EVALUATION OF THE HEAT PULSE VELOCITY TECHNIQUE FOR MEASUREMENT OF SAP FLOW IN RAIN-FOREST AND EUCALYPT FOREST SPECIES OF SOUTH-EASTERN AUSTRALIA

Sap flow in the stems of two cut saplings each of Eucalyptus maculata (a canopy eucalypt forest tree), Doryphora sassafras and Ceratopetalum apetalum (both canopy rainforest trees of south-eastern coastal Austr alia) was measured by the heat pulse velocity technique and compared w ith water uptake from a potometer. Scanning electron micrographs of wo unding caused by implantation of temperature sensor and heater probes into the sapwood showed that wounding was similar in rainforest and eu calypt species and was elliptical in shape, A circular wound has been implicitly assumed in previous studies, Accurate measurements of sapli ng water use were obtained using the smaller transverse wound dimensio n rather than the larger longitudinal dimension because maximum disrup tion of sap flow through the xylem vessels occurred in the transverse plane, Accurate measurements of sap flux were obtained above a minimum threshold sap velocity, These velocities were 15.7, 10.9 and 9.4 cm h (-1) for E. maculata, C. apetalum and D. sassafras, respectively, Belo w the threshold sap velocity, however, sap flow could not be accuratel y calculated from measurements of heat pulse velocity, The minimum thr eshold sap velocity appeared to be determined by probe construction an d xylem anatomy. Despite the elliptical wounding and inaccurate measur ement of sap flow below the threshold sap velocity, total sap flow ove r the experimental period for two saplings of each species was within 7% of water use measured by the potometer.