Recent variations in ecosystem water use efficiency to seasonal climate variability in China's key tropical-subtropical transitional zones

Understanding how terrestrial ecosystem water use efficiency (WUE) responds to climate change is critical to accurately representing the carbon-water cycle processes. However, the dynamics of WUE under seasonal climate variations and biome-specific characteristics remain still unclear. In this study, we integrated two satellite-based retrieval algorithms to estimate gross primary productivity (GPP) and evapotranspiration (ET). Such indexes served as input to quantify ecosystem WUE (GPP/ET) and explore its dynamics during the dry and wet seasons from 2001 to 2018 in China’s key tropical to subtropical transitional zones, i.e., Yunnan Province. Results show large spatial heterogeneity and seasonal difference in WUE over the observational period. During the dry season, the increasing trends in GPP and ET have led to contrasting WUE patterns in forest and non-forest biomes, leading to positive and negative WUE trends, respectively. During the wet season, the declining trends in GPP occurring in combination with opposite trends in ET, have caused decreasing WUE consistently across all biomes except croplands, likely further modulated by human factors. The observed changes in WUE appear primarily driven by variations in air temperature (Ta) and vapor pressure deficit (VPD) during both dry and wet seasons. Overall, these results provide a valuable case for a better understanding of the carbon-water interplay in tropical-subtropical transitional zones and provide new insights to improve our capacity to predict the terrestrial ecosystem’s response to climate change.