Sap flow measurements by thermal dissipation method using cyclic heating: a processing method accounting for the non-stationary regime
Résumé
Context : 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. Aim: 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. Methods: 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. Results: 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). Conclusion: 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
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