MODEL FOR CALCULATING PHOTOSYNTHETIC PHOTON FLUX DENSITIES IN FOREST OPENINGS ON SLOPES

A model has been developed to calculate the spatial distribution of th e photosynthetic photon flux density (PPFD) in elliptical forest openi ngs of given slopes and orientations. The PPFD is separated into direc t and diffuse components. The direct component is calculated according to the opening and radiation geometries, and pathlength of the solar beam through the forest canopy. The diffuse component is obtained from the sky, tree, and landscape view factors. In this model, the distrib ution of foliage area with height and the effect of foliage clumping o n both direct and diffuse radiation transmission are considered.The mo del has been verified using measurements from six quantum sensors (LI- COR Inc.) located at different positions in a small clear-cut (0.37 ha ) in a 90-year-old western hemlock-Douglas fir forest.