Leaf energy balances: developments and applications

Abstract

Theory of leaf energy balances is outlined in the context of four applications: (1) prediction of leaf temperatures during clear nights; (2) estimation of transpiration rates and stomatal conductances of individual leaves by using pairs of coated and normal leaves; (3) measurement of leaf boundary-layer conductances using pairs of artificial, electrically heated leaves; and (4) use of method (2) to assess validity of whole-tree transpiration rates measured with large ventilated chambers. Good agreement was found between predicted and measured leaf temperatures on clear nights. Leaf temperatures were controlled mainly by loss of thermal radiation to cold skies and by gain of sensible and latent heat from surrounding air. Leaves with condensation on them were 1-2 °C warmer than dry leaves under otherwise similar ambient conditions. Free convection was unimportant relative to forced convection as a mechanism for heat transfer to leaves during calm, clear nights. Satisfactory estimates of single-leaf transpiration were obtained using pairs of coated and uncoated leaves provided both were equally exposed to incoming radiation. Electrically heated facsimile leaves gave satisfactory estimates of leaf boundary-layer conductances in the field. Large ventilated chambers had a small influence on measured transpiration rates according to estimates made with the Penman-Monteith equation for both chamber positions.