Rising atmospheric CO₂, cₐ, is expected to affect stomatal regulation of leaf gas‐exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas‐exchange that include maintaining a constant leaf internal CO₂, cᵢ, a constant drawdown in CO₂ (cₐ − cᵢ), and a constant cᵢ/cₐ. These strategies can result in drastically different consequences for leaf gas‐exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas‐exchange responses to varying cₐ. The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas‐exchange responses to cₐ. To assess leaf gas‐exchange regulation strategies, we analyzed patterns in cᵢ inferred from studies reporting C stable isotope ratios (δ¹³C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of cₐ spanning at least 100 ppm. Our results suggest that much of the cₐ‐induced changes in cᵢ/cₐ occurred across cₐ spanning 200 to 400 ppm. These patterns imply that cₐ − cᵢ will eventually approach a constant level at high cₐ because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant cᵢ. Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low cₐ, when additional water loss is small for each unit of C gain, and increasingly water‐conservative at high cₐ, when photosystems are saturated and water loss is large for each unit C gain.