A forest is vulnerable to drought and plays important roles in the regulation of carbon and water cycling in a terrestrial ecosystem. Atmospheric vapor pressure deficit (VPD) has been identified as an increasingly major factor in plant functioning and has been established as a main contributor to recent drought-induced plant mortality, independent of other drivers associated with climate change. However, most previous studies have focused on the effects of climate warming and CO₂ enrichment on vegetation growth, without considering the effects of an increased VPD on vegetation growth and evapotranspiration (ET) in forest ecosystems. This could lead to a large uncertainty in estimating the variability in forest carbon sinks. Based on the long-term satellite data, we investigated the response of the leaf area index (LAI) and ET to the VPD via a partial correlation analysis in this study. We also examined the temporal variability in the partial coefficients within a ten-year moving window. The results showed that over 50% of the region displayed a negative partial correlation between the LAI, ET, and VPD, and those pixels were mainly concentrated in North America and the plains of Eastern Europe. Regions with a negative trend of partial correlation in both the LAI and ET are mostly located in the plains of Eastern Europe and the Siberian Plain of western Russia, while the positive trend is mainly in South America. The plains of Eastern Europe are becoming drier, which was proved by the interannual trend of the Standardized Precipitation Evapotranspiration Index (SPEI) and soil water content (SWC). Additionally, the LAI and ET in those areas exhibited a significant positive correlation with the SWC based on the moving window average. This study suggests that the role of the VPD on vegetation will become increasingly prominent in the context of future climate change for the forest ecosystem.

Introduction: Atmospheric vapor pressure deficit (VPD) has been identified as a key factor influencing vegetation productivity, with spatially divergent positive and negative effects observed across global ecosystems. However, the mechanisms underlying these heterogeneous responses remain insufficiently understood. Methods: Here, we integrated long-term global satellite observations with multiple statistical approaches to investigate the drivers of divergent vegetation responses to VPD. We examined the responses of leaf area index (LAI) and evapotranspiration (ET) to VPD along spatial gradients of soil and atmospheric water availability, and further explored the roles of plant water-use strategies and atmospheric moisture conditions using mediation analyses. Results: Our results showed that vegetation responses of LAI and ET to VPD varied significantly along gradients of soil water content (SWC) and relative humidity (RH). When SWC and RH increased beyond critical thresholds, vegetation responses to VPD shifted from negative to positive for both LAI and ET. These transition patterns were closely associated with spatial variations in plant water-use strategies. Mediation analyses further revealed that changes in ET responses to VPD, jointly regulated by SWC and plant water-use strategies, were associated with variations in atmospheric moisture conditions, which in turn influenced vegetation responses to VPD. Specifically, in water-limited regions, conservative plant water-use strategies and low soil moisture were associated with reduced ET responses under increasing VPD, potentially contributing to lower atmospheric humidity and constrained vegetation productivity. In contrast, in water-rich regions, higher soil moisture and more water-intensive strategies were associated with positive ET responses to VPD, which may help maintain relatively humid atmospheric conditions and less constrained vegetation productivity. Discussion: Overall, we propose an integrated observational framework to reconcile spatially divergent vegetation responses to VPD, highlighting the importance of soil water availability, atmospheric moisture conditions, and plant water-use strategies in regulating vegetation–climate interactions. These findings may improve our understanding of ecosystem responses to ongoing climate change.