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.