Department of Ecology and Evolutionary Biology, University of California, United States
The processes that shape diversity have long been of interest in ecology, but progress on this question has become more critical as biodiversity is threatened by climate change. One future consequence is that precipitation is generally expected to become more irregular which may have profound effects on plant coexistence. Modern coexistence theory suggests that coexistence mechanisms, such as the temporal storage effect, may be important in communities experiencing fluctuating abiotic conditions, and therefore understanding these phenomena can help predict how communities will respond to future variability. To examine the effects of temporal variation on coexistence, here we explore changes in species interactions over time in an annual grassland in southern California with high interannual variability in rainfall. Specifically, we set out to answer Q1) Is species performance correlated across years? Q2) Can species functional traits help us understand differences in species responses to wetter verse dryer years? Each year we set up plots within a cleared and fenced in area, and sowed seeds before winter rains began. We recorded germination and seed production for several species over 12 years, which included extremely dry and wet years. Additionally, we measured 11 key leaf, stem, and root functional traits for each species.
Preliminary analyses of eight annual plant species over 12 years show evidence that species demographic responses are not perfectly correlated across years, indicating that species differ in which years they perform best. Variation in response concentrates intraspecific interactions relative to interspecific interactions and favors coexistence. Additionally, species differences in functional traits, especially specific leaf area, rooting depth, specific root length, and leaf N were well correlated with differences in species demographic responses, such that species with similar traits did best in the same years. This sheds light on how coexistence is currently maintained in the community and provides context for future responses. By connecting these responses to functional traits, we hope to make the study more broadly useful and identify traits indicative of species responses to changing water availability.