Constraints on transpiration from an evergreen oak tree in southern Portugal

Abstract

The experiment took place at a sparse evergreen oak woodland in southern Portugal. Seasonal courses of sap flow, measured in eight points of the stem of a Quercus rotundifolia tree, were monitored during a 2-year period. Plant water relations (predawn and midday leaf water potential, canopy conductance and whole-plant hydraulic conductance) as well as meteorological variables were also measured during the experimental period (May 1996–August 1998). All evidence showed that the plants remained well watered throughout the observation period. The highest transpiration rates occurred during the summer, when only vestigial amounts of rain fell on the shallow soil with a low water storage capacity. This could only be explained by the direct access of the root system to a 13 m deep water table. Although there was no increase in water stress during the summer drought, the transpiration rates showed an upper limit well below the atmospheric evaporative demand. This was consistent with the occurrence of a maximum limit for the root water uptake capacity determined by the summer value of whole-plant hydraulic conductance and by stomatal control, which prevented leaf water potential from falling below a cavitation threshold.