Assessing canopy biomass and vigor by model-inversion of bidirectional reflectances: problems and prospects

Determining the biomass of an incomplete canopy by inversion of a plant/soil model depends upon assessing the separate contributions of the soil and the leaves to the overall surface reflectance. The factors affecting the relative importance of these two surface components are: 1) the canopy optical thickness, 2) the canopy architecture (these first two factors control the gap function), 3) the magnitude and anisotropy of the leaf reflectance (which control single-scattering backscattering), 4) the magnitude and anisotropy of the leaf transmittance (which control forward scattering), and 5) the magnitude and anisotropy of the soil reflectance. An appropriate inversion is made more problematic by the fact that these factors can change over time in response to environmental influences. Applying a simple model, the authors present and discuss bidirectional reflectances for an incomplete canopy/soil surface. Isotropic leaf and soil reflectances produce quite different overall reflectance patterns than those based on measured leaf and soil reflectances.