Spatiotemporal Variation of Vegetation Water Use Efficiency and Its Response to Extreme Climate in Northwestern Sichuan Plateau

The Northwestern Sichuan Plateau, characterised by complex topography, is located on the northeastern edge of the Tibetan Plateau where ecosystems are rapidly changing in response to climate change. However, the spatiotemporal characteristics of water use efficiency (WUE) and its response to climate change remain unclear, especially different responses to changes in climate mean and extremes. In this study, we found that the seasonal variation of WUE for different types of vegetation demonstrates a “unimodal” pattern, with high values during May to September. The variation of WUE is largely dependent on altitude. The annual WUE showed an overall increasing trend from 2001 to 2021 for most types of vegetation, while a decreasing trend was found for wetland, and grassland had a stronger adaptability to altitude and climate change. Mean climatic variables exerted stronger impacts on WUE than climate extremes, and mean temperature change had a stronger impact on WUE than precipitation. However, the synergistic effects of climate extremes and mean climate exerted significant impacts on WUE, with extreme cold events and mean precipitation events generally being unfavourable for an increase in WUE. The WUE of cropland and forest land demonstrated more sensitive responses to climate extremes than other types. The response of the same type of vegetation to climate extremes has obvious regional heterogeneity. However, the diurnal range of temperature, the length of the growing season, and the persistent humidity index had significant influences on the change of WUE for the entire region.