REFLECTANCE PROPERTIES OF WEST-AFRICAN SAVANNA TREES FROM GROUND RADIOMETER MEASUREMENTS .2. CLASSIFICATION OF COMPONENTS

The reflectance properties in the red and near-infrared of three Sahelian tree species were measured using a pole-mounted radiometer, and classified depending on their location over the canopy. A geometrical description of the patterns of shadow and sunlight on and beneath a model tree when viewed from above is given and six components defined. These components are: (1) sunlit soil, (2) sunlit canopy over sunlit soil, (3) sunlit canopy over shaded soil, (4) shaded canopy over shaded soil, (5) shaded canopy over sunlit soil and (6) shaded soil. The classification differs from the four-component model of Li and Strahler in that the presence or absence of shadow on the ground is considered. The average reflectances differ between components in the red, but in the near-infrared no statistical differences were observed. The results for the red waveband suggest that the presence of shadow on the ground under these fairly open canopies is important in determining the reflectance of the components. Average whole canopy reflectances were estimated using the geometrical model to calculate the areas of the different components within the canopy. Similarly, the spatial average reflectance from a landscape with a known population of trees and known soil was estimated. For the three species and locations measured here it was found that the tree canopies are dark in the red waveband with respect to the soil, but have little or no effect on the near-infrared.