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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.