Optimal stomatal theory predicts CO₂ responses of stomatal conductance in both gymnosperm and angiosperm trees

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (A_net) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO₂ (eCO₂), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO₂ on iWUE and its components A_net and stomatal conductance (g_s). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf–air vapour pressure difference (D). We expected smaller g_s, but greater A_net, responses to eCO₂ in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO₂ in all PFTs, and that increases in A_net had stronger effects than reductions in g_s. The USO model correctly captured stomatal behaviour with eCO₂ across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g₁) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO₂ conditions.