Stomatal conductance in mature deciduous forest trees exposed to elevated CO2

Stomatal conductance (g s) of mature trees exposed to elevated CO2 concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO2 compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO2 treatment. During the 2003 drought in the third treatment year, the CO2 effect became reversed in Carpinus, resulting in higher g s in trees exposed to elevated CO2 compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO2 enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO2-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO