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

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (A ) 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 (eCO ), 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 on iWUE and its components A and stomatal conductance (g ). 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 , but greater A , responses to eCO in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO in all PFTs, and that increases in A had stronger effects than reductions in g . The USO model correctly captured stomatal behaviour with eCO across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO conditions.