Resource-poor environments drive more prevalent habitat associations in ectomycorrhizal tree species: Evidence from tropical, subtropical, and subalpine forest dynamics plots

ABSTRACT Niche differentiation is a fundamental mechanism underlying tree species distribution patterns and coexistence in forest communities. Beyond the topographic heterogeneity that shapes species local distributions, recent studies demonstrate that resource niche differentiation mediated by mycorrhizal types may also influence local community structure, and we still lack knowledge to assess their importance for species coexistence. In this study, we employed torus-translation tests to analyze species-habitat associations in three 20-ha forest dynamics plots: tropical seasonal rainforest, subtropical semi-humid evergreen broad-leaved forest, and subalpine cold-temperate coniferous forest, with particular emphasis on the habitat association pattern of ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) tree species. We found that 60.4% of species in the tropical forest, 55.8% in the subtropical forest, and 31.8% in the subalpine forest exhibited significant associations with at least one habitat type, indicating that topographic niche differentiation plays a major role in maintaining species coexistence. Furthermore, we observed a consistent pattern across all three forests: EcM tree species (90.9% in the tropical forest, 75.0% in the subtropical forest, and 100.0% in the subalpine forest) exhibited higher proportions of habitat associations than AM tree species. This divergence resulted from their contrasting resource utilization strategies. Specifically, EcM tree species were positively associated with certain topographic features (e.g., ridges), and exhibited a significant negative correlation with soil nutrients, indicating that they are primarily distributed in resource-poor environments. Such areas exhibit intensified environmental filtering, resulting in tighter habitat associations for EcM than AM tree species. Our findings demonstrate that mycorrhizal-mediated niche partitioning plays a crucial role in shaping forest diversity patterns and promoting species coexistence.