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

Biogeographical variation in community assembly processes forms the basis
of the latitudinal gradient of biodiversity by driving b-diversity.
Classical studies on community assembly predict environmental filtering
affecting b-diversity more strongly at higher latitudes, where
productivity is lower and abiotic stress is stronger. Contrary to this
prediction, recent evidence indicates that plant community composition at
higher latitudes exhibits more spatially clustered distributions
independently of background environments, suggesting the importance of
spatial processes, such as priority effects. In this study, we propose a
hypothesis that resolves this paradox by considering plant-soil feedback
and biogeographic variations in the dominant mycorrhizal type: we predict
that 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 >10º 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 likely due to
positive plant-soil feedback. Consequently, (iii) tree species composition
at higher latitudes was better explained by spatial variables suggesting
the importance of priority effects. Consistent with the predictions of the
plant-soil feedback theory, these patterns were more pronounced in
understory than in canopy communities. Taken together, our results lend
support to our hypothesis that biogeographic variation in tree community
assembly patterns is defined by mycorrhizal types and plant-soil feedback,
thereby resolving a paradox in the latitudinal gradient of plant community
assembly. Our work highlights that plant mycorrhizal type underlies the
determinants of b-diversity which is a critical component of the
latitudinal gradient of diversity.