Geometric-optical modeling of blue oak woodland in California

Summary form only given. Reflectance properties of the blue oak (Quercus douglasii) woodland and savanna were analyzed to determine the contribution of tree canopy cover, shadows, and topography to variations in reflectance in both summer and winter imagery. The goals were to refine a procedure for satellite inventory of hardwood resources in California and to develop a geometric-optical canopy reflectance model for obtaining more detailed biophysical information from scanning radiometer data for this vegetation formation. Thematic mapper data from September 1986 (leaf on, understory senescent) and December 1987 (leaf off, understory green) for seventy-seven 3600 m² plots located on rugged terrain in the Coast Ranges, San Luis Obispo County, were analyzed. Slope geometry and basal area were measured in the field, and tree canopy cover was measured from air photos. The Li-Strahler geometric-optical canopy reflectance model was used to predict the amount of shadow cast by the trees based on crown and illumination geometry, and adjusted for overlap. The oak canopy was modeled as an ellipsoid on a stick, and the shape was parameterized from field data. It was assumed that the sclerophyllous tree canopy would be dark and spectrally similar to its shadow in the absorptive wavebands in summer and in the scattering (infrared) waveband in winter. Therefore, the spectral model could be aggregated to two components, figure (tree plus shadow) and background (understory), and the pixel reflectance modeled as a function of the cover of the figure.