A triple-probe heat-pulse method for measurement of thermal diffusivity in trees

Although thermal diffusivity is a crucial parameter for sap flow calculations in both the heat field deformation and the heat ratio method, it is seldom measured on a routine basis. This paper presents a theory based on thermodynamic principles to determine both axial and tangential diffusivity in sapwood. By measuring the temperature response after application of a heat pulse at a short axial and tangential distance from a line heater, axial and tangential thermal conductivity as well as volumetric heat capacity of the sapwood can be derived from a theoretical model. From these parameters, axial and tangential diffusivity can easily be determined. Sensitivity analysis and results of an experiment on European beech (Fagus sylvatica L.) confirm the applicability of the method. The obtained thermal diffusivities ranged from 2.7 × 10−7 m2 s−1 to 2.2 × 10−7 m2 s−1 for a relative water content (moisture per dry weight) ranging from 0.47 to 0.90, respectively. This was on average 22% lower than when applying the common methodology based on wood core sampling.