We enriched in CO2 the canopy of 14 broad-leaved trees in a species-rich, ca. 30-m-tall forest in NW Switzerland to test whether elevated CO2 reduces water use in mature forest trees. Measurements of sap flux density (JS) were made prior to CO2 enrichment (summer 2000) and throughout the first whole growing season of CO2 exposure (2001) using the constant heat-flow technique. The short-term responses of sap flux to brief (1.5-3 h) interruptions of CO2 enrichment were also examined. There were no significant a priori differences in morphological and physiological traits between trees which were later exposed to elevated CO2 (n=14) and trees later used as controls (n=19). Over the entire growing season, CO2 enrichment resulted in an average 10.7% reduction in mean daily JS across all species compared to control trees. Responses were most pronounced in Carpinus, Acer, Prunus and Tilia, smaller in Quercus and close to zero in Fagus trees. The JS of treated trees significantly increased by 7% upon transient exposure to ambient CO2 concentrations at noon. Hence, responses of the different species were, in the short term, similar in magnitude to those observed over the whole season (though opposite because of the reversed treatment). The reductions in mean JS of CO2-enriched trees were high (22%) under conditions of low evaporative demand (vapour pressure deficit, VPD <5 hPa) and small (2%) when mean daily VPD was greater than 10 hPa. During a relatively dry period, the effect of elevated CO2 on JS even appeared to be reversed. These results suggest that daily water savings by CO2-enriched trees may have accumulated to a significantly improved water status by the time when control trees were short of soil moisture. Our data indicate that the magnitude of CO2 effects on stand transpiration will depend on rainfall regimes and the relative abundance of the different species, being more pronounced under humid conditions and in stands dominated by species such as Carpinus and negligible in mono-specific Fagus forests.