Plant-mycorrhizal associations may explain the latitudinal gradient of plant community assembly

Biogeographical variation in community assembly patterns forms the basis of large-scale biodiversity gradients. Previous studies on community assembly have suggested the dominant role of environmental filtering at higher latitudes. However, recent evidence indicates that plant community distributions at higher latitudes exhibit more spatial clustering, suggesting the importance of spatial assembly processes. In this study, we propose a hypothesis that resolves this paradox by incorporating biogeographic variations in dominant mycorrhizal types: the increasing prevalence of ectomycorrhizal (EcM) trees with latitude contributes to the spatially clustered distribution of plants, as EcM trees tend to exhibit positive plant-soil feedback. We analyzed a large-scale standardized dataset of Japanese forests covering a latitudinal gradient of >10{degrees} and found that (i) the proportion of EcM trees was higher at higher latitudes, and (ii) EcM tree-rich communities exhibited more spatially clustered distributions. Consequently, (iii) the tree species composition at higher latitudes was better explained by spatial variables. Moreover, consistent with predictions derived from the plant-soil feedback theory, these findings were particularly evident in the understory rather than the canopy communities. This study demonstrates that plant-soil feedback influences plant community distributions at metacommunity scales, thereby giving rise to a paradox within the latitudinal gradient of plant community assembly.