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 vcvc. A simulation model of heat transport for the sensor inserted in the tree showed that vcvc is linearly related to the average temperature difference (ΔTa) between the 2 needles from negative vcvc 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 ΔTa measurements after the pulse, then obtaining empirically the function vc=f(ΔTa) in its linear domain, when vcvc is still measurable with the traditional CHPM. The calibrated function can then be used to calculate vcvc below the minimum velocity measurable by the CHPM as determined by the measuring system resolution. The vc=f(ΔTa) function is sensor-specific and depends only on sensor characteristics and thermal properties; its empirical fit allows not only estimating vcvc 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.