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 elev ated CO2 concentration ([CO2]) on European forest tree species were analyse d using meta-analysis and modelling. Meta-analysis was used to determine me an responses across the data sets, and data were fitted to two commonly use d models of stomatal conductance in order to explore response to environmen tal conditions and the relationship with assimilation. Meta-analysis indicated a significant decrease (21%) in stomatal conductanc e in response to growth in elevated [CO2] across all studies. The response to [CO2] was significantly stronger in young trees than old trees, in decid uous compared to coniferous trees, and in water stressed compared to nutrie nt stressed trees. No evidence of acclimation of stomatal conductance to el evated [CO2] was found. Fits of data to the first model showed that growth in elevated [CO2] did no t alter the response of stomatal conductance to vapour pressure deficit, so il water content or atmospheric [CO2]. Fits of data to the second model ind icated 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 re sponse 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 modell ing studies of responses of forest trees to elevated [CO2].