Southeastern Naturalist C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 602 2015 SOUTHEASTERN NATURALIST 14(4):602–611 Association between Roads and the Distribution of Microstegium vimineum in Appalachian Forests of North Carolina Christina Manee1, 2, W.T. “Duke” Rankin3, Gary Kauffman4, and Greg Adkison1,* Abstract - Microstegium vimineum (Japanese Stiltgrass) is an invasive, annual C4 grass that frequently forms dense populations along roads in the eastern US. We examined data from a survey that included 768 forested sites in western North Carolina, and carried out a transplant experiment to test (1) if the distribution of Japanese Stiltgrass is associated with roads and (2) if roadsides differ from forest interiors in terms of the frequency, abundance, and individual vigor of the species. Japanese Stiltgrass abundance was positively associated with total road length within watersheds. The species was much more common and abundant on roadsides than in forest interiors. Greenhouse-established individuals of Japanese Stiltgrass that we transplanted onto roadsides grew larger than those we transplanted in forest interiors. The 2 groups had similar survival rates. Our results suggest that roads promote the spread of Japanese Stiltgrass and that individuals and populations are more robust on roadsides than in forest interiors. However, the species can grow in forest interiors, suggesting its lower abundance and size there may result from limitations in dispersal, germination, or resource acquisition. Introduction Microstegium vimenium (Trin.) A. Camus (Japanese Stiltgrass) is a highly invasive grass generally associated with roadsides in the eastern US. Japanese Stiltgrass is native to eastern Asia and has rapidly spread throughout the eastern US since its introduction, which probably occurred in the early 1900s (Fairbrothers and Gray 1972). It is now a common roadside species in many portions of its adopted range (Cole and Wetzin 2004). Unlike most invasive plant taxa in the eastern US, Japanese Stiltgrass also invades undisturbed forest communities, forming extensive mats in moist, shady environments, and these populations can threaten the diversity of native species (Adams and Engelhardt 2009, Leicht et al. 2005). Japanese Stiltgrass diminishes herbaceous diversity, stunts early growth of native trees, and potentially alters the successional trajectory of forest communities (Bauer and Flory 2011, Flory and Clay 2010, Oswalt et al. 2007). The mechanisms behind the ability of Japanese Stiltgrass to invade both roadside habitats and forest interiors are not clear. Although the species is thought to colonize primarily mesic, shaded areas, some studies have found a positive correlation between Japanese Stiltgrass success and high-light or open-canopy conditions 1Department of Biology, Western Carolina University, Cullowhee, NC 28723. 2Deptartment of Biology, Asheville-Buncombe Technical Community College, Asheville, NC 28801. 3USDA Forest Service, Southern Region (R8), Atlanta, GA 30309. 4USDA Forest Service, Asheville, NC 28801 *Corresponding author - gadkison@wcu.edu. Manuscript Editor: Richard Baird Southeastern Naturalist 603 C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 (Barden 1987, Cole and Weltzin 2004, Flory et al. 2007, Gibson et al. 2002). In addition, Japanese Stiltgrass is a C4 species, which means it possesses a physiological pathway typically associated with dry, open environments, and is capable of taking advantage of brief sunflecks (Barden 1987, Horton and Neufeld 1998). Commongarden experiments suggest Japanese Stiltgrass can survive and grow across a wide variety of environmental conditions, leading Flory et al. (2011) to conclude that few habitats are immune to invasion. We conducted a survey and a transplant experiment to examine Japanese Stiltgrass distribution along roadsides and in forest interiors that parallel the roadsides. Specifically, we asked how strongly Japanese Stiltgrass distribution (i.e., frequency and abundance) is associated with roads in Appalachian forests of North Carolina and if its vigor and distribution varies with distance from roadsides and into adjacent forests. We asked these questions to better understand the likelihood of Japanese Stiltgrass spreading from roadsides into surrounding, undisturbed forest communities. Methods The southern Appalachian mountains of western North Carolina are well-suited for this study because there are many roadsides and forest communities available for sampling across