Heat Field Deformation Sensors for Sap Flow Measurements in Small Stems

The Heat Field Deformation (HFD) method was originally developed for sap flow measurements in tree organs with diameters larger than 2-3 cm. The design consists of 4 needles of various lengths that must be inserted into the conducting xylem of the tree. The ability to measure sap flow in smaller stems with diameters less than 2 cm was also investigated through experimentation during the original development. An alternative configuration of the classical HFD method was constructed that involved a non-invasive design. This configuration was known affectionately as the "HFD Baby Sensor". Several modifications of HFD Baby sensors were trialled. Sensor designs with external (exo-sensors) and internal (endo-sensors) heating were investigated in an attempt to replicate the HFD methodology on small and herbaceous stems to ensure water relations at any position or organ on the plant could be measured and compared using the same principle. However, this information is practically unknown. This presentation aims to make up this "gap" and demonstrate the ability of the HFD method to measure sap flow also in stems lower than 2 cm in diameter.