Scale Dependence of Woody Plant β-Diversity in a Tropical Rainforest Metacommunity

β-diversity is a central concept to understanding community assembly, yet how its decomposition components and underlying drivers vary with spatial extent remains insufficiently understood. We used a nested metacommunity framework to examine the scale dependence of total β-diversity and its species replacement and richness difference components, and to identify how environmental and spatial predictors contribute across scales. We analyzed woody plant diversity in 250 plots arranged on a contiguous 1 × 1 km grid in a tropical rainforest on Hainan, China. By aggregating nearest-neighbor plots into metacommunities, we constructed a continuous gradient of spatial extent and quantified total β-diversity (BDtotal), species replacement (Repl), and richness difference (RichDif) for each metacommunity realization. β-diversity increased modestly with spatial extent, but its internal structure was remarkably stable: species replacement consistently dominated total β-diversity, whereas richness differences contributed less and changed little across scales. Environmental heterogeneity was positively associated with β-diversity, primarily through replacement rather than richness differences. The variation explained by both environmental predictors and spatial structure increased with extent before approaching a plateau. Spatial structure explained the largest share of variation in BDtotal, Repl, and RichDif, while the relative importance of environmental predictors shifted with scale: soil variables were more influential at finer extents, whereas bioclimatic variables gained importance at broader extents. Environmental predictors were more strongly associated with species replacement than with richness differences, whereas spatial structure explained both components to a similar degree. Overall, increasing spatial extent amplified the magnitude of β-diversity more than it altered the balance between its components, revealing species replacement as a scale-robust feature of tropical forest metacommunities. These findings show that tropical forest β-diversity emerges from the joint effects of environmental heterogeneity and spatial structure, and they highlight the need to conserve spatially dispersed, compositionally complementary habitat mosaics rather than focusing solely on local richness hotspots.