Drying crisis under greening: a study on the drought trigger thresholds of vegetation carbon fixation loss in karst areas.

Global warming has intensified the frequency of droughts. Once droughts exceed a critical threshold, they pose a severe threat to vegetation growth and limit its carbon fixation capacity. However, current research still has an insufficient understanding of the probabilistic thresholds at which drought triggers the loss of vegetation carbon fixation (VCF) capacity under sustained greening, especially lacking a systematic comparison of the differences between karst areas (KA) and non-karst areas (NKA) under complex hydrogeological conditions. For this purpose, this study uses the Copula method and conditional probability to derive the VCF loss and its trigger thresholds under different drought scenarios from 1982 to 2022, and explores the key driving factors influencing these thresholds. The results show that: (1) Compared to non-karst areas (NKA), VCF in the KA of southern China is more sensitive to drought, with a higher response intensity and a shorter lag time. (2) The probability of VCF loss in KA is significantly higher than in NKA, and the drought trigger threshold is also higher. (3) The VCF thresholds in both KA and NKA are primarily driven by the "temperature (TEM)- moisture" coupling. However, KA show higher TEM sensitivity and dependence on deep soil water storage (SWC3-4), while in NKA, the thresholds are influenced by multiple factors, including shallow soil water storage (SWC1-2), atmospheric vapor pressure deficit (VPD), and soil organic carbon (SOC). From the perspective of probabilistic thresholds, the results reveal the hidden drought vulnerability of VCF capacity in humid greening regions, and can provide a quantitative basis for maintaining regional carbon sink stability, implementing zonal vegetation management, and promoting proactive drought risk prevention and control.