Tree mycorrhizal associations determine how biodiversity, large trees, and environmental factors drive aboveground carbon stock in temperate forests

Experimental and observational studies have elucidated that an amalgamation of biotic (e.g., biodiversity, large trees) and abiotic factors (e.g., climate, soil) jointly determine tree aboveground carbon stock within forest ecosystems. Yet, a pivotal factor potentially influencing these carbon repositories might be the specific tree mycorrhizal associations, especially given that ectomycorrhizal (EM) tree species frequently comprise more large-sized trees, translating to an augmented carbon reserve, as opposed to arbuscular mycorrhizal (AM) counterpart. However, how biotic and abiotic factors determine forest carbon through regulating AM vs. EM tree carbon stock is still elusive across large regions. Here, we examined a forest inventory data consisting of over 90,000 trees from 631 forest plots (30m × 30m each). Our objective was to explore how biodiversity (i.e., species diversity and ecological uniqueness), large trees (top 1% tree diameters), and environmental factors (e.g., climate and soil nutrients) differently regulate AM and EM, and thus, total tree aboveground carbon stock of temperate forests in northeast China. Our findings illuminated that large trees had consistent enhancement effect on AM and EM tree carbon repositories. However, the effects of biodiversity and environmental factors on carbon stock were opposite between AM and EM trees. Specifically, the two components of biodiversity were positively associated with AM tree carbon stock while negatively associated with EM tree carbon stock. Environment heterogeneity (i.e. mean annual temperature and soil nutrients) also exhibited contrasting impacts on AM and EM tree carbon stock. Consequently, when integrating AM and EM tree carbon stock into total carbon stock, the consistent effect of large trees on AM vs. EM trees was strengthened and most important, while the opposite effect of biodiversity or environment factors was diluted. In summary, this study emphasized a mycorrhizal viewpoint to better understand the determinants of overarching aboveground carbon profile across regional forests.