Drought trigger thresholds for different levels of vegetation loss in China and their dynamics

Frequent meteorological droughts can negatively impact terrestrial ecosystems by controlling the opening and closing of vegetation stomatal and altering vegetation structure to limit the ability of vegetation to sequester carbon. Due to the lagging and accumulation effects of drought on vegetation growth, drought trigger thresholds for different levels of vegetation loss are still unclear, which is very important for accurately assessing the future impacts of drought on terrestrial ecosystem. Therefore, this study proposed a framework to investigate drought trigger thresholds under various vegetation losses based on copula theory and conditional probabilities, and assessed the dynamics of drought trigger thresholds and possible causes, based on the random forest model. In addition, we used multiple GPP and soil water datasets for the analysis to ensure the robustness of relevant findings. The results show that: (1) there is a generally positive correlation between GPP and SPEI in China, and the response time of vegetation to drought is mostly on a short time scale (less than or equal to 4 months); (2) drought trigger thresholds are also higher in eastern China, with lower vegetation resistance and significantly higher risk of vegetation productivity loss than in other regions; (3) the trigger thresholds in northeastern China show a decreasing trend, with vegetation resistance gradually increasing. CO2 fertilization enhances vegetation drought resistance, but the magnitude of resistance increase is reduced due to the adverse effects of water stress and VPD on vegetation. The findings of this study may advance our comprehension of terrestrial ecosystem vulnerability and response to drought, and further provide scientific guidance for watershed water allocation, drought preparedness and risk management.