Roles of Soil and Atmospheric Dryness on Terrestrial Vegetation Productivity in China - Which Dominates at What Thresholds

Low soil moisture (SM) and high vapor pressure deficit (VPD) can reduce vegetation productivity (gross primary productivity (GPP)) and weaken terrestrial carbon stock. However, the roles of SM and VPD on GPP vary widely in terms of (a) which one dominates the impact (dominance) and (b) the specific threshold values at which they are activated (thresholds). This study adopted the copula method to investigate the dominance and thresholds of SM and VPD on terrestrial vegetation productivity reduction (GPP reduction) across China from 1982 to 2018. The results indicate that SM predominantly drives the GPP reduction across 71% of China's vegetation zones, with the SM thresholds decreasing from the south to the north. Vapor pressure deficit dominated the GPP reduction primarily in the croplands with low VPD thresholds, such as the North China Plain and Northeast Plain. Moreover, the SM and VPD thresholds have significant difference across different ecosystems, with higher SM thresholds (activated under wetter soil conditions) in the forests and lower VPD thresholds (activated under more humid atmospheric conditions) in the croplands. Compared with the non-irrigated croplands, irrigation can significantly reduce SM thresholds and increase VPD thresholds by relieving water stress from soil and atmospheric dryness. The structural equation modeling further demonstrates the dominant influence of SM and VPD on GPP reduction. This study explicitly identified the spatial distribution of the dominance and thresholds of soil and atmospheric dryness across different ecosystems. It could enhance our understanding of terrestrial ecosystem sensitivity to drought and provide guidance for drought management in different ecosystems.