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 (gs) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (gsmax) and constrain the dynamic responses of gs. 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 gsmax 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
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