Increased atmospheric water demand reduces ecosystem water use efficiency

Abstract

Revealing the drivers and underlying mechanisms of ecosystem water use efficiency are crucial for forecasting the impact of future climatic change on ecosystem carbon and water dynamics. Recent decades have been characterized by rising temperature worldwide, leading to a rapid increase in vapor pressure deficit. However, how elevated vapor pressure deficit would modulate the water use efficiency and the underlying mechanisms remain not fully understood. In this study, the spatial responses of water use efficiency to vapor pressure deficit were assessed through observations from 109 eddy covariance flux towers. Spatially, the negative sensitivity coefficients of water use efficiency to vapor pressure deficit significantly decreased with increasing aridity index, indicating that higher decreasing rate in water use efficiency with increased vapor pressure deficit in wet regions than that in dry regions. Compared to other environmental factors, vapor pressure deficit was the dominant driver regulating the water use efficiency variations, accounting to 19.93 %. More importantly, water use efficiency in response to vapor pressure deficit was dominated by more significantly negative gross primary productivity than the evapotranspiration. By the end of this century, vapor pressure deficit will increase by approximate 1.00 kPa, resulting in decreases of the average water use efficiency by 0.57–1.65 g C/kg H₂O under the highest emission scenario. This finding improves our understanding of future climate change on ecosystem carbon and water dynamics and provides valuable insights into predicting vegetation growth and managing ecosystems under a more extreme climate.