Evaluation of the heat pulse velocity method for measuring sap flow in Pinus patula

Information on the water use of Pinus patula plantations is required to predict the impact of forest plantations on water resources in South Africa. The heat pulse velocity (HPV) method is a promising technique for measuring water use by trees, and has been shown to measure sap flows accurately in a variety of hardwood trees. This method has not been sufficiently verified for pine trees where the presence of a strongly-defined ring structure in the sapwood gives rise to a complex radial pattern of sap flow. The purpose of this study was to compare water uptake by cut trees to simultaneous HPV sap flow measurements in the same tree. Fourteen trees were used for this comparison. Results showed that HPV sap flow estimates consistently overestimated cut-tree uptake by an average of 49%. The bias is attributed to heat averaging across non-conducting latewood rings. Water uptake was found to be highly correlated to the product of under-bark cross-sectional area and woundcorrected mean HPV, and it is suggested that this empirical relation provides a more appropriate way of estimating water use by this species.