Climate and stand development shape aboveground biomass dynamics in China’s natural forests: Implications for adaptive management

Forest expansion does not necessarily translate into commensurate gains in woody biomass, because canopy development and biomass accumulation can diverge across space and time. Understanding this divergence is important for adaptive management of natural-forest carbon under climate change. Here, we quantified aboveground biomass (AGB) dynamics across China’s natural forests from 2010 to 2021 using eight observation epochs. We combined Theil–Sen slope estimation with the Mann–Kendall test (TS-MK), matched-year comparisons with independent AGB products, relative baseline-age stratification, and spatially validated XGBoost–SHAP models. The TS-MK analysis revealed a near-balanced spatial mosaic, with increasing trends covering 51.43% of natural-forest area and decreasing trends covering 46.73%. Areas of significant enhancement combined faster canopy development with greater AGB per mapped tree, whereas mapped tree density was lower. AGB trajectories also shifted with stand development: positive trends were more frequent in relatively younger stands, while stable or declining trajectories became more common toward the older end of within-stratum age distributions. Comparisons with independent AGB products showed moderate and temporally comparable agreement, and spatial validation confirmed stable model performance ( R 2 = 0.797 −0.826). Growing-season relative humidity was the most consistent predictor, while stand age, terrain, and recurrent hydrothermal interactions shaped year-specific responses. Physiological suitability indices provided modest predictive gains but captured nonlinear thermal suitability beyond mean annual temperature. Together, these findings show that natural-forest AGB dynamics emerge from coupled canopy development, stand age, and hydrothermal context, supporting trajectory-specific monitoring and adaptive management.