Functional traits and the mechanisms of species coexistence in an annual plant community

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A fundamental aim of ecology is to understand the processes that maintain species diversity within communities. While biologists have long understood that differences between species in traits such as a bird's bill shape or the rooting depth of plants can maintain diversity in communities by promoting specialization and reducing competition, the generality of this assumption has not been widely tested. The goal of this research is to evaluate plant traits, such as leaf and fruit sizes, as predictors of the processes that can lead to either species coexistence or the exclusion of species though competition. The research will be conducted in a plant community adapted to specific soils in California. Testing if the links between such traits and coexistence can predict patterns of trait variation in natural communities can advance the field of ecology because it will link recently developed theoretical approaches for understanding species coexistence with advances in our understanding of plant functional traits. This research will help to anticipate the effects of global changes on biodiversity, and education and outreach activities will improve the nation's scientific human resources.The project will evaluate how plant functional traits predict competitive outcomes in annual plant communities on serpentine soils in California. It will do so at the local scales of species interaction and also at broader spatial and temporal scales where environmental heterogeneity can substantially influence coexistence. Spatially distributed removal and common garden experiments and observations will be used to parameterize models and test hypotheses on traits (e.g., specific leaf area, leaf N content, and integrated water use efficiency) and fitness at multiple scales. A neighborhood interaction experiment will parameterize competition coefficients in a mathematical model, which will be a basis for further hypothesis testing using comparisons to null models. At a broader spatial scale, population processes (e.g., seed production, germination, and survival) and plant functional traits will be measured across a range of biotic and abiotic gradients. Analyses of both these observations and the results of the interaction experiments will evaluate how well the model applies to natural conditions of environmental heterogeneity; the statistical analyses will use null models for trait dispersion patterns at multiple scales. Theoretically, the study will connect traits to the stabilizing niche differences and average fitness differences that actually drive coexistence, competitive exclusion, and community pattern. The project represents a significant advance over previous studies by firmly incorporating functional traits into the species coexistence framework, a framework that applies to a wide range of problems in community ecology. This project will support the training and mentoring of one postdoctoral researcher and one graduate student; foster international collaboration between students; augment undergraduate education; and develop outreach for early career scientists.