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 (ΔT) between a heated and an unheated probe when sap flow is zero (ΔTmax). Classically, ΔTmax has been estimated from the maximum predawn ΔT, assuming that sap flow is negligible at nighttime. However, because sap flow may continue during the night, the maximum predawn ΔT value may underestimate the true ΔTmax. No alternative method has yet been proposed to estimate ΔTmax when sap flow is non-zero at night. A sensitivity analysis is presented showing that errors in ΔTmax 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 ΔTmax. By rearranging Granier's original formula, an optimization method to compute ΔTmax from simultaneous measurements of diurnal ΔT 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 Myrica faya Ait., a broadleaf species. We show that, although ΔTmax values obtained by the proposed method may be similar in some instances to the ΔTmax 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 Čermák et al. (1973) heat balance method and the T-max heat pulse system of Green et al. (2003).