Ecologists frequently use functional traits as a means of exploring how communities are structured, and recent advances in functional and theoretical ecology have demonstrated that trait differences can influence species coexistence in unexpected ways. However, most trait-based ecology analyses have assumed that functional traits do not vary among individuals within a species. In particular, it remains unclear how phenotypic plasticity in functional traits in response to competitor identity and density may influence community dynamics. For example, trait plasticity in response to competition could alter stabilizing niche differences and fitness differences, which jointly determine competitive outcomes. To explore this issue, we measured functional traits of annual plants experimentally planted in the field with both high and very low densities of competitors to assess the potential for intraspecific trait shifts in response to competitors. Specifically, we measured leaf, root, and whole plant functional traits of 15 species. As previous work at the site has quantified the niche and fitness differences of the species pairs and shown how these quantities relate to differences in functional traits, we frame our results in terms of potential changes to competitive outcomes.
We measured specific leaf area, leaf dry matter content, leaf area, specific root length, and canopy shape for >200 individuals growing in the two competitive treatments. In preliminary analyses of a subset of six species, we found species-specific changes in key leaf functional traits in response to the presence of competitors. These traits are known to be related to both stabilizing niche differences (which promote species coexistence) as well as fitness differences (which drive competitive exclusion). These results highlight the important role that plastic trait changes in response to competitors may play in altering the outcome of species interactions.