ESTIMATING STAND WATER-USE OF LARGE MOUNTAIN ASH TREES AND VALIDATIONOF THE SAP FLOW MEASUREMENT TECHNIQUE

Mountain ash (Eucalyptus regnans F.J. Muell.) forest catchments exhibi t a strong relationship between stand age and runoff, attributed inter alia to differences in tree water use. However, the tree water use co mponent of the mountain ash forest water balance is poorly quantified. We have used the sap flow technique to obtain estimates of daily wate r use in large mountain ash trees. First, the sap flow technique was v alidated by means of an in situ cut tree experiment. Close agreement w as obtained between the sap flow estimate of water use and the actual uptake of water by the tree from a reservoir. Second, we compared the variability in sap velocity between a symmetric and an asymmetric tree by using multiple sap flow loggers. In the symmetric tree, velocity w as fairly uniform throughout the xylem during the day, indicating that accurate sap flow estimates can be obtained with a minimal number of sampling points. However, large variations in sap velocity were observ ed in the asymmetric tree, indicating that much larger sampling sizes are required in asymmetric stems for an accurate determination of mean sap velocity. Finally, we compared two procedures for scaling individ ual tree sap flow estimates to the stand level based on stem diameter and leaf area index measurements. The first procedure was based on a r egression between stem diameter and tree water use, developed on a sma ll sample of trees and applied to a stand-level census of stem diamete r values. Inputs to the second procedure were tree water use and leaf area of a single tree and the leaf area index of the stand. The two pr ocedures yielded similar results; however, the first procedure was mor e robust but it required more sampling effort than the second procedur e.