Drivers and Divergent Impacts of Rising Vapor Pressure Deficit on Global Vegetation Productivity

Atmospheric vapor pressure deficit (VPD) is a critical climate variable influencing vegetation productivity and the global carbon cycle. With climate warming, VPD has been increasing globally, but its effects on gross primary production (GPP) remain poorly understood, leading to uncertainties in predicting terrestrial ecosystem responses. This study classified the causes of VPD increase into three different types: temperature-driven, combined temperature and relative humidity-driven, and relative humidity-driven, to investigate the spatial heterogeneity and drivers of VPD impacts on GPP using three datasets-FLUXCOM_GPP, GOSIF_GPP, and VPM_GPP-spanning 2000 to 2018. By integrating trend analysis, partial correlation techniques, and random forest models, the results reveal a distinct latitudinal gradient in the relationship between VPD and GPP, characterized by a "Z-shaped" pattern. Near the equator and at high latitudes, VPD positively influences GPP, whereas in mid-latitudes, the relationship is predominantly negative. This variation is shaped by the climatic background and the interplay of water and energy-related factors. For example, in regions with synchronous changes in temperature and humidity, VPD effects on GPP tend to be neutral or positive. Conversely, asynchronous changes exacerbate negative effects, particularly in humidity-driven regions. This study provides a mechanistic understanding of the drivers of interannual GPP variability across different climatic contexts, implying the importance of biodiversity in shaping vegetation responses to climate extremes and affecting the overall ecosystem vulnerability and global carbon cycle under future warming.