UCLA

This dissertation aims to contribute to our understanding of plant coexistence and explore how global change could disrupt these dynamics thus altering the composition of future communities. I have attempted to answer these broad ecological questions by studying the coexistence mechanisms operating in an annual grassland in coastal southern California that experiences high interannual variability in climate, particularly in precipitation. I have explored multiple mechanisms of coexistence operating in the community and the physiological bases of the interacting plant species to make my results more broadly applicable. Each chapter also discusses how the results can inform our predictions about how plant communities will continue to respond to global change.

Chapter one explores how interactions between neighboring species are altered by changes in rainfall. Precipitation patterns have long been known to shape plant distributions but how changes in these patterns effect species interactions and thus community composition is less understood. As precipitation patterns across the globe are altered by global change, understanding how interactions like competition between plants is impacted will help us anticipate potential community composition changes. We studied how changes in precipitation altered competitive dynamics by studying the direct effects of changes on individual species, as well as, by the changing strength of competitive interactions between species. We grew six annual species under two rainfall conditions with varying densities and identities of competitors. We parameterized a population growth model that allowed us to determine stabilizing niche differences and fitness differences between species pairs which determine their ability to coexist. We found that reduced precipitation had little direct impact on species grown alone, but it qualitatively shifted predicted competitive outcomes for 10 of 15 species pairs. We also found that species that were more similar in their functional traits were less likely to experience changes in their competitive outcomes than species that were less similar.

In chapter 2, we investigated the mechanism that might be driving the changes in species competitive interactions that we found with changes in precipitation. We hypothesized that species flowering phenology (timing) might contribute to species ability to coexist by separating resource intensive periods for species over the growing season. These critical temporal dynamics could be disturbed if changes in precipitation affect the flowering phenology of some species and not others. We found that changes in rainfall shift some species flowering phenology, but sensitivity differed among neighboring species. Four of seven species we studied started and/or peaked flowering earlier in response to reduced water availability. The idiosyncratic responses among neighboring species has the potential to disrupt temporal coexistence mechanisms because it alters the flowering overlap between species pairs. We found the species pairs whose competitive interactions changed in the experiment described in chapter one had larger differences in their phenological responses to reduced rainfall than pairs whose competitive outcomes did not change. This shows that species pairs whose flowering time overlap changed more, were more likely to experience a change in their competitive interaction. Therefore, current temporal spacing between peak flowering times likely contributes to coexistence in the community and if changes in rainfall disrupt this, species may lose their ability to coexist, altering the composition of the community.

Chapter 3 explores coexistence at a broader timescale and investigates how multiple mechanisms of coexistence operate simultaneously. Southern coastal California experiences high interannual variation in rainfall. Modern coexistence theory suggests that coexistence mechanisms, such as the temporal storage effect, may be important in communities experiencing fluctuating abiotic conditions. To examine the effects of temporal variation in abiotic conditions on coexistence, we studied an annual grassland community that experiences high interannual variation in precipitation. We found that species demographic rates from the last 15 years, including germination rate and low-density fecundity, are rarely strongly positively correlated with other species in the community, indicating that species differ in which years they perform best, and therefore likely specialize on distinct abiotic conditions. Variation in response to interannual differences in rainfall concentrates intraspecific interactions relative to interspecific interactions and favors coexistence. Additionally, we found that species differences in functional traits, especially rooting depth, water use efficiency, and leaf nitrogen were well correlated with differences in species demographic responses, such that species with similar traits did best in the same years. Taken together this deepens our understanding of coexistence in the community and provides greater context for how plant communities may respond to future increases in climatic variability.