The effect of environmentally induced stem temperature gradients on transpiration estimates from the heat balance method in two tropical woody species.

Commercially available sap flow gauges were used to evaluate the performance of the stem heat balance (SHB) technique for measuring sap flow in coffee (Coffea arabica L. cv. Yellow Catuai) and koa (Acacia koa Gray) plants under greenhouse and field conditions. Transpiration rates measured gravimetrically and with the SHB technique were similar in greenhouse tests, provided that insulation in addition to that supplied by the gauge manufacturer was applied to reduce radiant heating in the vicinity of the sap flow gauges. Unrealistic estimates of transpiration rates were sometimes obtained under both field and greenhouse conditions as a result of negative stem temperature differentials from below to above the gauge heater, even in the absence of power applied to the heaters. It was possible to correct this environmentally induced bias by means of additional stem insulation that minimized the rate of change in stem temperature, or by applying simple corrections using the DeltaT values for unheated gauges operated as blanks. In the field, where dense canopies reduced the radiant energy load on stems, temperature corrections were unnecessary, because DeltaT values in unheated gauges were near zero.