Determination of sap flow in Douglas-fir trees using the heat pulse technique

Abstract

A modified heat pulse technique was used to determine volumetric sap flow in 15–17 m tall Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) trees. Laboratory calibration of the heat pulse technique, accomplished by passing water through 200 mm long by 77 mm diameter stem sections with a gas pressure apparatus, showed an underestimation of the actual water flow rate by 47%. Using a six-thermistor temperature sensing probe inserted radially to a depth of 60 mm, field measurements of sap flux density were found to change with depth into the sap wood. Simultaneous measurements using the temperature sensing probes inserted in three azimuthal directions (0, 120, and 240°) showed good agreement during the daytime in three trees, while in another tree the ratio of the three sap flux densities gradually changed during the daytime. Hourly values of sap flow rate in two different trees, obtained using the laboratory calibration factor were 29 and 53% larger than corresponding foliar transpiration rates estimated using measurements of stomatal conductance, tree leaf area and vapour pressure deficit. Using a microprocessor-based data acquisition system, the technique was successfully used to monitor the course of sap flow rate over a 4-month period.