© 1994 Heron Publishing—Victoria, Canada
The effect of environmentally induced stem temperature gradients on transpiration estimates from the heat balance method in
two tropical woody species
Marco V. Gutiérrez (1), Robin A. Harrington (2), Frederick C. Meinzer (3, 4) and James H. Fownes (2)
1. Department of Horticulture, University of Hawaii at Manoa, Honolulu, HI 96822, USA / 2. Department of Agronomy and Soil Science, University of Hawaii at Manoa, Honolulu, HI 96822, USA / 3. Crop Science Department, Hawaiian Sugar Planters' Association, P.O. Box 1057, Aiea, HI 96701, USA / 4. Author to whom correspondence should be addressed / Received March 29, 1993
Summary
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 ΔT 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 ΔT values in unheated gauges were near zero.
Keywords:
Acacia koa, Coffea arabica, coffee, koa, sap flow, stem heat balance technique.
