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Background and Aims Studies have indicated that plant stomatal conductance (g(s)) decreases in response to elevated atmospheric CO2, a phenomenon of significance for the global hydrological cycle. However, g(s) increases across certain CO2 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 CO2.

Methods Using (1) an extensive, up-to-date synthesis of g(s) responses in free air CO2 enrichment (FACE) experiments, (2) in situ measurements across four biomes showing dynamic g(s) responses to a CO2 rise of similar to 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 CO2.

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 CO2 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 g(s) under elevated CO2 in hot dry conditions. Contrary to the general assumption, positive g(s) responses to elevated CO2, 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 CO2, a finding that has important implications for predicting future vegetation feedbacks on the hydrological cycle at the regional level.