INVESTIGATION OF THE FEASIBILITY OF USING TRUNK GROWTH INCREMENTS TO ESTIMATE WATER USE OF EUCALYPTUS GRANDIS AND PINUS PATULA PLANTATIONS

INVESTIGATION OF THE FEASIBILITY OF USING TRUNK GROWTH INCREMENTS TO ESTIMATE WATER USE OF EUCALYPTUS GRANDIS AND PINUS PATULA PLANTATIONS
809/1/01
October 2001

EXECUTIVE SUMMARY

Background

The water use of forest plantations remains a topical subject in light of recent changes to water law, as well as continuing debate over the hydrological impacts of various types of land use. While sap flow and micrometeorological techniques exist to allow direct measurement of forest water use, their usefulness in resolving land use disputes is currently limited for the following reasons:

They are complex, and expensive in terms of both equipment and time. Funds are mostly insufficient to permit measurements of water use over a useful period of time. Measurements can generally only be sustained for a small fraction of a forest rotation.
Forest water use is known to be affected by a wide range of weather, forest stand and site factors, and therefore our ability to extrapolate from relatively few research sites to the entire forestry region is limited. Growth and water use is especially sensitive to the availability of soil water at a site, which is mostly impractical to measure because of deep rooting depths, highly variable stoniness and depth to bedrock, and uncertainty over lateral soil water flow.

An alternative approach to the estimation of plantation water use was suggested by the results of a previous investigation into the relation between annual trunk volume increments and cumulative annual water use of Eucalyptus grandis trees. The study was conducted in Kruisfontein Plantation (White River district, Mpumalanga), and involved a full year of growth and sap flow measurement in 17 trees growing on a wide range of site quality. A surprisingly tight, linear relation was found between annual volume growth increment and cumulative annual sap flow, suggesting that this measure of the efficiency of water use did not differ significantly among the trees. Further data from a Pinus patula experiment in the White River area (Witklip Plantation) revealed a similar efficiency of water use.