Permafrost degradation: A critical driver of aboveground carbon sink loss in China's boreal forests

Permafrost degradation is emerging as a major disturbance factor affecting boreal forests, altering vegetation and carbon sinks in these ecosystems. Yet, the specific influence of permafrost degradation on the carbon budget of boreal ecosystems remains inadequately understood. Here, we reconstructed annual Aboveground Biomass (AGB) stocks across the boreal forests of Northeast China (NEC) from 1988 to 2023 using multi-source satellite data. Although total regional AGB increased by 210.47 Tg over the study period, pronounced spatial heterogeneity was observed in relation to permafrost stability. Partial correlation and sensitivity analyses indicate that soil freeze–thaw dynamics exert the dominant control on AGB variability in continuous and discontinuous permafrost zones, where earlier thaw generally promotes forest growth. In contrast, biomass declines were primarily concentrated in the sporadic permafrost zone. These patterns reflect a transition from energy-limited to water-limited growth conditions under prolonged thaw, whereby the positive effects of extended growing seasons are increasingly offset by thaw-induced hydrological decoupling and associated physiological drought. Additionally, we compared the output from process-based models with satellite observations. Comparisons with 21 CMIP6 Earth system models further reveal that 15 models systematically overestimate regional AGB stocks. This bias suggests that current models inadequately represent thaw-related hydrological constraints on vegetation growth, leading to an overestimation of forest carbon sink strength. Together, our results demonstrate that permafrost degradation constitutes a critical constraint on boreal forest carbon accumulation and underscore the need to incorporate permafrost-mediated soil hydrological processes into Earth system models. • Spatial divergence in carbon dynamics reveals a physiological tipping point. • Prolonged thaw triggers a shift from energy- to water-limited growth dynamics. • Earth System Models overestimate carbon sinks by missing hydrological constraints.