The effects of environmental factors on stability of forest carbon sink exceeded biotic factors

The stability of forest carbon sinks is pivotal for mitigating carbon-climate feedbacks and achieving climate objectives. Emerging evidence suggested that this stability was diminished by climate change and extreme weather events, thus jeopardizing the carbon sequestration capacity and enhancing forest vulnerability. Yet, how biotic factors react to the effects of environmental change on it remains unknown. Here, we integrate long-term (7-29 years) flux and micrometeorological observations of 48 forest sites from ecosystem observation networks (FLUXNET2015, AmeriFlux and ICOS) with interpretable machine learning algorithm (SHAP values) to show how biotic factors and environmental factors impacts stability of forest carbon sink, quantified by critical slowing down indicators (i.e., temporal autocorrelation, TAC), and comparatively analyzed the differences between high- and low-stability forests. Our analysis revealed that environmental factors (i.e., mean annual temperature MAT; mean annual temperature, MAP; carbon dioxide concentration, C_a; incoming shortwave radiation, SW; vapor Pressure Deficit, VPD; soil water availability, the Priestley-Taylor coefficient, α) held a substantially greater impact (the cumulative mean SHAP values) on TAC than biotic factors (i.e., marginal water cost of carbon gain, G₁; canopy photosynthetic capacity, A_max; reference canopy conductance, G_cref; carbon sink capacity, NEP), with most of this influence ascribed to MAT, MAP, C_a, G₁, and NEP. High-stability forests more intensively presented in conditions with relatively warm and humid long-term climate, coupled with moderately conservative water-use strategies and carbon sink capacity. Although CO₂ fertilization effects increased the positive effects of moderate G₁ to carbon sink stability, however, higher C_a markedly diminished its stability. Our findings underscore the necessity for caution regarding the detrimental impacts of sustained rise in C_a, gradual warming and drying of the long-term climate, and extreme atmospheric and soil drought events on the stability of forest carbon sinks, given the current limitations of vegetation's biotic factors to adapt to these changes.