New approach for measuring low sap velocities in trees

The compensation heat-pulse method (CHPM) for measuring sap velocity in tree stems shows a poor performance for low velocities. Here we present a new procedure, that extends the validity of the technique down to zero and even for small negative values of the corrected heat-pulse velocity nu(c). A simulation model of heat transport for the sensor inserted in the tree showed that nu(c) is linearly related to the average temperature difference (Delta T(a)) between the 2 needles from negative nu(c) up to 30 cm h(-1). The relation was confirmed for olives and pistachio trees in the field with no apparent variation for very long periods (months). The procedure, named calibrated average gradient (CAG), consists in averaging the Delta T(a) measurements after the pulse, then obtaining empirically the function nu(c) = f (Delta T(a)) in its linear domain, when nu(c) is still measurable with the traditional CHPM. The calibrated function can then be used to calculate nu(c) below the minimum velocity measurable by the CHPM as determined by the measuring system resolution. The nu(c) = f (Delta T(a)) function is sensor-specific and depends only on sensor characteristics and thermal properties; its empirical fit allows not only estimating nu(c) for low transpiration conditions, but may also serve to check possible variations in sensor performance or changes in thermal properties of xylem tissue caused by wounding, advocating the need to re-install sensors in a new position. (C) 2008 Elsevier B.V. All rights reserved.