Ectomycorrhizal tree relative abundance is associated with higher aboveground biomass but reduced species richness and tree-size variation in temperate–subtropical transition forests

Ectomycorrhizal (EcM) tree species often dominate temperate and warm-temperate forests, yet how variation in their relative abundance is associated with species diversity and aboveground biomass (AGB) remains unclear. We analyzed a total of 6 ha of permanent forest monitoring plots in the temperate–subtropical transition zone of China. Using bivariate regressions, Gaussian linear mixed-effects models, and mixed-effects piecewise structural equation modeling, we evaluated the direct and indirect relationships linking mycorrhizal relative abundance, species richness, tree-size variation, and AGB. In the mixed-effects model, large EcM tree relative abundance and species richness accounted for 42.66% and 31.46% of the explained fixed-effect variation in AGB, respectively. The piecewise structural equation model further showed that large EcM tree relative abundance had the strongest direct positive association with AGB (β = 0.570, p < 0.001), whereas species richness had a slightly greater total effect than large EcM tree relative abundance (0.466 vs. 0.425). Large EcM tree relative abundance was negatively associated with arbuscular mycorrhizal tree relative abundance (β = − 0.509, p < 0.001) and tree-diameter variation (β = − 0.803, p < 0.001), but its total indirect effect on AGB was negative (− 0.145), indicating that the positive EcM–AGB relationship was dominated by the direct pathway rather than mediation through tree-size variation. These results indicate that mycorrhizal composition, tree diversity, and tree-size attributes are associated with AGB through distinct pathways in this temperate–subtropical transition forest.