Tree Leaf Energy Balance

Detached leaves of three tree species, Quercus velutina, Magnolia grandiflora, and Prunus caroliniana, were exposed in transpiring and non—transpiring situations under high radiation intensities to a range in wind velocities. Convective heat losses from these leaves were determined as a residual in the energy balance equation, with all other energy exchange components being measured or known. The leaves responded to their external environment in accordance with accepted heat transfer theory. Leaf—air temperature differences were maximum for non—transpiring leaves in still air, and minimum for transpiring leaves exposed to high wind velocities. Convective heat transfer coefficients, calculated from the measurements of convection, were compared with those determined from heat transfer theory. The forced convection coefficients agreed closely with those predicted by theory. The free convection coefficients, however, were close to the theoretical values only for the larger leaves. They increased to about twice the predicted values as leaf size decreased. Reasons for this difference are discussed. The ratios between convection and latent heat exchange (Bowen ratios) for the individual leaves were different from published values for whole forest stands. These differences are considered in discussing the relative role of individual heat transfer processes in various portions of the tree crown.