REFLECTANCE OF VEGETATION AND SOIL IN CHIHUAHUAN DESERT PLANT-COMMUNITIES FROM GROUND RADIOMETRY USING SPOT WAVEBANDS

The spatially averaged reflectance of partially vegetated land surface can be modeled as an area-weighted mixture of the reflectances of dif ferent components or classes of objects (plants, shadow) on a backgrou nd (soil, grass). We sampled the spectral reflectance of the shaded an d unshaded components of Chihuahuan desert plant communities (shrubs, soil, subshrubs, and perennial grasses) in the SPOT wavebands using a hand-held radiometer. We examined the mean reflectance differences bet ween components to evaluate their spectral separability. Shrub canopy and shaded components have similar reflectance in the visible waveband s. However, in the near-infrared band, which is strongly scattered by green plant canopies, the shaded canopy and shaded background componen ts were similar to each other and lower than either sunlit background or sunlit canopy. When reflectance measurements were transformed to no rmalized ratio (NDVI, SAVI) and orthogonal green vegetation indices, t he shaded and sunlit portions of each component (canopy and soil) were similar but the shaded components were intermediate between their sun lit counterparts. Different soil types and plant species with differen t life forms (e.g., shrubs, grasses) and phenologies exhibited differe nt reflectance characteristics. However the broadband reflectances of the three dominant shrub species were similar at the end of the growin g season, in spite of their differences in morphology.