Stomatal conductance of forest species after long‐term exposure to elevated CO₂ concentration: a synthesis

• Data from 13 long-term (> 1 yr), field-based studies of the effects of elevated CO₂ concentration ([CO₂ ]) on European forest tree species were analysed using meta-analysis and modelling. Meta-analysis was used to determine mean responses across the data sets, and data were fitted to two commonly used models of stomatal conductance in order to explore response to environmental conditions and the relationship with assimilation. • Meta-analysis indicated a significant decrease (21%) in stomatal conductance in response to growth in elevated [CO₂ ] across all studies. The response to [CO₂ ] was significantly stronger in young trees than old trees, in deciduous compared to coniferous trees, and in water stressed compared to nutrient stressed trees. No evidence of acclimation of stomatal conductance to elevated [CO₂ ] was found. • Fits of data to the first model showed that growth in elevated [CO₂ ] did not alter the response of stomatal conductance to vapour pressure deficit, soil water content or atmospheric [CO₂ ]. Fits of data to the second model indicated that conductance and assimilation responded in parallel to elevated [CO₂ ] except when water was limiting. • Data were compared to a previous meta-analysis and it was found that the response of g_s to elevated [CO₂ ] was much more consistent in long-term (> 1 yr) studies, emphasising the need for long-term elevated [CO₂ ] studies. By interpreting data in terms of models, the synthesis will aid future modelling studies of responses of forest trees to elevated [CO₂ ].