© 1995 Heron Publishing—Victoria, Canada
Estimating stand transpiration in a Eucalyptus populnea woodland with the heat pulse method: measurement errors and sampling strategies
Thomas J. Hatton, Stephen J. Moore and Peter H. Reece
CSIRO Division of Water Resources, GPO Box 1666, Canberra ACT 2601, Australia / Received June 28, 1994
Summary
Sap flow measurement techniques, such as the heat pulse (compensation) method, are practical means for estimating the water
use of individual trees and are often the only reasonable alternative for measuring forest and woodland transpiration in complex
heterogeneous terrain. The need to scale estimates of water use from a sample of individual stems to a stand (population)
of known area may be satisfied by applying scalars of flux based on tree size or domain. We estimated the aggregate errors
in applying the heat pulse technique to the estimation of stand transpiration in a poplar box (Eucalyptus populnea F.J. Muell.) woodland in southeastern Queensland, Australia, by a combination of precision analyses, experimental validation
and Monte Carlo simulations of sampling errors. Errors in sap flux density measurements were approximately 13%. The potential
error in the flux estimates for individual stems with stratified sampling of sap flux density with depth and bole quadrant
based on four sensors was an additional 25%. Conducting wood area, diameter at 1.3 m, leaf area and domain based on Ecological
Field Theory all proved excellent scalars of flux at the stand level. With a sample size of six trees stratified by diameter,
coefficients of variation in scaling to the stand level were approximately 5% for any of these scalars. The greatest potential
source of error in estimating stand transpiration by the heat pulse method was in the measurement of the fluxes of individual
stems; scaling these measurements to a homogeneous stand of trees involved less uncertainty.
Keywords:
sap flow, scaling, water use.
