Solar radiation absorption by leafless hardwood forests

Abstract

Variations in the solar radiation regime of a deciduous forest in winter have never been analyzed. When snow is on the ground, roughly 65% of the incident radiation is absorbed by stems and branches and 20% is reflected; the remaining 15% is absorbed by the snow. However, these proportions vary greatly according to the solar zenith angle, θ, the ratio of direct to diffuse radiation, the ground or snow albedo, and canopy characteristics. A canopy model has been developed based on a uniformly absorbing crown space and a stem space of randomly arranged vertical cylinders. The direct or beam radiation transmission of the stand, t, is then: t= exp(−P c secθ−P s tanθ) The crown-space parameter, P c includes a crown absorption coefficient and must be found empirically, but the stem-space parameter, P s, is fully specified by the basal area, mean diameter, and mean tree height. With the beam radiation fraction and snow albedo as further independent variables, the model gives the fractions absorbed by the canopy and by the snow and gives the albedo of the stand. Field tests using silicon photocells in three forests demonstrated the validity of the model.