Relative Controls of Vapor Pressure Deficit and Soil Water Stress on Canopy Conductance in Global Simulations by an Earth System Model

Climate change impacts soil water (SW) and vapor pressure deficit (VPD), critically influencing land-atmosphere water vapor and CO2 exchange. There is a lack of model-based global evaluation of the relative roles of the aforementioned water stresses in limiting canopy conductance G(c). Using the Energy Exascale Earth System Model (E3SM), we conducted four land-atmosphere coupled simulations for the historical period with and without a plant hydraulics scheme (PHS) and under present-day and quadrupling of CO2 concentrations. By fitting an empirical model of G(c) to the E3SM simulations, we evaluated the relative dominance of the limitation of G(c) by water stresses caused by VPD and SW in the growing season. The empirical model is based on a multiplicative algorithm that adjusts G(c) according to VPD, SW saturation, and their interaction. Our results show positive trends of VPD over land during the historical period, with large differences between simulations with/without PHS. Despite the differences, G(c) is more limited by SW than VPD except in grasslands when SW saturations are above the 60th quantiles, and the fraction of grassland limited by VPD becomes small under dry soil conditions. Ignoring SW and VPD interaction may underestimate the relative role of VPD on G(c) by as much as 33% in the dry SW quantile. Based on our uncalibrated results, including PHS may accentuate SW stress limitation on G(c) by simulating wetter soil and higher evapotranspiration that lower VPD. Elevated CO2 reduces G(c) by increasing water use efficiency and ameliorates SW stress on G(c). Plain Language Summary In response to dry air and/or dry soil, plants close the pores (stomata) on their leaves to prevent water loss, excessive of which can harm or kill them. It is not clear whether plants close pores more due to air dryness or soil dryness, especially when both conditions are present. As air dryness and soil dryness may change with global warming, understanding their relative controls on stomatal closure is important for understanding plant response. Here we apply a simple model to decompose how much each dryness contributes to the stomata closure calculated from an Earth system model. Our model results show there is a high probability that soil dryness dominates stomatal closure more than air dryness when both conditions are present in the dry period, whether in the present-day condition or hypothetical future warming. Including water movement in the soil and the water transport within the plant in the Earth system model can increase the relative influence of soil dryness on stomatal closure because plant physiological response can help reduce air dryness.