Global shortwave, photosynthetically active, net and allwave radiation was measured above, and at several levels within an oak—hickory forest with instruments mounted on a moving tram system. Profiles of radiation flux densities were quantified using extinction coefficients based on the Beer—Bouguer law. Data are reported here from periods when the forest was both fully-leafed and leafless.
In the fully-leafed forest the solar radiation components are attenuated exponentially in the following manner: ;, where PAR is photosynthetically active radiation, Q∗ is net radiation, K↓ is shortwave radiation and Q↓ is allwave radiation. PAR attenuation is greater than that for the other components because leaves preferentially absorb PAR. This preferential absorption causes the ratio, PAR/K↓, to decrease from 0.49 above the canopy to 0.27 at the forest floor.
During the leafless phenoseason, the radiation components are attenuated exponentially as follows: Q∗ > K↓ = PAR > Q↓.
K↓ and PAR are attenuated in a similar manner during this phenological phase because no leaves are present to absorb PAR preferentially. The magnitude of the attenuation coefficients for Q∗, K↓ and PAR is much greater during winter leafless period because solar elevation angles are lower and the canopy consists of dark, opaque, woody biomass.
Shortwave beam radiation is not attenuated in an exponential manner. Consequently, extinction coefficients for beam radiation, γ(S), were computed separately for the upper canopy and lower canopy. A comparison between measured and modeled γ(S) show periods of reasonable agreement and disagreement. Deviations from theory are attributed to clumping and gaps in the canopy.