Soil microbes drive phylogenetic diversity-productivity relationships in a subtropical forest

The relationship between plant diversity and productivity, and the mechanisms underpinning that relationship, remain poorly resolved in species-rich forests. Here we combined extensive field observations and experimental manipulations in a subtropical forest to test how species richness (SR) and phylogenetic diversity (PD) interact with the putative root-associated pathogens, and how these interactions mediate diversity-productivity relationships. We show that (1) both SR and PD were positively correlated with biomass for both adult trees and seedlings across multiple spatial scales, but productivity was best predicted by PD, (2) significant positive relationships between PD and productivity were observed in non-sterile soil only, and (3) root fungal diversity was positively correlated with plant PD and SR, while the relative abundance of putative pathogens was negatively related to plant PD. Our findings highlight that soil pathogenic fungi play a key role in tree diversity-productivity relationships, and that increasing phylogenetic diversity may counteract negative effects of plant-soil feedback.