Host Developmental Stage and Vegetation Type Govern Root EcM Fungal Assembly in Temperate Forests

Ectomycorrhizal (EcM) fungi are critical mediators of forest succession, yet the relative contributions of stochastic (neutral) and deterministic (niche-based) processes in shaping their communities are still poorly understood. We investigated the assembly processes in root EcM fungal communities across juvenile and adult coniferous (<i>Abies nephrolepis</i>, <i>Picea jezoensis</i>, and <i>Pinus koraiensis</i>) and broadleaf (<i>Acer mono</i>, <i>Betula platyphylla</i>, and <i>Quercus mongolica</i>) tree species in northeastern China. Employing neutral theory modeling, alpha and beta diversity metrics, and a random forest analysis, we identified patterns of EcM fungal community assembly and the specific taxa associated with developmental stages of various hosts. Neutral processes contributed to the variation in fungal communities, with adult trees showing a higher explanation power (more than 33% of variation) compared to juvenile trees (less than 7% of variation), reflecting a successional shift in assembly mechanisms. Dispersal dynamics was pronounced in juveniles but diminished with host age. Additionally, alpha diversity increased with host age and was slightly moderated by host identity, while beta diversity reflected stronger effects of host age (PERMANOVA R² = 0.057) than host identity (R² = 0.033). Host age and identity further structured communities, with distinct taxa varying between juvenile vs. adult, and coniferous vs. broadleaf hosts. Our results demonstrate that host maturity drives a transition from deterministic to stochastic assembly, modulated by tree species identity, improving our understanding of plant-fungal dynamics during forest succession.