Functional traits have been widely used to study the structure and dynamics of terrestrial plant communities. However, the connections between functional trait variation and the mechanisms promoting species coexistence have often been ambiguous or poorly justified from the perspective of ecological theory. In addition, we have a poor understanding of the extent to which individual traits contribute to multiple coexistence mechanisms, such as local resource partitioning or the temporal storage effect. In order to strengthen our understanding of the mechanisms through which trait variation contributes to species coexistence, we have developed a series of experiments with serpentine annual plants in coastal California. These projects allow us to assess variation in plant lifetime fitness in response to a range of abiotic and biotic conditions, and therefore to quantify the potential for species coexistence at a range of spatial and temporal scales. Specifically, here we assess the extent to which functional trait differences can be used to understand coexistence at the scale of neighborhood interactions, across an edaphically and climatically heterogeneous landscape, and across years (corresponding to plant generations) where rainfall and other climatic properties vary from year to year.
Results/Conclusions
Over several years of experiments, we have gathered data on interactions between >25 annual plant species, as well as detailed sampling of functional and physiological traits related to carbon gain, water use, above and belowground architecture, dispersal ecology, and phenology. In earlier experiments at the neighborhood spatial scale, we found that many traits drive competitive dominance, but that the niche differences promoting coexistence were correlated with a multi-trait measure capturing species differentiation in multiple ecological dimensions. We also found that few of our species pairs were predicted to be able to coexist indefinitely at the neighborhood scale, implicating the role of coexistence mechanisms operating beyond the neighborhood scale in allowing persistence across the landscape over time. Using 24 common gardens arrayed across the landscape, we have found that species demographic responses to spatial environmental variation are partially decoupled, showing that subsets of species have contrasting environmental preferences. The degree to which species have common demographic responses to the environment was correlated with similarity in several functional traits, including phenology and leaf economics traits. Taken together, these studies experimentally show how it is possible for single traits (such as phenology) to shape coexistence at both local and landscape scales.