Global temperature changes are predicted to influence the distributions of plants and can have significant consequences for population genetic structure. Both the nature of these consequences and the processes that shape them are of interest for both conservation genetics and the development of realistic management programs. Rapid range expansion occurs on short temporal scales not conducive to conventional phylogeographical analyses. This paper presents the findings from a population genetic study of the invasive grass Microstegium vimineum Trin. A. Camus throughout the James River Basin of Virginia. Genotypic analysis using Amplified Fragment Length Polymorphism (AFLP) molecular markers were used to test for evidence of rapid range expansion and the effects associated with colonization and spread of Microstegium vimineum. Within the James River Basin three genetically distinct clusters were identified that were not clearly associated with natural geographic boundaries and recent founder events were also inferred. The James River Basin also appears to act as a corridor for long-distance dispersal events. These findings contribute to our knowledge of the genetic consequences of rapid range expansion for invasive species, and more importantly, native species. Contrary to several studies, the present research also indicates that long-distance dispersal is not rare and can be a major contributor to the genetic structure following range expansion.

Invasions by alien understory plant species have gradually become a worldwide challenge in maintaining forest biodiversity and ecosystem productivity. Notably, Microstegium vimineum, a C4 grass native to Japan, has increased dominance in mixed deciduous hardwood forests of the southeastern United States, largely due to its shade tolerance and ability to spread through rhizomes. Microstegium vimineum also proliferates after prescribed fire, a common management tool used in the region to restore forests to desired conditions, and may compete with hardwood seedlings, hindering restoration efforts. Using two upland deciduous hardwood forest research sites in Marshall and Tate Counties, Mississippi, this study measured competitive effects of Microstegium vimineum on native hardwood seedling growth and survival with and without prescribed fire treatments. A single burning had no significant effect on Microstegium vimineum abundance, while the effect of Microstegium vimineum on seedling growth and survival varied by species.

<p>Species invasions are more prevalent than ever before. While the addition of a species can dramatically change critical ecosystem processes, factors that mediate the direction and magnitude of those impacts have received less attention. A better understanding of the factors that mediate invasion impacts on ecosystem functioning is needed in order to target which exotic species will be most harmful and which systems are most vulnerable. The role of invasion on nitrogen (N) cycling is particularly important since N cycling controls ecosystem services that provision human health, e.g. nutrient retention and water quality.</p><p>We conducted a meta-analysis and in-depth studies focused on the invasive grass species, Microstegium vimineum, to better understand how (i) plant characteristics, (ii) invader abundance and neighbor identity, and (iii) environmental conditions mediate the impacts of invasion on N pools and fluxes. The results of our global meta-analysis support the concept that invasive species and reference community traits such as leaf %N and leaf C:N are useful for understanding invasion impacts on soil N cycling, but that trait dissimilarities between invaded and reference communities are most informative. Regarding the in-depth studies of Microstegium, we did not find evidence to suggest that invasion increases net nitrification as other studies have shown. Instead, we found that an interaction between its abundance and the neighboring plant identify were important for determining soil nitrate concentrations and net nitrification rates in the greenhouse. In field, we found that variability in environmental conditions mediated the impact of Microstegium invasion on soil N pools and fluxes, primarily net ammonification, between sites through direct, indirect, and interactive pathways. Notably, we detected a scenario in which forest openness has a negative direct effect and indirect positive effect on ammonification in sites with high soil moisture and organic matter. Collectively, our findings suggest that dissimilarity in plant community traits, neighbor identity, and environmental conditions can be important drivers of invasion impacts on ecosystem N cycling and should be considered when evaluating the ecosystem impacts of invasive species across heterogeneous landscapes.</p> / Dissertation

Invasive species are reported as one of the top current and future concerns for the health and functioning of native ecosystems. In response, identification of invasive plant impacts are one of the top most studied aspects of plant invasions. Yet we still know very little about invasive plant impacts, and many land managers remove invasive plants not because of known negative impacts, but because of the general negative connotation associated with invasions. Here, I develop and utilize integrative methods to more holistically measure the ecological impacts of invasive plants. I develop a meta-analysis of current invasive plant impact literature, I integrate independent ecosystem metrics into holistic measures of total ecological impact, I examine the potential for legacy and temporal effects in newly established and recently managed invasions, and I measure ecological impact of co-invaded ecosystems. Through these studies, I find that magnitude, and not direction, of impact better represents actual ecosystem changes when evaluating invasions holistically. I also find invasive plant management may not only fail to remove long term legacy effects, but may in some cases further negatively impact the plant community. Finally, I find that co-invaded systems are not intrinsically worse off than single invaded systems. Collectively, these studies help to better our understanding of the impacts of invasive plants and their implications for management, and show that simple attempts at eradication may not always be desirable. / Ph. D.

