Nighttime water use in an irrigated Eucalyptus grandis plantation.

Sap flow measurements showed that a well-watered four-year-old plantation of Eucalyptus grandis (Hill ex Maiden) at Wagga Wagga, New South Wales, Australia, used 0.8 mm of water between 2100 and 0500 h on the midwinter night of July 30. Sap flow ceased for 2 to 3 h after sunset before recommencing at high rates that reached a maximum of 0.3 mm per h between 0200 and 0300 h. This pattern is inconsistent with the replenishment of tissue water reserves depleted during the day. Moreover, maximum leaf conductance at night was about 20 times maximum cuticular conductance values reported in the literature, which strongly suggests that stomata were partly open and that there was substantial water loss by way of the foliage. In an 8-month period from late winter to mid-autumn, comparable rates of nighttime water use were observed on only one other occasion. However, water use at rates of 0.1 mm per h or more occurred on 24 other nights. Almost 70% of the variation in nighttime sap velocity was explained by nighttime mean vapor pressure deficit and nighttime mean wind speed. Total recorded nighttime water use of the plantation was 48 mm, or 5% of total transpiration during the 8-month study. In view of the insensitivity of heat pulse measurements at low sap flows, this value may be an underestimate of actual nighttime transpiration.