a wide range of ecological conditions. The region contains extensive areas of montane forest communities, including many public lands, some of which are managed for commodities and others of which are managed for conservation purposes. Also, the study area is well known for its diverse herbaceous flora—the plant species likely most threatened by Japanese Stiltgrass. We accessed previously unpublished data (USDA Forest Service, Asheville, NC) from the region that allowed us to examine the association between roads and the regional distribution of Japanese Stiltgrass to a degree unmatched in the existing literature. Landscape survey We examined data from a survey of invasive plants coordinated by Gary Kauffman (USDA Forest Service, Asheville, NC). The survey examined 28 watersheds in Pisgah and Nantahala National Forests, NC, during the growing seasons of 2002 and 2003. A total of 768 sites was sampled, each with a set of 3 plots that paralleled the same 100 m of road—1 on the roadside, 1 at the ecotone between roadside and forest, and 1 in the forest interior. Plot sets were established every 1.61 km (1 mile) along roads sampled in 2002 and every 0.8 km (0.5 mi) along roads sampled in 2003. If a road was shorter than 1.61 km in the 2002 survey or shorter than 0.8 km in the 2003 survey, the plot set was placed at the midpoint of the road’s length. Roadside plots started at the edge of the roads and extended 1–3 m toward the forest, depending on the distance to the ecotone between the roadside and the forest. In other words, some roadside plots were as small as 100 m x 1 m, and others were as large as 100 m x 3 m. Ecotone plots were 3 m wide and started where roadside plots ended (i.e., ecotone plots were 100 m x 3 m). Forest-interior plots were 10 m wide and began 30 m from the edge of the ecotone plots (i.e., forest-interior plots were Southeastern Naturalist C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 604 100 m x 10 m). A technician walked the plots and visually estimated the abundance of Japanese Stiltgrass in each plot as percent cover. Not all plot sets were walked by the same technician; thus, variation of cover-estimates among technicians may have caused some variance in our data. We averaged Japanese Stiltgrass abundance over all plots in each drainage. After log-transforming the mean values to meet assumptions of normality and homoscedasticity, we used single-factor ANOVA to examine the effect of plot location (roadside, ecotone, forest interior) on these mean values of abundance. We used Spearman rank correlation to test for an association between total road length and untransformed mean values of abundance from each watershed. We determined the total road length in each watershed by summing the lengths of all roads using 3D Analyst’s Surface-Area tool in ArcGIS 9.2 (ESRI, Redlands CA). We carried out all statistical analyses with R 3.0.1 (R Foundation for Statistical Computing, Vienna, Austria). Transplant experiment In May 2006, we planted Japanese Stiltgrass seeds in potting soil (6 seeds per 85-ml cup, except 1 cup in which we unintentionally planted 7 seeds), allowed them to germinate and grow under greenhouse conditions until 4 August 2006, then randomly assigned the resulting plugs to 1 of 4 sites in Nantahala National Forest and to 1 of 2 treatments (roadside or forest interior) at each site. We planted 10 plugs 0–1 m from the road edge (the roadside treatment) and 10 plugs 18–20 m from the road edge (the forest-interior treatment) at each of 4 sites. To increase homogeneity between plots at each site, we avoided roads with steep banks. One of the sites, Moses Creek, is a Liriodendron tulipifera L. (Yellow Poplar) forest community on a floodplain. Coward Ridge is an upland site above Moses Creek. It supports a Quercus (oak)-Carya (hickory) forest community. Bryson Branch and Whiterock Creek are also upland sites. Bryson Branch is an upland hardwood–Pinus strobus L. (White Pine) forest community. Whiterock Creek is an oak-hickory forest community. Forests at the 4 sites were at least 50 years old (USDA Forest Service, Asheville, NC, unpubl. data). Seventy of the 80 plugs survived transplantation. Consequently, the design became unbalanced with 6 forest-interior plugs and 10 roadside plugs at Moses Creek, 9 interior plugs and 7 roadside plugs at Coward Ridge, and 10 interior plugs and 9 roadside plugs at both Bryson Branch and Whiterock Creek. On 4 August at each transplant location, we measured ground-level light intensity as