Species, trait, and phylogenetic associations in the alpine tundra: Insights into the processes that structure plant communities

Understanding the factors that structure plant communities has been central to the study of ecology and recently, trait-based approaches have been advocated as an avenue to explain the different processes suggested to affect community assembly. This dissertation uses trait, species, and phylogenetic approaches to understand the processes that structure alpine plant communities. Chapter 1 investigates how a species (Castilleja occidentalis) with a specific suite of traits impacted community structure and ecosystem function. I found that while Castilleja has only a minor effect on diversity it boosted productivity via its effects on nutrient cycling. Chapter 2 examines the pattern of species, trait, and phylogenetic associations along a productivity gradient to relate these results to the different processes suggested to affect species coexistence within plant communities. I found that less productive communities were structured by environmental filters, belowground competition, and dispersal, while more productive communities were structured by aboveground competition and priority effects. Chapter 3 examines the pattern of species and trait associations in areas of the alpine tundra that vary in geographic isolation. Incorporating geographic isolation into this work allows for a greater understanding of the role of dispersal in affecting community structure. I found that more isolated sites were more similar to each other because dispersal could not counter deterministic local extinctions. Finally, Chapter 4 mechanistically explores a core assumption of trait-based approaches: that there will be greater competition between species with similar traits. I found that species that are more similar in five key traits compete more and that this competition can be mediated by trait plasticity. This research adds to growing evidence of the power of trait based approaches in understanding the factors that structure plant communities. Additionally, this work helps to resolve the historical mismatch between the fine resolution in ecological field work and the broad resolution of biographical models. Lastly, this research advances my scientific understanding about important controls on plant biodiversity in a region known for its extraordinary natural resources, the alpine tundra.