Neighborhood interactions and soil chemical properties co-regulate woody plant species diversity across different relative densities of arbuscular mycorrhizal (AM)-associated tree species in subtropical natural forests of southern China

Mycorrhizal fungi play a functional role in nutrient absorption and transfer in ecosystems, influencing plant survival and interspecific interactions. However, it is unclear how different types of mycorrhizae affect symbiotic tree species in subtropical regions. To investigate the patterns of tree species diversity and the driving factors across different mycorrhizal-dominated communities in the Wuyi Mountains of China, we established four plots (20 m × 20 m) at each interval along a gradient of increasing arbuscular mycorrhizal (AM) tree dominance (10% to 90%), corresponding to a concurrent decline in ectomycorrhizal (EcM) tree dominance. Partial least squares path modeling (PLS-PM) was applied to quantify the direct and indirect effects of environmental variables on woody plant species diversity along a gradient of relative density of AM-associated tree species. Our results demonstrated that (1) species diversity significantly increased with the rising relative density of AM-associated trees; (2) increasing relative density of AM-associated trees was accompanied by distinct variations in community phylogenetic structure, spatial aggregation, competition intensity, conspecific negative density dependence (CNDD) strength, stand structural diversity, and soil chemical properties among plots; (3) the partial least squares path model revealed that the relative abundance of AM-associated trees promoted woody plant diversity through two indirect mechanisms: first, by enhancing community phylogenetic diversity, thereby reducing competitive exclusion and facilitating species coexistence; and second, by intensifying CNDD among AM trees, which decreased community aggregation and further mitigated the negative effects of competition, ultimately enhancing species diversity. These findings highlight the pivotal role of AM tree dominance in shaping diversity patterns, offering key insights for forest management and biodiversity conservation in subtropical ecosystems.