photosynthetic photon flux density (PPFD) using a hand-held light meter (Apogee Instruments Quantum Meter, Apogee Instruments, Inc., Logan, UT). We harvested Japanese Stiltgrass on 1 October 2006. We recorded the height (cm) of the tallest plant and the number of surviving shoots in each plug before harvest, dried the plugs at 60 °C to constant weight, and measured total plant biomass to the nearest 0.0001 g. We calculated mean individual biomass (i.e., mean biomass per plant) in each plug by dividing total plant biomass by the number of surviving individuals. We used a Pearson chi-squared test with Yate’s correction Southeastern Naturalist 605 C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 to determine if the number of surviving shoots was associated with plot location at each site, and Pearson correlation to test for an association between the amount of light and the proportion of transplants that survived. We analyzed proportions rather than counts because 1 of the plugs started out with 7 individuals rather than 6. To test for an effect of plot location on plant mass per individual and maximum plant height, we used ANOVA with plot location (i.e., roadside vs. forest interior) as a fixed effect and site as a block effect. Normality and homoscedasticity were achieved by log-transforming plant mass per individual and maximum plant height prior to analysis. We did not conduct inferential analysis to compare light at different plot locations because we measured light level only once at each plot location. All statistical analyses were carried out with R 3.0.1 (R Foundation for Statistical Computing, Vienna, Austria). Results Landscape survey Japanese Stiltgrass occurred in 759 roadside plots, 489 ecotone plots, and 159 forest-interior plots. Its abundance was usually greater in roadside plots than other plots (Fig. 1). Abundance exceeded 10% cover in 195 roadside plots, in contrast to 19 ecotone plots and 5 forest-interior plots (Fig. 2). Mean abundance of Japanese Stiltgrass in roadside plots was 3–4 times its mean abundance in ecotone plots and forest-interior plots (Fig. 3). Japanese Stiltgrass abundance in roadside plots and ecotone plots increased with the amount of road in a watershed (Fig. 4). However, the association was relatively weak (Spearman r = 0.38, P = 0.05) on roadsides, and relatively moderate (Spearman r = 0.51, P = 0.006) in ecotones. Figure 1. Percent cover of Japanese Stiltgrass in each plot by category: roadside, ecotone, and forest-interior. The species occurred in 759 roadside plots, 489 ecotone plots, and 159 forest-interior plots. Southeastern Naturalist C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 606 Figure 2. Number of roadside plots ( ), ecotone plots ( ), and forest-interior plots ( ) with particular ranges of Japanese Stiltgrass cover. For example, 151 roadside plots and 117 ecotone plots were 3–5% covered by Japanese Stiltgrass, whereas only 27 forest-interior plots had 3–5% Japanese Stiltgrass cover. Figure 3. Mean (dots) and median (horizontal line) abundance of Japanese Stiltgrass calculated by aggregating data for all watersheds after calculating each watershed’s mean per-plot abundance and omitting plots with no Japanese Stiltgrass cover. Error bars associated with the means are 95% confidence intervals. Q1 identifies the 25th percentile and Q3 identifies the 75th percentile. Also included are results from ANOVA that tested the effect of plot location on the log-transformed percent-cover values that were aggregated for this figure. Figure 4. Mean abundance of Japanese Stiltgrass in relation to the total length of roads. Each circle in the figure is the mean percent cover of Japanese Stiltgrass in one of 27 watersheds and the corresponding total length of roads in the watershed. Also included are results from Spearman rank correlations. Data from a 28th watershed (the Chattooga drainage) were included in the analyses but omitted from the graphs because Chattooga’s total amount of road (3701 km) was more than triple that of the next-largest drainage and including it would have reduced readability of the graphs. Mean Japanese Stiltgrass cover was 6% in Chattooga’s roadside plots, 1% in Chattooga’s ecotone plots, and 0% in its forest-interior plots. Southeastern Naturalist 607 C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 Transplant experiment All plants in 10 of the experiment’s 80 plugs died within 2 weeks of being transplanted, leaving 70 plugs with a total of 421 plants in the experiment (69 plugs x 6 