PNNL

Abstract

Although there is mounting concern about how have high vapor pressure deficit (VPD) and low soil moisture (SM) affect ecosystem productivity, their relative importance is still under debate. Here, we comprehensively quantified the relative impacts of these two factors on gross primary production (GPP) using observations from a global network of eddy-covariance towers (87 sites and 826 site years) and via two approaches (sensitivity analysis and linear regression model). Both approaches agree well that high VPD limits GPP reductions to a greater extent than SM at many sites over the growing season. However, the limitation of high VPD and low SM on GPP reduction varies with climate and plant functional types, where the humid and mesic sites and forests and grasslands are dominated by VPD while the semi-arid and arid sites, shrublands and savannas are dominated by SM. We then applied a best-fitted stomatal model to understand the underlying mechanisms. The dominant role of VPD and SM is closely related to the stomatal water use efficiency slope, which is higher in more arid environments and water-limited ecosystems. We further emphasized the important role of SM on GPP during extreme droughts, and highlighted the possible underestimation of the SM effects for phreatophytes when using surface-layer SM only. Our results shed light on a better understanding of the impacts of VPD and SM on vegetation productivity, with vital implications for modeling the response and feedback of ecosystem dynamics to current and future climate.