Quantifying the effects of spatial environmental variation and soil microbes on plant community dynamics

Understanding the processes that determine the diversity and dynamics of plant communities is a longstanding challenge in ecology. Many studies have inferred the role of demographic processes by studying patterns of functional trait variation in natural communities, but studies explicitly linking such functional trait differences to demographic processes are lacking. There has also been a growing realization that the dynamics of plant communities are also influenced by the composition of the soil microbial community, but despite hundreds of empirical studies, predicting the influence of soil microbes on the diversity and dynamics of natural plant communities remains a challenge. In my dissertation I couple ecological theory with field and greenhouse experiments to build a more complete and generalizable understanding of the processes that control plant biodiversity. In Chapter One, I ask whether community-wide shifts in three key plant functional traits across an environmental gradient reflect variation in the trait-performance relationship across the landscape. To address this question I coupled observational data of variation in plant composition and functional with experimental data on species performance across the same landscape. I asked whether observed trait-environment interactions in the experimental data match observed patterns of trait variation. I found that shifts in community-weighted mean traits generally reflect the direction of trait-environment interactions. But on the whole, the interactions we found were weak, and by themselves might not be sufficient to explain community-wide shifts. This supports the value of plant functional traits for predicting species responses to environmental variation, and highlights a need for more detailed evaluation of how trait-performance relationships change across environments to improve such predictions. Chapters Two and Three focus on how soil microbes can influence diversity in plant communities. Chapter Two begins with a re-analysis of a classic framework that has been extensively used to study how feedbacks between plants and soil microbes can influence species coexistence. A great deal of existing theoretical and empirical work has shown that soil microbes can promote plant coexistence when they generate stabilizing feedback loops, or can drive exclusion when they generate destabilizing feedback loops. I applied insights from modern coexistence theory to show that existing work has largely neglected another avenue by which plant-soil feedbacks can mediate plant coexistence, by driving average fitness differences between plants. This chapter also extends classic models of plant-soil feedback to include more biological detail to show how the effects of plant-soil feedback on plant coexistence depends critically on how plants interact with each other through other processes like resource competition. In the final chapter of my dissertation, I applied the insights from Chapter Two to ask how plant-soil feedbacks influence diversity in southern California annual grassland communities. I conducted a greenhouse experiment to quantify microbially mediated stabilization and fitness differences among fifteen pairs of annual plants. We found that soil microbes frequently generate negative frequency-dependent dynamics that stabilize plant interactions, but they simultaneously generate large average fitness differences between species. The net result is that if the plant species are otherwise competitively equivalent, soil microbes would often drive exclusion among the focal species. This work illustrates the importance of quantifying microbially mediated fitness differences, and points to important avenues for future studies on how soil microbes shape plant diversity