The directional reflectance distributions for both a hardwood and pine forest canopy at Beltsville, Maryland, were measured in June as a function of sun angle from a helicopter platform using a hand-held radiometer with AVHRR band 1 (0.58-0.68 Mm) and band 2 (0.73-1.1 Mm). Canopy characteristics were measured on the ground. The reflectance distributions are reported and compared to the scattering behavior of agricultural and natural grassland canopies. In addition, the three-dimensional

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... e transfer model of Kimes was used to document the unique radiant transfers that take place in forest canopies due to their special geometric structure. Measurements and model simulations showed that the scattering behavior of relatively dense forest canopies is similar to the scattering behavior of agricultural crops and natural grasslands. Only in more sparse forest canopies with significant spacing between the tree crowns (or clumps of tree crowns) does the scattering behavior deviate from homogeneous agricultural and natural grassland canopies. This clumping of vegetation material has two effects on the radiant transfers within the canopy: A) it increases the probability of gap to the understory and/or soil layers that increases the influence of the scattering properties of these lower layers; and B) it increases the number of low transmitting clumps of vegetation within the scene causing increased backscatter and decreased forward scatter to occur relative to the homogeneous case. Both effects, referred to as phenomenon A and B, respectively, tend to increase backscatter relative to forward scatter. For typical forest canopies, the peak backscatter reflectance can be increased as much as 30 percent relative to an equivalent homogeneous canopy due to phenomenon A and 35 percent due to phenomenon B. The combined effect of phenomenon A and B can cause typical increases of 65 percent or higher. It is hypothesized that these phenomena are especially important in sparse conifer forests, such as boreal forests, that account for 50 percent of the world's forest area.