Structural diversity primarily drives forest biomass along altitudinal gradients in subtropical forests

Empirical evidence indicates that structural diversity plays a crucial role in enhancing forest biomass. However, the mechanisms through which forest biomass is jointly influenced by structural diversity, biodiversity, and stand factors across environmental gradients remain controversial. Using 115 plots (comprising 7000 trees from 279 species) sampled from subtropical forest along an altitudinal gradient in the Hou’he national nature reserve in central China, we analyzed the effects of structural diversity on forest biomass in comparison with biodiversity (taxonomic, phylogenetic, and functional diversity), stand density, soil fertility, and climatic factors using structural equation model and hierarchical partitioning analysis. Our results illustrated that: (i) structural diversity, rather than biodiversity or stand density, primarily promoted forest biomass along an altitudinal gradient. (ii) Biodiversity and soil fertility indirectly increased forest biomass via structural diversity, likely through providing more species with diverse ecological strategies that enhance structural diversity, while stand density indirectly reduced biomass through structural diversity. (iii) The positive direct effect of structural diversity on forest biomass was consistently strong across low-, middle-, and high-altitudinal zones. Meanwhile, the total positive effect of soil fertility and biodiversity on forest biomass decreased from low- to high-altitudinal zones. Our results suggest that structural diversity plays a pivotal role in mediating the effects of other abiotic and biotic factors on forest biomass, highlighting the need for future research priorities to examine how biodiversity, stand factors, soil fertility, and climate influence structural diversity, which increases the complementarity of canopy and soil spatial niches.