Stomatal conductance and elevated CO2

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

The location of FACE studies included in the assessment.
The location of FACE studies included in the assessment. Fifty-one FACE studies are shown (most overlap on this scale). Most FACE studies are located in northern hemisphere locations between 30 and 60°N. FACE studies which did not, to the authors’ knowledge, document gs changes were not included. See Materials and Methods for all cited studies used.

Field observations are corroborated by an extensive synthesis of gs responses in free air CO2 enrichment (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.