Abstract: Mixed forests represent the largest contributor to China’s forest carbon sink, yet their spatio-temporal changes in biomass carbon stocks remain insufficiently quantified, limiting the accuracy of national carbon sink assessments under the carbon neutrality target. Here, we integrated National Forest Inventory (NFI) data with multi-source remote-sensing products to quantify the changes in biomass carbon stocks of mixed forests across China from 1999 to 2018. The NFI dataset comprises approximately 26,000 plots nationwide, with plot sizes ranging from 600 to 800 m². Biomass carbon changes were attributed to distinct ecological and management processes, including natural growth, succession from pure forests, afforestation-driven expansion, and conversion-related losses. We found that biomass carbon stocks in China’s mixed forests increased continuously from 968.63 ± 109.2 Tg C (1 Tg C = 10¹² g C) in 1999 to 3833.39 ± 278.3 Tg C in 2018. The conversion of pure stands into mixed forests accounted for approximately 84.8% of this increase (2709.15 ± 320.2 Tg C), followed by area expansion (25.8%; 713.33 ± 136.9 Tg C) and natural growth (19.7%; 598.7 ± 122.7 Tg C), partially offset by an estimated 30.4% loss (− 929.78 Tg C) due to conversion from mixed forests to other forest types. Our findings provide empirical evidence for the dominant role of forest-type transitions in driving China’s forest carbon accumulation and underscore the necessity of incorporating mixed-forest management and dynamic forest-type transitions into national climate mitigation and carbon sink projections.

Key words: Mixed forests, Biomass carbon stocks, Spatio-temporal dynamics, Forest management