Abstract: Mycorrhizal associations are critical to forest productivity and biomass carbon stock. Ectomycorrhiza (EcM) and arbuscular mycorrhiza (AM) associations, the dominant types in forest soils, differ in their nutrient acquisition and soil feedback mechanisms. However, previous studies mainly focused on single-species stands or experimental forests, leaving the influence and underlying mechanism of mycorrhizal associations on forest carbon stocks at regional scales unclear. Based on data from 64,303 trees across 1226 natural forest plots in the tropical/subtropical regions of southwest China, we found that aboveground carbon stock levels peaked at intermediate proportions (53.8%) of EcM trees, suggesting that mixed mycorrhizal strategies, characterized by the coexistence of EcM and AM trees, are more effective in enhancing carbon stocks than single-mycorrhizal type. Mechanistically, this effect may be primarily attributed to the contrasting impacts of mycorrhizal associations on species diversity and stand structure, suggesting that mycorrhizal-mediated niche complementary in spatial resource allocation and nutrient acquisition drives forest aboveground carbon accumulation. In addition, in EcM-dominated forests, the negative impact of increasing EcM tree proportions on carbon stocks diminishes as precipitation rises. These findings highlight the importance of simultaneously incorporating mycorrhizal associations and forest attributes in carbon sequestration strategies and ecosystem management practices.

Key words: Mycorrhizal associations, Forest carbon stocks, Complementarity effect, Species diversity, Stand structure