TRANSPIRATION FROM A MATURE EUCALYPTUS-GLOBULUS PLANTATION IN PORTUGAL DURING A SPRING-SUMMER PERIOD OF PROGRESSIVELY HIGHER WATER-DEFICIT

The rates of transpiration from a mature Eucalyptus globulus Labill. s tand in Portugal were evaluated during a drying period of the spring-s ummer 1994. Transpiration was measured by the Granier sap flow method and estimated by the Penman-Monteith model. During the experimental pe riod daily transpiration varied between 3.64 and 0.50 mm day(-1). For high-transpiration days, a good agreement was observed between Penman- Monteith estimates and sap flow measurements, both on a daily and on a n hourly basis. However, for low-transpiration days, the Penman-Montei th model overestimated transpiration in comparison with the sap flow m ethod. The diurnal variation of sap flow was then smoother and lagged behind the estimates of the Penman-Monteith model. E. globulus showed an efficient control of transpiration losses during dry periods throug h a progressive stomatal closure. As soil moisture deficit increased, the daily maximum stomatal conductance decreased from 0.46 to 0.14 cm s(-1). The results also show that, on a seasonal basis, stomatal condu ctance and daily transpiration were mainly related to predawn leaf wat er potential and, thus, to soil moisture content.