Community assembly in revegetated serpentine grassland: Functional vs. taxonomic perspectives

Effective restoration depends on understanding and manipulating the process of community assembly. Functional traits offer potential for better understanding of ecological processes, but of the many traits to measure, which ones are most relevant for community assembly? We used a California serpentine grassland as a model ecosystem and focused on plant traits most closely related to resource use to try to better understand the role of complementarity in structuring plant communities. We divided species into eight groups depending on their phenology (a proxy for rooting depth and timing of maximum resource demand), annual vs. perennial (a proxy for root structure and storage), and nitrogen-fixation status. We then compared species and functional composition across topographic gradients in natural grassland, and over time (5 years) in experimental plots that were allowed to colonize naturally from different initial community compositions. Resource availability differed significantly across topographic gradients in natural grassland, as did species composition. However, functional group composition was more similar than expected at random, suggesting that complementarity played a role in structuring communities in all these microenvironments. Similarly, functional composition across experimental plots converged over time, despite initially strong differences and despite continued differences in species composition. Our results suggest that multiple community processes interact to determine composition: 1) complementarity among functional groups across major strategies of resource use; 2) competition among species within a given functional group, the outcome of which is determined by the particular environmental conditions; and 3) stochastic processes related to dispersal limitation for functionally similar species.