Asymmetrical pegs in square holes? Functional and phylogenetic determinants of plant community assembly in temperate forest understories

Despite advances in community assembly theory, uncertainties remain regarding how ecological and evolutionary processes shape species distributions and communities. We analyzed patterns of occurrence for 139 herbaceous plant species across 257 forest stands in Wisconsin (USA) to test predictions from community assembly theory. Specifically, we applied Bayesian phylogenetic linear mixed effects models (PGLMMs) to examine how functional traits and phylogenetic relationships influence plant distributions along environmental gradients and how functional similarity and phylogenetic relatedness affect local species co-occurrence. Leaf height, specific leaf area, and seed mass mediate species distributions along edaphic, climatic, and light gradients. In contrast, functional trait similarity and phylogenetic relationships only weakly affect patterns of local co-occurrence. These results confirm that broad-scale plant distributions are largely shaped by ecological sorting along environmental gradients but suggest deterministic assembly rules based on niche differentiation and complementary resource use may not govern local species co-occurrence in homogeneous environments. Statement of authorshipJB conceived the idea for the study. DL, SJ, and DR collected the vegetation and functional trait data. JB analyzed the data with assistance from DL. KC, KS, TG, and DW secured funding for research and oversaw data collection. JB wrote the first draft of the manuscript, all authors contributed to manuscript revisions. Data accessibility statementUpon acceptance, data will be archived at Figshare (https://figshare.com/) and scripts used to analyze the data will be shared on Github (https://github.com/jaredjbeck/).