Continuous measurements of the energy balance were made during the 1991 growing season over a dwarf willow‐birch forest located near Churchill, Manitoba. Intensive measurements of stomatal conductance for several species were taken on three fair‐weather days. These represented a wide range of air temperatures and leaf‐to‐air vapour pressure deficits and allowed the quantification of the surface‐atmosphere interaction. Modelling evaporation on a 0‐5 h basis can be performed accurately using a modified version of the Penman‐Monteith combination model coupled to a submodel of stomatal conductance. With a vegetated surface cover of about 90 per cent at full leafing, vegetation plays an important role in the overall moisture flux because 80 per cent is a result of transpiration. Simulating various vegetation‐change scenarios shows that species composition, through differences in stomatal behaviour, has a marked effect on evaporation.