plants, and 1 plug x 7 plants). Two hundred fifty-five of the 421 plants survived until harvest. Analyzed across all sites, Japanese Stiltgrass survival was higher along roadsides than in forest interiors (χ2 = 12.68, P = 0.0004; Table 1), but this result was likely caused by the large difference between roadside and interior survival at Moses Creek. The other 3 sites differed little in survival of roadside versus forestinterior transplants (Table 1). When we omitted Moses Creek from the analysis, the proportion of plants that survived was positively related to ambient light (r = 0.79). However, there was a 6% chance (P = 0.06) of detecting a correlation as large as or larger than r = 0.79 among the plants in our experiment even if the association did not exist in the statistical population represented by those plants. The correlation dropped to r = 0.19 when we included Moses Creek in the analysis, and there was a 70% chance (P = 0.7) of detecting a correlation as large as or larger than r = 0.19 among our experiment’s plants even if the association did not exist in the statistical population represented by our experiment’s plants. Roadside transplants were heavier and taller than forest-interior transplants (Fig. 5). Harvested roadside transplants were typically 2–3 times heavier and up to twice as tall as harvested forest-interior transplants. Light was higher in the roadside transplant sites at Bryson Branch and at Whiterock Creek, but it differed little between transplant sites at Coward Ridge and not at all at Moses Creek (Fig. 5). We found no association between light and either plant mass or height. Discussion Given the current distribution of Japanese Stiltgrass in the eastern US and its life-history characteristics, the species appears to have a 2-stage invasion process. For new invasions, there is a landscape-level dispersal facilitated by anthropogenic disturbance associated with road construction and maintenance followed by a community-level dispersal as the plants invade undisturbed forests from roadsides (Mortensen et al. 2009). This second stage appears Table 1. Survivorship of Japanese Stiltgrass following transplantation to roadside and forest-interior sites. Abbreviations include the number of individuals that survived to the end of the experiment (#Surv), the number that died before the end of the experiment (#Died), and the percent that survived to the end of the experiment (%Surv). Roadside Forest Interior Site #Surv #Died %Surv #Surv #Died %Surv χ2 P Bryson Branch 33 21 61 41 20 67 0.24 0.6 Whiterock Creek 33 21 61 28 32 47 1.84 0.2 Moses Creek 55 5 92 15 21 42 26.01 less than 0.0001 Coward Ridge 24 18 57 26 28 48 0.45 0.5 Overall 145 65 69 110 101 52 11.91 0.0006 Southeastern Naturalist C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 608 Figure 5. Observed light (only 1 observation per plot location), mean plant mass, and mean height for each plot location. Values of mean plant mass and mean height were calculated by aggregating plugs from each plot location. Error bars associated with the means are 95% confidence intervals. Also shown are results for the effect of plot location on log-transformed plant mass and height from ANOVAs in which plot location was a fixed effect and site was a block effect. Southeastern Naturalist 609 C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 to be facilitated by the wide ecological amplitude of Japanese Stiltgrass but constrained by limitations in dispersal, germination, or resource acquisition (Huebner 2010, Nord et al. 2010, Oswalt and Oswalt 2007). Our study focused on the effects of roads as dispersal corridors and differences between roadside and forest habitats for supporting the survival and growth of Japanese Stiltgrass. Roads that cross topographically complex montane ecosystems provide extensive and varied habitat for a species that requires edge conditions for dispersal, germination, or growth. Similar to several other studies of Japanese Stiltgrass distribution in association with roads, our work shows the species to be primarily a roadside species (Christen and Matlack 2006, Cole and Weltzin 2004, Flory and Clay 2009, Mortensen et al. 2009). For example, Japanese Stiltgrass occurred in 99% of roadside plots but only 34% of forest-interior plots, and its abundance was roughly 4 times greater in roadside plots than in forest-interior plots. Also, its abundance in roadside and ecotone plots was positively associated with the total length of roads in watersheds. Moreover, Japanese