Distribution of photosynthetically active radiation in the canopy of a loblolly pine plantation.
Photocells were traversed at 6 canopy heights in a 15-yr-old plantation in North Carolina. When incident radiation was predominantly direct-beam, a bimodal distribution of photosynthetically active radiation in the canopy was observed. One set of data had nearly the same irradiance as incident radiation, indicating sunfleck penetration. When incident radiation was diffuse and the photocells naturally shaded, a unimodal distribution was observed. An exponential relation was found between extinction of radiation and cumulative leaf area index; the extinction coefficient was dependent on solar altitude. A model of sunfleck penetration was used to estimate relative gap frequency, and indicated that at the top of the forest the canopy was not closed. Radiation intercepted by the entire live crown was similar to that expected for a canopy with randomly distributed foliage. From authors' summary.