The aim of this study was to investigate ecophysiological responses of young beech seedlings to doubling of atmospheric CO2 concentration ([CO2]: 350 and 700 μmol.mol-1 air) combined with a moderate drought. Beech seedlings were grown for one year in semi controlled greenhouses under controlled [CO2] and non limiting nutrient fertilization. Under doubled [CO2], net CO2 assimilation (A) was stimulated while stomatal conductance (gw) was not affected. 15Nitrogen uptake was increased as well as the water use efficiency. Biomass accumulation of shoots and roots was significantly stimulated without a change in root:shoot ratio. The specific hydraulic conductance of shoots and hydraulic conductivity of roots was not affected by [CO2]. After 20 days of moderate drought, C assimilation was still increased in response to doubled [CO2] and stomatal conductance (gw) was not affected, while 15N uptake showed no change. Under doubled [CO2], growth of water stressed seedlings was increased more than control one (350 μmol.mol-1, well irrigated). However, the root:shoot ratio was decreased by 20% at both [CO2]. Instantaneous water use efficiency of water stressed seedlings was increased. However, WUE was unchanged when it was calculated using 13C of leaves, a second measure of water-use efficiency. The specific hydraulic conductance of shoots did not change but hydraulic conductivity of roots was decreased by drought under both CO2 treatments. [CO2] had no direct effect on stomatal functioning of beech leaves of seedlings, in contrast to results on other species such as oaks. In reforestation context, this lack of antiperspirant effect of CO2 could have major implications for survival and adaptation to drought of beech seedlings during the next decades.