UC Riverside

Global climate change is driving the rapid redistribution of plant species, and, in dryland ecosystems in particular, we lack an understanding of how species have and will continue to respond to novel abiotic conditions. This gap exists largely because, while the performance of plant species is influenced by abiotic conditions, interactions with co-occurring species at the community level are also key determinants of their persistence and success. Thus, to predict the novel plant assemblages of the future, we require a more mechanistic understanding of how plant species respond to variation in both abiotic and biotic conditions simultaneously. In this dissertation, I investigate how plant functional traits of populations can explain interspecific responses to long-term climate change (Chapter 1), how biotic interactions at the level of the community interact with abiotic drivers to structure the functional composition of plant assemblages (Chapter 2), and how the plant functional diversity of the surrounding neighborhood influences the interaction outcome of a focal species across drought conditions (Chapter 3). To address these questions, I utilized fine-scale observational plant community data from a steep elevational gradient in the desert mountains of southern California and a greenhouse experiment manipulating community trait diversity across contrasting abiotic conditions. In Chapter 1, I documented substantial range shifts among perennial species across a large aridity gradient and showed that functional traits related to resource use and biotic interactions are predictive of range dynamics spanning the last forty years. In Chapter 2, I discovered that both competition and facilitation are ubiquitous in plant communities and that their relative prevalence varies with abiotic conditions to structure the functional trait composition of plant communities. In Chapter 3, I show that both the community functional composition of neighboring species, the trait values of a focal species, and the distance between the two are all important determinants of the net interaction outcome of a focal species when growing in diverse assemblages. Overall, my dissertation highlights that plant functional traits sampled at the appropriate scale can lend predictability to species’ distributions and community-level interspecific interactions and that patterns of functional trait composition can be explained by accounting for diverse interactions and how they change across environmental gradients.