Habitat-area thresholds structure multidimensional bryophyte diversity and environmental associations across fragmented karst mountains

Habitat fragmentation creates isolated patches in which conservation-relevant area thresholds may differ among biodiversity dimensions and ecological groups. Using 11,106 bryophyte specimens from 105 fragmented karst mountains in southwestern China, we quantified species richness (SR), phylogenetic diversity (PD), and functional richness (FRic), and combined breakpoint models, regression, random forests, and hierarchical partitioning to evaluate habitat-area thresholds and size-dependent environmental associations. All three biodiversity dimensions showed SIE patterns. Overall SR and PD thresholds converged near 4.41 ha, whereas FRic showed a higher threshold of 31.92 ha. SR-derived taxon-specific thresholds ranged from 3.12 ha for pleurocarpous mosses to 6.82 ha for liverworts; all five thresholds used to classify small-area and large-area karst mountains fell within their bootstrap 95% confidence intervals. Environmental associations also shifted across size domains: vegetation, elevation, and shape often had larger coefficients in large-area mountains, whereas isolation was relatively important in several small-area mountain random-forest models. Habitat area was therefore associated with both multidimensional biodiversity patterns and changes in measured biodiversity-environment relationships. These results support a scale-dependent, taxon-specific view of habitat thresholds and suggest that conservation based only on species richness may underestimate the area needed to retain functionally diverse assemblages.