Functional diversity of chemical defense and morphological traits and reveal biotic and abiotic drivers of tree community assembly

Background/Question/Methods

: Models of community assembly often focus on environmental conditions that either select for a narrower diversity of functional traits than expected by chance (underdispersion) or promote niche differentiation and a broader diversity of functional traits than expected by chance (overdispersion). However, these patterns have primarily been explored with morphological functional traits that do not explicitly incorporate biotic processes such as trophic relationships. Chemical defense traits provide an alternative perspective on community assembly that is driven by plant interactions with natural enemies. At the community scale, plant-enemy interactions should increase overdispersion in plant chemical traits when variation in chemical defense at local scales minimizes suitable hosts for natural enemies. Here, we leverage recent advances in plant metabolomics to examine community-wide variation in leaf secondary metabolites related to defenses against natural enemies. We examine the functional diversity of leaf secondary metabolites and traditional plant morphological traits at the local community scale in three temperate forest dynamics plots distributed across North America. Specifically, we ask: 1) how patterns of morphological and chemical defense trait functional diversity change along environmental gradients of soil resources and topography; and 2) what can we infer about community assembly processes from these two suites of traits?

Results/Conclusions

: We find that chemical defense traits are often orthogonal to morphological traits, adding a unique dimension to functional trait space. As predicted, chemical defense traits tended to be overdispersed relative to null expectations while morphological traits tended to be underdispersed. Chemical defense traits and morphological traits also showed contrasting patterns along environmental gradients at each site. When considering individual traits, community weighted mean trait values and deviations in functional dispersion were both weakly related to environmental conditions. Taken together, these results show that community composition is determined both by species sorting across environmental gradients (underdispersed morphological traits) and biotic interactions among plants and natural enemies (overdispersed chemical defense traits). Investigating multiple dimensions of functional trait composition can reveal the importance of bottom up and top down mechanisms of community assembly.