Stomatal conductance and elevated CO₂

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 by Purcell et al. was to demonstrate that under certain environmental conditions, g_s can increase in response to elevated CO₂.

Field observations are corroborated by an extensive synthesis of g_s responses in free air CO₂ enrichment (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 (r2 = 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).

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.