The importance of conduction versus convection in heat pulse sap flow methods

Heat pulse methods are a popular approach for estimating sap flow and transpiration. Yet, many methods are unable to resolve the entire heat velocity measurement range observable in plants. Specifically, the Heat Ratio (HRM) and Tmax heat pulse methods can only resolve slow and fast velocities, respectively. The Dual Method Approach (DMA) combines optimal data from HRM and Tmax to output the entire range of heat velocity. However, the transition between slow and fast methods in the DMA currently does not have a theoretical solution. A re-consideration of the conduction/convection equation demonstrated that the HRM equation is equivalent to the Péclet equation which is the ratio of conduction to convection. This study tested the hypothesis that the transition between slow and fast methods occurs when conduction/convection, or the Péclet number, equals one, and the DMA would be improved via the inclusion of this transition value. Sap flux density was estimated via the HRM, Tmax and DMA methods and compared with gravimetric sap flux density measured via a water pressure system on 113 stems from 15 woody angiosperm species. When the Péclet number ≤ 1, the HRM yielded accurate results and the Tmax was out of range. When the Péclet number > 1, the HRM reached a maximum heat velocity at approximately 15 cm hr -1 and was no longer accurate, whereas the Tmax yielded accurate results. The DMA was able to output accurate data for the entire measurement range observed in this study. The linear regression analysis with gravimetric sap flux showed an r2 of 0.541 for HRM, 0.879 for Tmax and 0.940 for DMA. With the inclusion of the Péclet equation, the DMA resolved the entire heat velocity measurement range observed across 15 taxonomically diverse woody species. Consequently, the HRM and Tmax are redundant sap flow methods and have been superseded by the DMA.