THE INFLUENCE OF GROWTH RINGS IN PINUS-PATULA ON HEAT PULSE VELOCITY AND SAP FLOW MEASUREMENT

Heat pulse velocity (HPV) apparatus was used to investigate the relation between the radial HPV profile and the position of growth rings in two Pinus patula trees (aged 4 and 7 years), growing in Frankfort State Forest in the eastern Transvaal, South Africa. Successive measurements of HPV at 3.0 mm intervals along each of four radii in each tree revealed a cyclical pattern, with maxima and minima correlated with summer and winter wood, respectively. We recommend that probe emplacement in fast-growing pines with relatively wide growth rings should be to randomly-assigned depths in the sapwood. Confidence limits associated with sample means of wound-corrected HPV obtained with 4, 8 or 12 probe sets were generated by simulating the repeated selection of a randomly-chosen HPV measurement from each sapwood radius. We conclude from this analysis that a minimum of 12 point measurements per tree is required to obtain an acceptable estimate of sap flow. In older trees where a decline in HPV is expected towards the centre of the trunk, sampling must be modified to avoid bias caused by deeper measurements representing proportionately less of the total sapwood area than those taken at more shallow depths in the sapwood.