Increasing stomatal conductance inresponse to rising atmospheric CO2

Background and Aims Studies have indicated that plant stomatal conductance (g s) decreases in response to elevated atmospheric CO 2, a phenomenon of significance for the global hydrological cycle. However, g s increases across certain CO 2 ranges have been predicted by optimization models. The aim of this work was to demonstrate that under certain environmental conditions, g s can increase in response to elevated CO 2. Methods Using (1) an extensive, up-to-date synthesis of g s responses in free air CO 2 enrichment (FACE) experiments, (2) in situ measurements across four biomes showing dynamic g s responses to a CO 2 rise of ∼50 ppm (characterizing the change in this greenhouse gas over the past three decades) and (3) a photosynthesis-stomatal conductance model, it is demonstrated that g s can in some cases increase in response to increasing atmospheric CO 2. Key Results Field observations are corroborated by an extensive synthesis of g s responses in FACE experiments showing that 11.8 % of g s responses under experimentally elevated CO 2 are positive. They are further supported by a strong data-model fit (r 2 = 0.607) using a stomatal optimization model applied to the field g s dataset. A parameter space identified in the Farquhar-Ball-Berry photosynthesis-stomatal conductance model confirms field observations of increasing gs under elevated CO 2 in hot dry conditions. Contrary to the general assumption, positive gs responses to elevated CO 2, although relatively rare, are a feature of woody taxa adapted to warm, low-humidity conditions, and this response is also demonstrated in global simulations using the Community Land Model (CLM4). Conclusions The results contradict the over-simplistic notion that global vegetation always responds with decreasing g s to elevated CO 2, a finding that has important implications for predicting future vegetation feedbacks on the hydrological cycle at the regional level.