UC San Diego

Phenology, the timing of life-history transitions, is a crucial component of plant adaptation in seasonal environments because it determines the environmental conditions that each developmental stage interacts with. Adaptive evolution in phenological traits will contribute to the long-term persistence of plant species in the face of climate change and plant species invasions. The timing of emergence from seed is a highly adaptive phenological trait and has sizeable impacts on plant community structure, yet the potential for this trait to evolve and the subsequent effects on plant communities is unclear. The goal of my dissertation research is therefore to improve our understanding of how various factors shape the evolution of emergence timing and its impacts on plant communities. Using two California grasses, the native perennial Stipa pulchra (Hitchc.) Barkworth and the exotic annual Bromus diandrus (Roth), and a suite of other native and exotic herbaceous species as a study system, I carried out a series of field and greenhouse experiments that form the basis of each dissertation chapter. In Chapter 1, I demonstrate that a genetic trade-off between within- and among-year emergence timing can constrain evolutionary responses to joint selection on both traits, resulting in Pareto optimal trait combinations in more arid environments. In Chapter 2, I demonstrate that variation in the surrounding plant community influences the strength of selection for earlier emergence, although the selective effect of communities can vary between species. I also show that the strength of selection for earlier emergence can be influenced by the origin of neighbors, suggesting that changes in community composition are likely to alter patterns of selection on this trait. In Chapter 3, I demonstrate that mammalian herbivory weakens directional selection for earlier emergence in competition independently of reductions in mean fitness. In Chapter 4, I demonstrate that species that emerge earlier in the growing season are more susceptible to mammalian herbivory, particularly when herbivory occurs early in the growing season, and that early-season herbivory increases the relative performance of later-emerging native species when growing with earlier-emerging exotics. Together, these results demonstrate that the evolution of emergence timing is constrained by a genetic trade-off between its within-year and among-year components. Furthermore, these results demonstrate that biotic interactions, such as competition and herbivory, play an important role in determining selection on emergence timing within species and the community-level impacts of variation in emergence timing among species.