© 1997 Heron Publishing—Victoria, Canada
Estimating stand water use of large mountain ash trees and validation of the sap flow measurement technique
R. A. Vertessy (1, 2), T. J. Hatton (1), P. Reece (1, 2), S. K. O'Sullivan (2, 3) and R. G. Benyon (2, 4)
1. CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia / 2. Cooperative Research Center for Catchment Hydrology, GPO Box 1666, Canberra, ACT 2601, Australia / 3. Monash University, Wellington Road, Clayton, Victoria 3168, Australia / 4. Melbourne Water, Box 4342, Melbourne, Victoria 3001, Australia / Received November 13, 1996
Summary
Mountain ash (Eucalyptus regnans F.J. Muell.) forest catchments exhibit a strong relationship between stand age and runoff, attributed inter alia to differences in tree water use. However, the tree water use component of the mountain ash forest water balance is poorly
quantified. We have used the sap flow technique to obtain estimates of daily water use in large mountain ash trees. First,
the sap flow technique was validated by means of an in situ cut tree experiment. Close agreement was 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
was 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 observed 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 individual tree sap flow estimates to the stand level based on stem diameter and leaf
area index measurements. The first procedure was based on a regression between stem diameter and tree water use, developed
on a small sample of trees and applied to a stand-level census of stem diameter 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 procedures yielded similar results;
however, the first procedure was more robust but it required more sampling effort than the second procedure.
Keywords:
Eucalyptus regnans, flow variability, forest catchment, runoff, sap velocity, stand age.
