Stem temperature influence on heat pulse sap flux density measurements

While natural temperature gradients between measurement needles have been thoroughly investigated for continuous heat based sap flow methods, the natural changes in stem temperature because of varying ambient temperature and radiation have, with exception of the Sapflow+ method, so far never been mentioned as a possible error-inducing factor for heat pulse based methods. By modelling both the theoretical equation for an ideal instantaneous and step pulse and applying a finite element model including actual needle dimensions and wound effects, the influence of varying stem temperature on heat pulse based methods was investigated. It was shown that the Heat Ratio method was clearly influenced, while for the Heat Pulse Velocity and Tmax method changing stem temperatures did not lead to significantly different results. For the Heat Ratio method, rising stem temperatures during measurements led to lower heat velocity values while descending stem temperatures led to higher heat velocities and highly erroneous or even imaginary results for high flows. These errors can, fortunately, easily be prevented by including a temperature correction in the data analysis procedure, calculating the slope of the natural temperature change based on the measured temperatures before application of the heat pulse. Results of a greenhouse and outside experiment show the influence of this correction for low and average sap flux densities