Stiltgrass individuals that grew in forest-interior plots were smaller and had lower survival than individuals growing in roadside plots. Our results also indicate that Japanese Stiltgrass is capable of surviving in undisturbed forests, especially in forest communities characterized by higher light levels such as our Bryson Branch site, which had the second-highest light level (14 μmol/ms) and the second-highest survival (67%) in the study. Bryson Branch is an upland site, dominated by a combination of hardwoods and White Pine. It does not have the high moisture levels generally associated with Japanese Stiltgrass survival (Warren et al. 2011). Other studies have shown that the C4 physiology of Japanese Stiltgrass (Barden 1987, Horton and Neufeld 1998) may be responsible for its ability to survive in forest interiors by taking advantage of brief sun flecks. Transplanted Japanese Stiltgrass survival was highest (92%) on the roadside at Moses Creek. This result strongly influenced our analysis of survival at all sites combined. In fact, there was no effect of habitat (i.e., roadside vs. forest interior) at the other 3 sites, all of which are uplands. Moses Creek probably stands out because it was the only floodplain community in the experiment, and the roadside at Moses Creek was close to the creek and more of a floodplain habitat than a forest-interior site. The high survival rates of Japanese Stiltgrass on the roadside at Moses Creek is consistent with the expectation that moist habitats are more susceptible to invasive plant species (Brown and Peet 2003). Survival rates in the experiment were based on healthy seedlings whose growth had been established in controlled conditions for 3 months prior to placement in the experiment’s forested sites. Thus, our results do not address the roles of litter depth, disturbance, reproduction, germination, or survival of newly emerged seedlings. Other studies, however, have shown all of these factors are important to the biology and spread of Japanese Stiltgrass (Cheplick 2010, Christen and Matlack 2009, Flory and Clay 2009, Huebner 2011, Marshall and Buckley 2008, Nord et al. 2010, Oswalt and Oswalt 2007). Although Japanese Stiltgrass is often considered an aggressive invader of roadsides and forest communities, Rauschert et al. (2010) Southeastern Naturalist C Manee, W.T. Rankin, G Kauffman, and G. Adkison 2015 Vol. 14, No. 4 610 showed that its spread is relatively slow. Warren et al. (2011) suggested that the invasive ability of the species may be limited at certain stages by light, soil moisture, leaf-litter, and competing species. Germination and establishment of Japanese Stiltgrass are limited by leaf-litter depth (Marshall and Buckley 2008, Nord et al. 2010, Oswalt and Oswalt 2007) and are constrained to some degree by the amount of surrounding vegetation (Barden 1987), seed depth in the soil, and moisture availability (Gibson et al. 2002). Japanese Stiltgrass may also be limited by dispersal. Its seeds have no apparent adaptations for dispersal other than being suitably sized and shaped to temporarily attach to animals (Tu 2000). Road construction and use undoubtedly affects Japanese Stiltgrass distribution patterns and probably facilitates its spread from roadsides into adjacent forests (Rauschert et al. 2010). Gibson et al. (2002) argued that Japanese Stiltgrass may be dispersal-limited and need disturbance to disperse into uninfested areas because of its connection to vectors associated with roadside habitats. Our results support an emerging consensus that roads are important vectors for the spread and establishment of Japanese Stiltgrass and that road density may be a useful component of inavasibility assessments by conservation scientists (e.g., Forman and Alexander 1998). A possible explanation of our results is that the distribution of Japanese Stiltgrass in a landscape with abundant roadside habitat reflects differences in dispersal success between roadsides and forest interiors. Another explanation emphasized in the literature is that Japanese Stiltgrass in forests produces fewer seeds and exhibits lower germination rates. Seeds could be dispersing from roadside populations, but have minimal success establishing in forest interiors. Our study cannot conclusively distinguish between these 2 hypotheses, but future research that includes seed traps to determine dispersal capability along with corresponding germination studies might shed light on the matter. 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