INCLUDING THE HEAT-STORAGE TERM IN SAP FLOW MEASUREMENTS WITH THE STEM HEAT-BALANCE METHOD

The importance of the change in stem temperature and therefore the heat storage term in the stem heat balance method of measuring sap flow is examined. Results from a range of measurements on a model stem, potted sunflower plants in a glasshouse, and Guiera senegalensis shrubs in the Sahel, Niger, are presented. A novel analysis of the heat balance in zero flow conditions allows the accurate determination of the gauge radial conductance and the stem segment heat capacity, both of which are required for accurate sap flow measurement with good dynamic resolution in low flow conditions. In high sap flow conditions the change in heat storage constitutes only a small component of the balance, and can be neglected, especially for small stems. The improved accuracy and dynamic resolution for stems of any size if heat storage is included allowed the measurement of low night-time flows during rehydration, and of redistribution of water between stems of G. senegalensis bushes in the field following rain.