Sap flow measurements by thermal dissipation method using cyclic heating: a processing method accounting for the non-stationary regime

Abstract

The thermal dissipation method to measure sap flow in tree stems can be used with cyclic heating to reduce electricity consumption and/or to account for natural temperature gradients. Nevertheless, errors in sap flow estimation can be introduced because the thermal equilibrium has not been reached at the measurement time. We propose a method to assess this error and to estimate sap flow density. It is based on Granier’s (Ann For Sci, 42:193–200, 1985) non-species-specific calibration. This work was performed on silver fir trees (Abies alba Mill.) with low sap flux densities (maximum of 0.68 L dm−2 h−1). To estimate the error, we developed a calibration procedure using the experimental set-up in trees. This approach is based on a pair of sensors having similar temporal patterns in sap flux density, one being under cyclic heating while the other is continuously heated. Applying Granier’s calibration without correction led to large errors (relative error reached 200%). After correction, the error was greatly diminished; it was lower than 0.042 L dm−2 h−1 when using short heating cycles (0.5 h). The correction was applicable to all silver fir trees monitored. However, this method can be easily repeated to investigate the validity domain of the correction.