The aim of the research is to quantify the “species” effect on the water use of a mixed stand of beech/douglas fir located in the Belgian Ardennes, in order to correctly estimate the water use efficiency (WUE, g C/ kg H2O) of the stand. The study site is part of the CarboEurope network (Vielsalm site), and has been equipped with eddy covariance technology since 1996. A first part of the research will characterize the water use and functioning of each species, the water balance of the stand and the risk of soil drought. A second part will adjust the sap flow measurements (tree/species scale) with the water vapour measurements (eddy covariance; stand scale). The last part will consist in evaluating the WUE at the stand scale and by species, by coupling water vapour fluxes with CO2 fluxes.