An alternative method to estimate zero flow temperature differences for Granier's thermal dissipation technique

Calibration of the Granier thermal dissipation technique for measuring stem sap flow in trees requires determination of the temperature difference (Delta T) between a heated and an unheated probe when sap flow is zero (Delta T-max) . Classically. Delta T-max has been estimated front the maximum predawn Delta T, assuming that sap flow is negligible at nighttime. However. because sap flow may continue during the night, the maximurn predawn Delta T value may underestimate the true Delta T-max. No alternative method has yet been proposed to estimate Delta T-max when sap flow is non-zero at night. A sensitivity analysis is presented showing that errors in Delta T-max may amplify through sap flux density Computations in Granier's approach, such that small amounts of undetected nighttime sap flow may lead to large diurnal sap flux density errors, hence the need for a correct estimate of Delta T-max. By rearranging Granier's original formula, ail optimization method to compute Delta T-max from simultaneous measurements of diurnal Delta T and micrometeorological variables, Without assuming that sap flow is negligible at night, is presented. Some illustrative examples are shown for sap flow measurements carried out on individuals of Erica arborea L., which has needle-like leaves, and Myricafaya Ait., a broadleaf species. We show that, although Delta T-max values obtained by the proposed method may be similar in some instances to the Delta T-max predicted at night. in general the values differ. The procedure presented has the potential of being applied not only to Granier's method. but to other heat-based sap flow systems that require a zero flow calibration, such as the Cermak et al. (1973) heat balance method and the T-max heat pulse system of Green et al. (2003).