Atmospheric Vapor Pressure Deficit Outweighs Soil Moisture Deficit in Controlling Global Ecosystem Water Use Efficiency

Abstract

High vapor pressure deficit (VPD) and low soil moisture (SM) lead to soil and atmospheric droughts, which can stress carbon-water coupling in terrestrial ecosystems. However, the strong collinearity between VPD and SM, particularly under certain climatic conditions, makes it challenging to disentangle their independent contributions to carbon and water dynamics in land-atmosphere interactions. This study aimed to clarify the long-term independent response of global vegetation carbon-water coupling, based on ecosystem water-use efficiency (WUE_E) and plant canopy water-use efficiency (WUE_Et), to decoupled VPD and SM from 1982 to 2100. WUE_E is defined as the ratio of ecosystem gross primary productivity to evapotranspiration, while WUE_Et is defined as the ratio of ecosystem gross primary productivity to vegetation transpiration. The results indicate that from 1982 to 2018, both before and after the decoupling of VPD and SM, over 64% of global vegetation zones experienced stronger atmospheric moisture stress from VPD than soil drought stress from SM, consistently impacting WUE_E and WUE_Et. The influence of VPD on WUE_E and WUE_Et gradually declined, while the influence of SM presented a tendency to increase. The small difference in the responses of WUE_E and WUE_Et to VPD and SM is attributed to the strong collinearity between WUE_E and WUE_Et. The effects of VPD and SM on WUE_E and WUE_Et varied across vegetation cover gradients, biomes, and climatic zones. As atmospheric and soil drought intensifies in the coming decades, the effects of VPD on WUE_E and WUE_Et stress are stronger than those of SM across all four socio-economic shared pathway (SSP) scenarios. In the high SSP scenarios (SSP5-8.5 for WUE_E and SSP3-7.0 for WUE_Et), the dominant influence of VPD is expected to expand.