The seasonal variability in the frequency distribution (FD) and higher order moments (e.g., standard deviation, skewness and kurtosis) of photosynthetically active radiation (PAR) penetration in an oak—hickory forest were examined. In a leafless, wintertime forest the FD is unimodal and skewed. In a leafing forest, the FD is highly variable and is strongly skewed. This results from higher solar elevation angles enhancing PAR penetration and new, expanding leaves causing penumbral shade and scattering the incoming radiation. The FD of PAR in the fully-leafed canopy is unimodal in the upper canopy, bimodal below crown closure and unimodal in the subcanopy trunkspace. Low solar elevation angles and leaf senescence cause the FD of PAR in the autumnal canopy to be uniformly distributed, as is indicated by its low kurtosis. FDs of PAR also provide a means of estimating sunlit leaf area. Estimated values of sunlit leaf area below crown closure agree within 15% with theoretical estimates derived with a negative binomial distribution.
