Topographic and biotic factors determine forest biomass spatial distribution in a subtropical mountain moist forest

A mechanistic understanding of factors driving forest biomass will help stewards manage carbon storage in forests. We examined the potential biotic and topographic factors in regulating subtropical forest carbon storage. We utilized data from the Badagongshan 25 ha (500 × 500 m) forest dynamics plot to examine the factors regulating the spatial variation of large trees and forest biomass. We mapped forest biomass and large tree biomass distributions and applied variation partitioning analysis to examine a suite of topographic and biotic factors related to the distributions. The average biomass of the 25 ha plot is 252.7 Mg/ha but varied substantially from 39.16 to 1024.53 Mg/ha in the 20 × 20 m quadrats. Overall, large tree (diameter at breast height ⩾25 cm) density accounted for 71% of variation in forest biomass distribution. Variance partitioning showed that biotic, topographic and spatial factors altogether explained 64.8% and 57.5% of the variation in the distribution of forest biomass and large tree density, respectively. Fractions of variance explained by the convexity and topographic wetness index (TWI) were much larger than other topographic variables in both distributions. For biotic variables, stem density and wood specific gravity were important in predicting forest biomass and large tree density distributions. Both biomass and large tree density showed an increasing trend with increasing convexity, stem density and wood specific gravity, but decreased as TWI increased. Convexity and TWI explained more variation among topographic variables, indicating that water deficiency may play an important role in shaping the distribution of forest biomass and large tree density. In conclusion, the crucial relationship between forest biomass and large tree density distribution should attract more attention, and suggests a mechanistic control of forest carbon storage that may help provide options in forest carbon management.