Abstract

Plant physiological adaptation to the global rise in atmospheric CO₂ concentration (CO₂) is identified as a crucial climatic forcing. To optimize functioning under rising CO₂, plants reduce the diffusive stomatal conductance of their leaves (g_s) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (g_smax) and constrain the dynamic responses of g_s. Here, we develop and validate models that simulate structural stomatal adaptations based on diffusion of CO₂ and water vapor through stomata, photosynthesis, and optimization of carbon gain under the constraint of a plant physiological cost of water loss. We propose that the ongoing optimization of g_smax is eventually limited by species-specific limits to phenotypic plasticity. Our model reproduces observed structural stomatal adaptations and predicts that adaptation will continue beyond double CO₂. Owing to their distinct stomatal dimensions, angiosperms reach their phenotypic response limits on average at 740 ppm and conifers on average at 1,250 ppm CO₂. Further, our simulations predict that doubling today's CO₂ will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m⁻². We conclude that plant adaptation to rising CO₂ is altering the freshwater cycle and climate and will continue to do so throughout this century.