Customizing single-point thermal dissipation sensors for characterizing radial profiles in situ

There is a growing recognition that uncalibrated use of thermal dissipation (TD) sap flow sensors impacts measurement accuracy. Thus, species-specific corrections are becoming more common to better leverage this method’s advantages for use in forest transpiration studies. The goal of this study was to develop a novel field methodology using a movable TD sensor to construct radial gradient characteristic curves that correct estimates of sap flux density (Js) in situ through precision placement of the single point sensor. We also investigated how the radial profile shapes varied with tree diameter. Using a mobile 10-mm TD heater sensor, the radial profile shape was characterized for individual trees by inserting and moving the probe as low as 1-mm depth increments, repeating trials to validate the method. Although the Gaussian radial profile shape was repeatable across replicates, the relative depth-to-peak flow differed greatly between trees, indicating that knowledge of depth-to-peak is critical for sensor placement in replicate trees of the same species. This novel field approach demonstrates high potential for improving Js estimates through precision placement of TD sensors.