Heat balance measurements - To quantify sap flow in thin stems only?

Two methods were used to measure the flow of sap through the xylem of a mature Norway spruce by balancing heat. One, the tissue heat balance (THB), had been designed for tree stems of large diameter and is based on the balance of heat within an internal compartment of tissue conducting the sap. The other, the stem heat balance (SHB), is applicable for stems of small diameter. It follows the principle of balancing heat within a section of the entire stem. Both methods were tested in the laboratory using simple experimental simulators of flow. In the field, comparative measurements were made within the crown of a mature Norway spruce. The THB was inaccurate mainly because (1) it failed to consider the dissipation of heat appropriately during measurements, and (2) assessing the temperature within the tissue of the stem in such a way that it represented the entire heated compartment was not practicable. The SHB yielded more realistic results, A procedure obtaining approximate results also by THB is proposed. It is a fundamental requirement of balancing heat that all its fluxes can be completely assessed. This is unlikely to be achieved for a compartment inside a stem as intended by THB. There the dissipation of heat is not traceable and varies as the flow of sap changes, An insulated section of an entire stem, however, is a compartment, which allows one to measure the SHB including all its major components. Thus, balancing heat appears to be suitable for quantifying the flow of sap in thin stems by SHB rather than in thick stems as intended by THB, unless complex calibrations are made.