18) 0.00 Solidago sp. L. 1.02 (2.04) 0.00 Toxicodendron radicans (L.) Kuntze 1.02 (2.04) 5.47 (5.31) Tradescantia subaspera Ker Gawl. 0.51 (1.02) 0.00 Uvularia perfoliata L. 0.00 0.51(1.02) Vaccinium sp. L. 0.00 2.55 (5.10) Viburnum acerifolium L. 0.00 5.27 (10.54) Viola sp. L. 2.04 (2.89) 0.51 (1.02) Vitis rotundifolia Michx. 0.51 (1.02) 11.73 (20.77) Wisteria fl oribunda (Willd.) DC. 0.00 2.38 (4.76) Plant species richness 11.75 (4.27) 15.50 (6.45) Plant species diversity 1.76 (0.28) 1.99 (0.43) 522 Southeastern Naturalist Vol. 8, No. 3 Insect family diversity was inversely related to plant species diversity (Fig. 3). Insect family diversity was not significantly related to mean air temperature, relative humidity, or canopy cover. Also, the abundance of insects was not significantly related to mean air temperature, relative humidity, canopy cover, percent herbaceous plant cover, M. vimineum percent cover, or plant species diversity. Discussion The most obvious difference, as well as statistically significant difference, was the increase in percent plant cover as a result of the inclusion of M. vimineum. Herbaceous plant cover was over 3 times higher in areas with M. vimineum than in those without, because of the addition of this species. Other research provides little evidence that M. vimineum alters hardwood forest communities, but this species still remains a species of management concern. While Cole (2006) illustrated that some hardwood tree species had lower germination in M. vimineum and Oswalt et al. (2004, 2007) found decreases in abundance of woody stems and growth of out-planted oak seedlings with increases in M. vimineum, there have been no assessments of overall changes in forest plant and insect communities with M. vimineum establishment. In addition, other arthropods, such as I. scapularis, may exhibit very little change, in terms of distribution and abundance, as a result of M. vimineum invasion (Carroll 2003). Based on the results of this study, Figure 3. Linear relationship between plant species diversity and insect family diversity in central hardwood forests. 2009 J.M. Marshall and D.S. Buckley 523 it appears that the overall plant community diversity, and not the abundance of M. vimineum, affects the diversity of insects. This finding differs from results for other invasive species which have changed insect communities associated with the invaded forest plant communities, resulting in decreased insect abundance and richness (Standish 2004). Without studies quantifying the feeding habits of insects on M. vimineum, there is no clear explanation for the increases in the abundances of Acrididae, Cicadellidae, and Gryllidae and reductions in the abundances of Blattellidae and Chrysomelidae with the presence of M. vimineum. However, general family characteristics may provide some information regarding the observed changes. Acrididae are usually generalist herbivores and respond to the increased overall plant cover and structure (Lawton 1983). Also, increases in Acrididae food quality has resulted in measurable increases in population size (Oedekoven and Joern 2000). Increases in abundances of Acrididae in areas with M. vimineum may have been associated with changes in plant structure due to the addition of M. vimineum to the community; however, further investigations are necessary to clarify this relationship. The reductions in abundance of Blattellidae in areas with M. vimineum may result from the changes that occur in the decomposition of litter under M. vimineum compared to native litter layers (Kourtev et al. 2002). In contrast, increases in Gryllidae individuals may also have been a result of the decomposition of litter under M. vimineum, selecting for the increased litter layer nitrogen content under M. vimineum stands (Ehrenfeld et al. 2001, Schädler et al 2003). The reduced herbaceous plant cover other than M. vimineum in areas where this species occurred may explain the reduced abundance of Chrysomelidae; there may have been fewer plants acceptable for feeding by individuals in this family. Typically, species within this family are specific to their food plant; oftentimes these species are monophagous or oligophagous (Arnett et al. 2002). In addition, species within the family Cicadellidae are also food-plant specific (Triplehorn and Johnson 2005) and were more abundant in areas with M. vimineum. Differences in the responses of these two host-specific taxa may be due to the differences of native plants in areas with and without M. vimineum. Plant species occurring only with or without M. vimineum may have played an important role in the distribution of insects from these two families. The negative relationship in insect family and plant species diversities in this study also concurred with several arthropod functional groups presented by Koricheva et al. (2004). This relationship between insect and plant diversities suggests that the overall plant community may be a more influential driver in the distribution of insects rather than the invasion of M. vimineum. The invasion of M. vimineum may alter decomposition rates and soil functionality (Ehrenfeld et al. 2001; Kourtev et al. 2002, 2003), but the presence of this species appeared to have little effect on the plant species richness or diversity. Other factors may have attributed to the 524 Southeastern Naturalist Vol. 8, No. 3 invasion by M. vimineum, such as forest management (Marshall and Buckley 2008). Such factors may have been the overarching cause of changes in plant species diversity, ultimately altering the insect community (Maskell et al. 2006, Palmer et al. 2004). Acknowledgments The authors thank Jason Corsberg, Richard M. Evans, Brien Ostby, Martin R. Schubert, and J. Mark Young for assistance in the planning and implementation of this study. The authors are also grateful to Dr. Craig Harper and John Gruchy for loaning the vacuum sampler used in this work, and to Dr. Jerome F. Grant for his review of an earlier version of this manuscript. Literature Cited Arnett, R.H. Jr., M.C. 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