Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. I. Trees and forest floor daily transpiration

Water use of a Quercus robur ( L.) declining stand was estimated from 1999 to 2001 by measuring independently tree canopy and 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