Continuous growth with fragile stability in China's forest biomass carbon storage since 2002.

Forest biomass carbon (BC) storage is an important means of mitigating climate change. However, the spatiotemporal patterns and stability of BC growth remain unclear in China. Based on the latest BC maps (2002-2021), we calculated the spatiotemporal dynamics of BC and used resilience indicators to reveal the stability of BC. We found that although China's long-term BC stock continued to increase, forests with higher BC density widely experienced BC density reductions, particularly in older forests over 70 years of age. While the BC density in deciduous needleleaf forests (DNF) was not high, these forests experienced the most severe BC density reductions. Approximately 53.4 % of forests in China experienced weakened resilience, with the most severe decline observed in DNF, which directly led to a slowdown in BC growth, accompanied by significant interannual fluctuations. More alarmingly, 10.4 % of forests (BC-, Resilience-) primarily with high BC density (average 28.3 tC/ha) underwent frequent and severe BC losses (118 Tg C) due to weakening resilience, and had become sources of biomass carbon. We suggested the potential for China's BC is significant, yet face risks of loss and transformation into a carbon source. We emphasized that although China's BC stock has continued to grow, it remains fragile and may be approaching its peak, facing a significant risk of substantial loss.