Projected droughts readily trigger vegetation decline

Drought is intensifying under a warming climate, while the lag of vegetation response and the drought trigger thresholds for relative vegetation loss remain insufficiently quantified. Here we provide a global assessment of vegetation responses to the Terrestrial Water Storage Drought Severity Index (TWS_DSI) under historical and future climate scenarios. Using five vegetation indicators, including Leaf Area Index (LAI), Fraction of Photosynthetically Active Radiation (FPAR), Normalized Difference Vegetation Index (NDVI), kernel Normalized Difference Vegetation Index (kNDVI), and Gross Primary Productivity (GPP), we quantified their lagged responses to drought during the historical period. The mean optimal lag times ranged from 8.20 months for LAI to 10.59 months for NDVI, with FPAR (8.78 months), GPP (9.80 months), and kNDVI (10.42 months) showing intermediate lag responses. Based on future LAI simulations under the SSP126, SSP370, and SSP585 scenarios, the mean lag varied from 5.15 to 10.48 months. At predefined vegetation loss levels (≤40 th , ≤30 th , ≤20 th , ≤10 th ), we identified the drought trigger thresholds for vegetation decline and mapped their spatial variability. Compared to the historical baseline, the average drought trigger thresholds increased by 30.0%, 36.3%, and 35.3% under the SSP126, SSP370, and SSP585 scenarios, respectively, indicating heightened vulnerability whereby even milder drought conditions in the future could trigger vegetation decline. Our findings characterize the temporal lags, threshold behavior, and projected changes of vegetation responses to terrestrial water deficits, providing a scientific basis for assessing ecosystem vulnerability and informing targeted ecological conservation and climate adaptation strategies.