Original article Evapotranspiration of a declining Quercus robur (L.) stand

herb layer transpiration. Two plots differing in density were compared. Oak daily sap flux density kinetic is well synchronised with potential evapotranspiration (PET) daily time course. Despite differences in density, stand structure and LAI spatial organisation, oak transpiration (T, mm d–1) is quite the same between plots. The declining trees are very responsive to the PET fluctuations, but their daily response is low (T ≤ 1 mm d–1; T/PET < 0.3). A combination of soil constraints and low, disorganised LAI could induce this low transpiration capability. According to its phenology, density and the above canopy closure, the herbaceous layer contributes to at least the same but often more water consumption than the oak (up to 2.9 mm d–1). Therefore it cannot be neglected in water balance calculations