Natural and anthropogenic disturbances have influenced forest stand structure and affected species compositions within forests for millennia. Disturbances such as fire, severe weather events, and forest management practices may result in significantly reduced tree biomass. Thus, these disturbances may lower canopy closure and stem density in support of forest management goals, such as open oak woodland restoration, or promote an increase in species richness within the forest understory. However, these types of disturbance may also produce the unwanted consequence of opening pathways for the invasion of non-native species. Once established, these non-native species may threaten native plant communities and biodiversity. Here, we elucidate how varied natural and anthropogenic disturbances have altered forest stand structure. One consequence of disturbance has been the increase in the presence and cover of non-native species within upland oak forest sites located in the Daniel Boone National Forest, Kentucky. The results of these studies suggest that timely and effective management strategies are needed to minimize non-native species impact on forested habitats following disturbance, and to promote the desired structural shifts that will sustain diverse habitats and support biodiversity.

<p><p>Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. <italic>Microstegium vimineum (Microstegium) </italic> is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of <italic>Microstegium</italic> on both plant diversity and N cycling in a riparian floodplain, paired plots of <italic>Microstegium</italic> hand-weeded and unweeded were established for three years. Plots without <italic>Microstegium</italic> increased from 4 to 15 species m<super>-2</super> and 90% of the newly establishing species were native. The <italic>Microstegium</italic> community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m<super>-2</super> year<super>-1</super>, respectively (p=0.05). Decomposition and release of N from <italic>Microstegium</italic> detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m<super>-2</super> year<super>-1</super>. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While <italic>Microstegium</italic> invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (&plusmn; 11% SE) Microstegium cover and species richness decreased to <8 species m<super>-2</super>. </p></p><p><p>To learn how <italic>Microstegium</italic> responds to increased N, we conducted a greenhouse competition experiment between <italic>Microstegium</italic> and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet <italic>Microstegium</italic> was most dominant in the high levels of N. </p></p><p><p>Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without <italic>Microstegium</italic> in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. <italic>Microstegium</italic> was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.</p> / Dissertation

Across the central and eastern United States, upland oaks are experiencing regeneration complications. This thesis comprises two studies evaluating challenges in regenerating upland oaks. One study evaluates upland Quercus spp., Carya spp., and Acer rubrum L. response to prescribed fire and impacts on seedling growth and mortality. This study found prescribed fire promoted oak growth when canopy cover was < 77%. Oaks displayed lower percent mortality and higher resprout capacity after fire compared to red maple. The second study investigates impacts of Microstegium vimineum presence on Quercus alba L., Liriodendron tulipifera L., and Acer rubrum L. seedlings under different light and moisture environments. Microstegium vimineum presence decreased growth of both sweetgum and red maple; white oak was less impacted, likely because of its conservative growth strategy and high-water use efficiency. Oaks’ conservative growth strategy may offer more resiliency compared to its competitors in response to prescribed fire and Microstegium vimineum.

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The spatial distributions of four plant species native to Asia, yet considered invasive in southwestern Virginia, were studied in order to produce predictive habitat maps. The study took place in the mountains to the north of Blacksburg, VA, on National Forest lands. A random GPS survey of each of the four species, Microstegium vimineum, Lonicera japonica, Rosa multiflora and Elaeagnus umbellate, was used in combination with a series of Geographic Information System (GIS) layers representing environmental variables (Elevation, Aspect, Roads, Trails, Streams, & Normalized Difference Moisture Index) to produce logistic regression models. After field- validating the models, the models were ranked according to usefulness, with the E. umbellate model proving most accurate. It is hoped that such GIS models will allow forest managers to more productively search for invasive species within their jurisdiction, by indicating sites more likely to provide habitat suitable to the invasive species described by the model. A non-GIS search for correlations between the study species' presence and field-collected discrete environmental variables was also included. Both Disturbance and Canopy Cover were considered for their effect upon Microstegium vimineum, Lonicera japonica, Rosa multiflora and Elaeagnus umbellate presence. Using Pearson's Correlation with the Canopy Cover data, and Chi-squared Correlation with the Disturbance data, only R. multiflora and E. umbellate showed significant correlation to decreasing canopy cover. / Master of Science