Functional trait differences and species coexistence in an annual plant community

Background/Question/Methods

A fundamental aim of ecology is to understand the processes that promote and maintain diversity within communities. While the processes enabling coexistence are well understood from a theoretical perspective, predicting the operation of these mechanisms in diverse, multi-species assemblages in nature remains a difficult endeavor. Particularly challenging is generalizing findings on the mechanisms of diversity maintenance from one system to another, or predicting how those mechanisms will change under future environmental conditions. One promising path forward is to integrate recently developed quantitative approaches for understanding species coexistence with advances in our understanding of plant functional traits. Functional trait approaches to plant communities offer a way to gain insight into strategy variation across species and to predict responses of species to global change. Yet thus far, ecologists have primarily connected functional traits to observational patterns in communities, not the theoretical drivers of coexistence and competitive exclusion with which functional traits are more mechanistically linked. Here we present a field experiment with serpentine annual plant communities designed quantify two of the key drivers of coexistence: stabilizing niche differences and relative fitness differences. We then test whether differences in key field-measured functional trait differences between species predict either of these two components.

Results/Conclusions

Relatively little coexistence was predicted to occur among the species pairs in the experiments based on one season of measurements, suggesting that broader-scale environmental heterogeneity and temporal fluctuations in climate likely contribute to species coexistence at the site. Functional traits were relatively poor predictors of stabilizing niche differences among species. However, a number of traits, including specific leaf area, rooting depth, phenology and maximum height were good predictors of relative fitness differences among species.  These results suggest that at small spatial scales of interactions, commonly measured plant functional traits primarily predict competitive dominance among species. Implications for functional and phylogenetic patterns within communities will be discussed.