The mechanisms generating community phylogenetic patterns change with spatial scale

Studies that test community assembly hypotheses in observational communities frequently evaluate patterns for plots or entire communities, yet studies that examine assembly patterns across spatial scales show that they are greatly influenced by scale. Here, we test the spatial dependency of patterns of relatedness and plant height for all individual herbaceous plants along five 40-m old-field transects (Southern Ontario, Canada). We identified each individual plant and measured its distance along the transect and its height, and we constructed a molecular phylogeny for all observed species. To uncover the scale at which community phylogenetic and trait similarities shift, we used partial Mantel correlograms and distance-based Moran Eigenvector Maps (dbMEMs). We found that communities shift from significantly overdispersed at relatively smaller scales (i.e., < 15 m) to spatially clustered at larger scales, showing that assembly mechanism influence depends on scale of observation. This pattern was observed for both phylogeny and height, but was the strongest when considering phylogeny only. These results reveal the importance of spatial scale when examining community phylogenetic or trait patterns, where finding support for one assembly mechanism at a single scale does not necessarily mean that other mechanisms are also not important for structuring community composition and diversity.