Extended Data Fig. 5: Comparison between EC observations and CMIP6 simulations.
From: Reduced water loss rather than increased photosynthesis controls CO2-enhanced water-use efficiency
a. Comparison of \({\beta }_{{\rm{iWUE}}}\). The horizontal dashed line indicates the spatial median \({\beta }_{{\rm{iWUE}}}\) (\({\widetilde{\beta }}_{{\rm{iWUE}}}\)) across 63 sites, derived from EC observations. Bars represent \({\widetilde{\beta }}_{{\rm{iWUE}}}\) derived from CMIP6 model ensembles, obtained by first averaging across CMIP6 models at each site and then calculating the spatial median across sites. Markers overlaid on the bars represent the individual \({\widetilde{\beta }}_{{\rm{iWUE}}}\) from each CMIP6 model. Error bars indicate the standard deviation of \({\widetilde{\beta }}_{{\rm{iWUE}}}\) across models. b. \(\beta\) comparison for direct CO2 effects on GPP and Gc. c. \(\beta\) comparison for total CO2 effects on GPP and Gc. Diagonal lines in b and c represent regimes in which GPP and Gc responses are proportionally balanced (\(\left|{\beta }_{{\rm{GPP}}}\right|=\left|{\beta }_{{\rm{Gc}}}\right|\)), indicating that CO2-enhanced iWUE is jointly driven by GPP stimulation and Gc reduction (referred to as the joint-driven regime). Regimes above the diagonal line indicate that CO2-enhanced iWUE is dominantly driven by increased GPP (\(\left|{\beta }_{{\rm{GPP}}}\right| > \left|{\beta }_{{\rm{Gc}}}\right|\), referred to as the GPP-dominance regime), whereas areas below the line represent the Gc-dominance regime (\(\left|{\beta }_{{\rm{Gc}}}\right| > \left|{\beta }_{{\rm{GPP}}}\right|\)), in which Gc reduction dominantly drives CO2-enhanced iWUE.
