Greening Nonlinearly Intensifies Drought Impacts on Grasslands of the Qinghai-Tibet Plateau

Grasslands on the Qinghai-Tibet Plateau have been greening under climate change, but it is still debated whether this change strengthens resistance to drought or heightens ecosystem sensitivity to water stress. Here, we applied three indicators of drought impact-loss probability, loss intensity, and drought threshold-to assess how trends in the Normalized Difference Vegetation Index (NDVI) relate to drought sensitivity and influence grassland net primary productivity (NPP). We find that greening areas cover about 75% of the grassland area, concentrated in the mid to late growing season and more prominent at lower elevations. Despite elevated tolerance thresholds, greening regions experience more frequent and severe drought-related losses than browning areas, forming a pattern of high exposure and high response, which was most evident during the mid-season and at elevations between 3000 and 3500 m. A clear shift in drought impacts was observed with elevation, with the most pronounced transition occurring between 3500 and 4500 m, where ecosystem stability began to decline. In addition, we identified a nonlinear relationship between NDVI trends and drought impacts, with losses increasing at low greening rates but declining beyond a turning point around 0.12 to 0.15 × 10^-2 year^-1, suggesting that greening can either exacerbate or alleviate stress depending on its magnitude. While the role of soil moisture varied with season, limiting NPP early in the season and supporting it later. These findings suggest that greening does not uniformly mitigate drought impacts. Rather, its effects depend on greening intensity, phenological stage, and elevation.