ABSTRACTCarbon storage in subtropical forests results from multiple interacting factors, including biodiversity attributes—such as species diversity, functional traits, functional diversity, and stand structural diversity—and environmental conditions like climate, topography, and soil characteristics. Biodiversity typically influences forest carbon through two primary mechanisms: niche complementarity (complementary utilization of resources among species) and selection effects (dominance of species with specific functional traits). However, the relative importance of these mechanisms in involving subtropical forest carbon storage under varying environmental conditions remains unclear. This study assessed diversity attributes within tree, shrub, and herb layers across three subtropical forest types: coniferous, coniferous/broad‐leaved mixed, and broad‐leaved forests. We quantified forest aboveground, belowground, and total carbon storage and examined the relationships between forest diversity and carbon storage, and the impact of environmental factors on these relationships. Our findings showed that broad‐leaved forests were more conducive to creating carbon storage. In subtropical forests, belowground carbon, a major component of carbon storage, was most affected by environmental factors, followed by species diversity. Functional diversity most strongly exerted influences on aboveground and total carbon stocks, followed by functional trait composition and environmental factors. Environmental factors directly affected forest carbon storage and also indirectly influenced it through plant functional attributes. Notably, the biodiversity of the herb layer exhibited a significant linear correlation with carbon storage. Our results indicate that niche complementarity and selection effects contribute to carbon fixation, with selection effects being more predominant. Based on these findings, we recommend that afforestation policies prioritize enhancing broad‐leaved tree species to improve carbon sequestration, as evergreen broad‐leaved forests represent the zonal climax vegetation, while also acknowledging the importance of understory vegetation. Thus, in the context of global climate change, it is imperative to thoroughly evaluate the impact of environmental factors on biodiversity and carbon sequestration in forest ecosystems.