Theory and practical application of heat pulse to measure sap flow

Heat pulse methods can be used for accurate measurements of sap flow in plant stems provided a reliable calibration procedure is used to relate the measured heat pulse velocity to the actual sap flow. This paper reviews the theory underpinning both the compensation and T-max heat pulse methods that use a linear heater and temperature probes inserted radially into the plant stem. These probes not only disrupt the sap stream, but they also alter the thermal homogeneity of the sapwood in the vicinity of the probes. The degree of disturbance depends on the size and geometry of the probes and the corresponding wound width of the nonconducting sapwood. A two-dimensional model of heat and water flow was used here to derive appropriate correction factors to account for the influence of both probe thermal properties and flow blockage. Wound width has a large influence on the heat pulse measurements while sensor material appears to have little or no influence. A table of correction factors is presented for both the compensation and T-max methods. These new correction factors are confirmed by comparing heat pulse measurements in the trunk of a willow (Salix alba L.) and a poplar (Populus deltoides W. Bartram ex Marsh), against actual rates of transpiration determined from measured weight loss of the trees growing in large lysimeters. On a daily basis, both heat pulse measurements were found to be within 5 to 10% of the actual transpiration. The compensation method accurately measured flows close to 2 cm/h. The T-max method had difficulty resolving any flows slower than about 10 cm/h.