In the context of ongoing increase of drought in temperate forests, forest managers consider the reduction of stand density (i.e. the number of standing trees at a given time) to limit soil water depletion. However, the reduction of tree canopy density increases light below canopy and allows the development of understorey vegetation. Knowing that the forest stand evapotranspiration (ET) is divided into three components: namely the soil, the understorey vegetation and the overstorey vegetation, i.e. trees. A lower consumption of trees could be compensated by a higher evapotranspiration (ET) of the understorey vegetation in particular during drought periods. Our objectives were (i) to quantify the ET of two common understorey plants Molinia caerulea Moench and Pteridium aquilinum Kuhn and, (ii) to characterize their impacts on soil water availability in mature Quercus petraea Liebl. stands.