Quercus ilex transpiration as affected by a prolonged drought period

The effect of an extended drought (from 1992 to 1995) on water relations was assessed on evergreen oak (Quercus ilex L.) in a dehesa ecosystem (Seville., Southern Spain). Diurnal and seasonal transpiration patterns were analysed at leaf (porometry) and whole-tree level (sap flow), focusing on the relationship between tree transpiration rates (E-t) and potential evapotranspiration rates (PET). Daily maximum E-t varied over the year, becoming higher between May and August, and lower between November and April. Annual E-t (169 - 205 mm y(-1)) accounted for less than 40 % of annual rainfall. The prolonged drought did not affect the water relations of the Q. ilex, mainly due to strong stomatal regulation avoiding the loss of water. Stomatal control was found in all seasons, although it was stronger in summer. This behaviour leads to low water consumption and low E-t/PET ratios throughout the year (0.05 to 0.27).