Patterns and mechanisms of tree recruitment in eastern United States forests

A major goal of ecology has been to understand the factors that contribute to the coexistence of species in communities. Particularly, the existence of a latitudinal gradient in species richness has been a focus of inquiry. Climatic variables such as temperature and precipitation correlate strongly with the gradient in species richness, but biological activity and ecological functions may vary with climate as well. In my PhD research, I investigated patterns of tree species diversity and the potential mechanisms that can affect those patterns across multiple spatial scales using multiple metrics.;First, through an experimental manipulation of an invasive plant species and vertebrate herbivory, I examined tree seedling performance and found that the invasion directly suppressed seedling performance and that there was an interaction with vertebrate herbivores. To examine potential microbial effects on tree seedlings, conducted greenhouse experiments with five native tree species. I found that seedling growth was reduced in conspecific soil compared to heterospecific soil for Platanus occidentalis, suggesting a negative plant-soil feedback that would conform to Janzen-Connell dynamics. To examine patterns of establishment and mortality of tree seedlings, field experiments at three mapped forests indicate many species display over-dispersed spatial patterns of recruitment near conspecific trees and seedlings experience greater mortality with increasing basal area of conspecifics which would lead to greater species coexistence. Finally, analyses of patterns of seedling establishment across the eastern half of the United States indicate that species that experiencing the greatest conspecific negative density dependence were regionally rare and that the average strength of conspecific negative density dependence was correlated with regional species richness.;Overall, the results of my dissertation chapters indicate the important roles of biotic mechanisms affecting species coexistence. The interaction of invasive plants and herbivores was clearly important for the performance of tree seedlings, but the most significant results of my research were to demonstrate that both large and small scale patterns of seedling establishment were indicative of conspecific negative density dependent interactions such that seedling mortality was positively correlated with increasing conspecific density. This is significant because the long-term coexistence of tree species is initially determined by seedling dynamics.