| This dissertation addresses the consequences of reciprocal interactions between the plant and the soil microbial communities, and how those interactions affect nutrient cycling and plant competition during exotic plant invasion. Each chapter is linked by the common theme of evaluating the importance of these feedbacks to the rate of plant invasion in the forest understory.;In the first two chapters, I utilize microcosms to evaluate the importance of leaf litter inputs for plant-soil feedback. The first chapter demonstrates that leaf litter from native and exotic plants create divergent soil microbial communities, altering soil enzyme activities and nitrogen cycling, which in turn affects the growth of native and invasive plants. However, while this plant-soil interaction affects growth rate, it does not change the competitive hierarchy or the success of the invasive plant. The second chapter shows how the effect of an exotic species' leaf litter on soil microbes varies over a range of invasion severity. Using litter mixtures ranging from 0% to 100% exotic litter, I show that ecosystem-level effects of invasion on carbon and nitrogen cycling are linearly related to the exotic plant density, while community-level effects on soil microbes are non-linear and very sensitive to low levels of invasion.;In the final chapters, I extend these results to examine whole-plant effects in more natural plant communities. The third chapter uses a large-scale field experiment to explore the temporal dynamics of invasion impacts. I show that the short-term impact of native and invasive plants on soil microbes is weak, while long-term effects are much stronger. However, after restoration of native plants, the legacy effect of invasion remains strong. Using experimental litter-removal, I also show that belowground plant litter more strongly influences the soil microbial community than aboveground litter. The fourth chapter examines how the diversity of the native community influences the invasion impact on soils. I show that while diversity has little direct effect, individual native plant species can influence how an exotic invasive shrub affects the soil ecosystem. Together, these results show that the importance of plant-soil feedbacks for exotic invasion is context- and scale-dependent, exhibiting nonlinear dynamics that depend on the native community and the degree of invasion, and vary in strength over time. |
