Measurement of low sap flux density in plants using the single needle heat pulse probe

Abstract

Heat pulse probes are used worldwide to quantify sap flow in plants. Probes consisting of a single needle inserted into a plant have a simple physical configuration relative to multi-probe methods. The single probe also minimizes physical damage to plant tissue. Single probe methods are therefore increasingly used for sap flux density measurements. However, at low sap flux density values, existing single probe heat pulse (SPHP) methods cannot be used for accurate measurements. To increase the sensitivity of SPHP to low sap flux densities, this paper describes and validates a simplified expression of sap flow and heat transport under various heat inputs and finite heat pulse time intervals. To test this theoretical development, measurements were collected from SPHP probes inserted into mature poplar forest trees (Populus tomentosa Carrière) during the 2020 growing season. The results show that the new method can successfully monitor low sap flux density: the lowest measured heat pulse velocity was 3 cm h−1, which is an improvement from the original SPHP method (20 cm h−1). The improved SPHP method had a root mean square deviation (RMSD) of 1.63 cm h−1 in comparison with the heat ratio (HR) method. The proposed SPHP method is therefore suitable for quantifying both low and high sap flux densities.