Beam enrichment of diffuse radiation in a deciduous forest

Abstract

By applying a model developed by Reifsnyder et al. (1971) to solar radiation data collected in and above an East Tennessee deciduous forest, solar radiation budgets are approximated for three forest levels within the winter leafless, spring leafless, summer fully leafed and autumnal fully leafed forest phenoseasons. Beam radiation dominates these budgets throughout the year and varies directly with solar elevations. Although beam radiation penetration is severely reduced in the fully leafed forest, this component still accounts for over one-half the total radiation received within the forest in early autumn, the phenoseason of minimal beam penetration. Beam enrichment of diffuse radiation within the forest is also highly dependent upon solar elevation and canopy biomass density. Maximum beam enrichment occurs in summer with the high solar elevation of that season and a fully leafed canopy. Minimum enrichment occurs in the leafless winter forest when solar elevations are lowest and canopy biomass density is minimal. The penetration of diffuse sky radiation is maximal in the leafless forest and minimal in the fully leafed forest but, because of limitations of the model used in this approximation, this quantity appears insensitive to changes in solar elevations. This limitation is a defect of the model but it should not substantially alter the indicated results.