The importance of tree species identity and diversity for biogeochemical cycles in forests is not well understood. In the past, forestry has widely converted mixed forests to pure stands while contemporary forest policy often prefers mixed stands again. However, the hydrological consequences of these changes remain unclear. We tested the hypotheses (i) that significant differences in water use per ground area exist among the tree species of temperate mixed forests and that these differences are more relevant for the amount of stand-level canopy transpiration ( E c ) than putative complementarity effects of tree water use, and (ii) that the seasonal patterns of E c in mixed stands are significantly influenced by the identity of the present tree species. We measured xylem sap flux during 2005 (average precipitation) and 2006 (relatively dry) synchronously in three nearby old-growth forest stands on similar soil differing in the abundance of European beech (pure beech stand, 3-species stand with 70% beech, 5-species stand with <10% beech). In summer 2005 with average rainfall, E c was 50% higher in the beech-poor 5-species stand than in the two stands with moderate to high beech presence (158 vs. 97 and 101 mm yr −1 ); in the dry summer 2006, all stands converged toward similar E c totals (128–139 mm yr −1 ). Species differences in E c were large on a sapwood area basis, reflecting a considerable variation in hydraulic architecture and leaf conductance regulation among the co-existing species. Moreover, transpiration per crown projection area ( E CA ) also differed up to 5-fold among the different species in the mixed stands, probably reflecting contrasting sapwood/crown area ratios. We conclude that E c is not principally higher in mixed forests than in pure beech stands. However, tree species-specific traits have an important influence on the height of E c and affect its seasonal variation. Species with a relatively high E CA (notably Tilia ) may exhaust soil water reserves early in summer, thereby increasing drought stress in dry years and possibly reducing ecosystem stability in mixed forests.