An improved heat pulse method to measure low and reverse rates of sap flowin woody plants

The compensation heat pulse method (CHPM) is of limited value for measuring low rates of sap flow in woody plants. Recent application of the CHPM to w oody roots has further illustrated some of the constraints of this techniqu e. Here we present an improved heat pulse method, termed the heat ratio met hod (HRM), to measure low and reverse rates of sap flow in woody plants. Th e HRM has several important advantages over the CHPM, including improved me asurement range and resolution, protocols to correct for physical and therm al errors in sensor deployment, and a simple linear function to describe wo und effects. We describe the theory and methodological protocols of the HRM , provide wound correction coefficients, and validate the reliability and a ccuracy of the technique against gravimetric measurements of transpiration.