Abstract

A principle response of C3 plants to increasing concentrations of atmospheric CO₂ (CO₂) is to reduce transpirational water loss by decreasing stomatal conductance (g_s) and simultaneously increase assimilation rates. Via this adaptation, vegetation has the ability to alter hydrology and climate. Therefore, it is important to determine the adaptation of vegetation to the expected anthropogenic rise in CO₂. Short-term stomatal opening–closing responses of vegetation to increasing CO₂ are described by free-air carbon enrichments growth experiments, and evolutionary adaptations are known from the geological record. However, to date the effects of decadal to centennial CO₂ perturbations on stomatal conductance are still largely unknown. Here we reconstruct a 34% (±12%) reduction in maximum stomatal conductance (g_smax) per 100 ppm CO₂ increase as a result of the adaptation in stomatal density (D) and pore size at maximal stomatal opening (a_max) of nine common species from Florida over the past 150 y. The species-specific g_smax values are determined by different evolutionary development, whereby the angiosperms sampled generally have numerous small stomata and high g_smax, and the conifers and fern have few large stomata and lower g_smax. Although angiosperms and conifers use different D and a_max adaptation strategies, our data show a coherent response in g_smax to CO₂ rise of the past century. Understanding these adaptations of C3 plants to rising CO₂ after decadal to centennial environmental changes is essential for quantification of plant physiological forcing at timescales relevant for global warming, and they are likely to continue until the limits of their phenotypic plasticity are reached.