Distribution of Photosynthetically Active Radiation in the Canopy of a Loblolly Pine Plantation

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Abstract
Photocells were traversed at 6 heights in a loblolly pine canopy to obtain instantaneous measurements of photosynthetically active radiation. When the incident radiation was predominantly direct-beam, a bimodal distribution of radiation in the forest canopy was observed. One set of data had nearly the same irradiance as incident radiation indicating sunfleck penetration in the canopy. The other set of data resulted when the photocells were naturally shaded. During incident conditions of mostly diffuse radiation, a single modality was observed in the canopy. Allometric relationships allowed the leaf area and its height distribution to be estimated. An exponential extinction of radiation with cumulative leaf area index was found. The exponential extinction coefficient was dependent on the solar attitude. A model of sunfleck penetration in the leaf canopies was tested to estimate the relative gap frequency of the forest. This analysis confirmed that at the top of the forest the canopy was not closed, while the radiation intercepted by the entire live crown was similar to that expected for a canopy with randomly distributed foliage.

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