Responses of gross primary productivity and normalized difference vegetation index to drought in northwestern China from 1982 to 2018.

The environment in northwestern China is fragile and extremely sensitive to climate change. Under increasingly frequent global drought events, it is critical to investigate the impacts of aridification on vegetation and their response mechanisms. Based on the standardized precipitation evapotranspiration index (SPEI), gross primary productivity (GPP), and normalized difference vegetation index (NDVI), we analyzed the impacts of drought characteristics (peak, intensity, and duration) on vegetation GPP and NDVI in northwestern China from 1982 to 2018 from interannual and seasonal scales. The results showed that during 1982-2018, drought peak, intensity, and duration all exhibited significant upward trends. Drought intensified in eastern Xinjiang and western Inner Mongolia, but eased in eastern Qinghai and southern Gansu. At the annual scale, the regions significantly correlated with drought intensity, GPP, and NDVI accounted for 7.8% and 7.4% of the total study area, respectively, while the regions significantly correlated with drought peak and GPP, and drought duration and NDVI accounted for 5.5% and 3.4%, respectively. At the seasonal scale, spring GPP was more susceptible to the cumulative effect of drought, while NDVI was more affected by drought in the current season. In summer and autumn, GPP was more driven by the lag effect of drought, while the NDVI was more affected by the lag effect and the drought of the current season, respectively. Drought intensity had the highest negative contribution rate to GPP change (38.6%), and drought peak had the largest negative contribution to NDVI change (42.6%). The drought duration had the largest positive contribution to GPP and NDVI changes, reaching 30.3% and 38.5%, respectively. Overall, the response of vegetation GPP and NDVI to drought in northwestern China were primarily characterized by lag effects, with drought intensity and peak dominating their variations, respectively. In the context of increasing drought, arid and semi-arid regions should enhance adaptability of ecosystems to drought stress through zonal and seasonal management.