A COMPARISON OF THE HEAT PULSE METHOD AND DEUTERIUM TRACING METHOD FOR MEASURING TRANSPIRATION FROM EUCALYPTUS-GRANDIS TREES

Estimates of transpiration obtained with the heat pulse velocity (HPV) method and the deuterium tracing method were compared in two Eucalyptus grandis Hill ex Maiden trees aged 18 months and 4 years, respectively. The HPV estimate of weighted mean daily flow rate differed from the deuterium mean by 7% in the older tree, and 35% in the younger tree. An analysis of possible sources of error revealed that the discrepancy in the younger tree may have arisen as a result of inadequate probe espacement radially through the sapwood, or from sapwood disruption caused when holes were drilled to release the tracer. We conclude that the two methods can provide very similar estimates of mean weighted transpiration rate in E. grandis trees if due precautions are taken to minimize known sources of error. The relative merits of the two techniques, which in many respects are complementary, are discussed.