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Increasing stomatal conductance in response to rising atmospheric CO₂

Purcell, C, Batke, S P, Yiotis, C, Caballero, R, Soh, W K, Murray, M, McElwain, J C
Annals of botany 2018 v.121 no.6 pp. 1137-1149
air, carbon dioxide, data collection, ecosystems, environmental factors, free air carbon dioxide enrichment, greenhouse gases, hydrologic cycle, models, prediction, stomatal conductance, vegetation
Studies have indicated that plant stomatal conductance (gₛ) decreases in response to elevated atmospheric CO₂, a phenomenon of significance for the global hydrological cycle. However, gₛ increases across certain CO₂ ranges have been predicted by optimization models. The aim of this work was to demonstrate that under certain environmental conditions, gₛ can increase in response to elevated CO₂. Using (1) an extensive, up-to-date synthesis of gₛ responses in free air CO₂ enrichment (FACE)experiments, (2) in situ measurements across four biomes showing dynamic gₛ responses to a CO₂ 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 gₛ can in some cases increase in response to increasing atmospheric CO₂. Field observations are corroborated by an extensive synthesis of gₛ responses in FACE experiments showing that 11.8 % of gₛ responses under experimentally elevated CO₂ are positive. They are further supported by a strong data-model fit (r² = 0.607) using a stomatal optimization model applied to the field gₛ dataset. A parameter space identified in the Farquhar–Ball–Berry photosynthesis–stomatal conductance model confirms field observations of increasing gₛ under elevated CO₂ in hot dry conditions. Contrary to the general assumption, positive gₛ 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). The results contradict the over-simplistic notion that global vegetation always responds with decreasing gₛ to elevated CO₂, a finding that has important implications for predicting future vegetation feedbacks on the hydrological cycle at the regional level.