Dominant effect and threshold response of soil moisture on global vegetation greening in the 21st century

Abstract

Extensive studies have revealed the compound impact of high Vapor Pressure Deficit (VPD) and low Soil Moisture (SM) on global Gross Primary Productivity (GPP), but their relative importance and nonlinear effects remain uncertain due to their covariation. Herein, based on the CMIP6 outputs, we examined the global GPP, SM, and VPD variation under baseline (1982–2014) and four future scenarios (2015–2100), clarified the dominant force on GPP by decoupling the effects of SM and VPD; further, we explored critical threshold and costs of dominant force on GPP, identified global drought zoning, and proposed optimization strategy. The results show that GPP will experience an increasing trend, and the growth rate is higher in the Northern Hemisphere; except for the SSP5-8.5, soil tends to be moist, but atmospheric dryness is obvious, especially in the Southern Hemisphere. SM dominates the GPP variation, but its impact influence differs across vegetation types and scenarios. There is an inverted U-shaped relationship between SM and GPP in forests and shrublands; GPP and SM exhibit positive reinforcing feedback in croplands and grasslands; but a negative feedback is found in savannas. The threshold and SM cost increase with rising CO₂, enhancing water use efficiency is the key to alleviating the high SM costs for vegetation growth in high CO₂ emission scenarios. SM-drier zones constitute more than 70% of the global land and expand significantly, particularly in forests and grasslands, highlighting the vulnerability of these vegetation to severer soil water deficits; SM in 12.25% to 20.8% of the vegetated land is within the elastic range, dominated by savannas, revealing its strong self-adaptive ability; SM-wetter zones occupy the smallest area and exhibit a marked decreasing trend, mainly in croplands and shrublands. For different vegetation, targeted mitigation strategies must be adopted to enhance their drought resilience.