Comment on “Sap flow measurements by a single thermal dissipation probe: exploring the transient regime” — Ann. For. Sci. 66 (2009) by Mahjoub et al. and “Sap flow measurement by a single thermal dissipation probe in transient regime: implementation of the method and test under field conditions” — Ann. For. Sci. 1-9 (2012) by Masmoudi et al.

A single‐probe heat pulse method for estimating sap velocity in trees

“…Apart from the heating scheme, there is another essential difference between heat pulse and TTD techniques: whereas SPHP is based on the fundamental heat conduction−convection equation (Eqn ), the groundwork of TTD remains empirical. Regardless of the method, thermal properties of sapwood are implicitly (TTD) or explicitly ( D x in Eqn ) present in the working equations (Vandegehuchte & Steppe, ). The empirical relationships used by TTD are, therefore, species‐specific and would require challenging ad hoc calibration procedures to provide reliable outputs (Wullschleger et al ., ).…”

“…() used a single heater probe to explore relationships between sap flux density and thermal indices from a cyclic heating scheme. Their best relationships were found in the cooling phase and presented two coefficients that are implicitly species‐specific (Vandegehuchte & Steppe, ). Do et al .…”

“…() re‐examined the issue and both provided approaches that apparently removed the species‐specificity of the method at the cost of requiring zero‐flow conditions. However, as recent evidence indicates, no continuous method based on Granier ()'s principles exists that is completely independent of the species (Wullschleger et al ., ; Vandegehuchte & Steppe, ).…”