A heat balance method for measuring sap flow in small trees

Abstract

SUMMARY The heat balance method of Sakuratani (1981) was adapted and improved for measuring sap flow in fruit trees of high density orchards. In this technique the trunk is surrounded by a heating jacket operating at constant power. Thermocouples are used to measure conductive and convective heat flux and therefore the sap flow rate from the heat balance. This method does not require any calibration before the setting up of the sap flow sensor. INTRODUCTION Several thermal methods have been used for measuring sap flow in the trunk of trees: heat pulse methods (Marshall, 1958; Swanson & Whitfield, 1981) and those that depend on the temperature of a linear heat source inserted in the xylem (Granier, 1985). These methods give the sap flux density (kg s −1 m −2 ). The estimation of the sap flow rate (kg s −1 ) involves the measurement of the cross-sectional area of the functional xylem. An improvement of the heat pulse method (Cohen, Fuchs & Green, 1981) avoids this additional measurement by estimating the radial profile of sap velocity, but assembling the sensor is very difficult. Heat balance methods were applied also on sectors of tree trunks by Daum (1967), Cermak, n, Kucera & Penka (1976). With all these methods, there is a question of how representative the measurements are of the overall flow rate in the trunk. Here we propose a direct measurement of the sap flow rate in the whole section of the stems or trunks as an improvement of the heat balance method (Sakuratani, 1981; Valancogne & Nasr, 1989a, 1989b).