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

Summary 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 CO 2 (eCO 2 ), 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 eCO 2 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 eCO 2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO 2 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 eCO 2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter ( g 1 ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO 2 conditions.