Seasonal variations in solar elevation, canopy phenology and leaf pigmentation result in a dynamic variation in the radiation regime within a deciduous forest. Measurements of several insolation components taken within an east Tennessee oak-hickory forest during eight combinations of season and phenological conditions of the canopy are presented. Insolation measurements were made with instruments mounted on a moving tram system at seven levels within, and at one level above the canopy.
The attenuation of solar radiation is least during the leafless phenoseason since only woody biomass is present to intercept and absorb it. Net radiation is attenuated to a greater extent than shortwave and photosynthetically active radiation, which are attenuated to a similar degree, since the sparse woody biomass does not effectively trap the outgoing reflected and radiated radiation. Solar elevation did not influence the attenuation of insolation within the leafless forest under the conditions studied.
The attenuation of insolation within the leafing, spring canopy increases progressively with increasing leaf area. The attenuation among the components, however, varies. Shortwave attenuation progresses at a faster rate with expanding leaf area than photosynthetically active radiation since newly expanded leaves are low in chlorophyll. The penetration of light into the leafing canopy is independent of solar elevations less than 40° and increases linearly with greater solar elevations.
The radiation regime within the fully-leafed canopy is rather static. For daily mean values, no effects of leaf age or seasonal variation in solar elevation are evident. Penetration of light into the canopy, however, increases when solar elevation exceeds 65°.
The attenuation of insolation components diminishes during the autumnal senescence—abscission phenoseason, in response to changes in leaf pigment and loss of foliage. Attenuation of insolation is greater in the autumn than during spring periods with similar leaf area since the sun is lower in the sky in the autumn.
The depletion of photosynthetically active radiation with depth is minimal during the leafless, early leafing and autumnal phenoseasons. Moderate depletion occurs during the late leafing period and maximal depletion occurs during the fully-leafed phenoseason.