Physical and Physiological Basis for the Reflectance of Visible and Near-Infrared Radiation from Vegetation

Knowledge of how solar radiation interacts with vegetation is necessary to interpret and process remote sensing data of agricultural and many natural resources. A plant leaf typically has a low reflectance in the visible spectral region because of strong absorption by chlorophylls, a relatively high reflectance in the near-infrared because of internal leaf scattering and no absorption, and a relatively low reflectance in the infrared beyond 1.3 ~ because of strong absorption by water. The reflectance of a plant canopy is similar, but is modified by the nonuniformity of incident solar radiation, plant structures, leaf areas, shadows, and background reflectivitics. Airborne sensors receive an integrated view of all these effects, and each crop or vegetation type tends to have a characteristic signature which permits its discrimination. When disease and physiological stresses directly affect the reflectance properties of individual leaves, the most pronounced initial changes often occur in the visible spectral region rather than in the infrared because of the sensitivity of chlorophyll to physiological disturbances. The primary basis for the detection of stress conditions in a crop or other plant community by aerial remote sensors often, however, is not a change in the reflectance characteristics of individual leaves, but a reduction in the total leaf area exposed to the sensors. This reduction can result from a direct loss of leaves, a change in their orientation, or an overall suppression of plant growth. In such eases the total infrared reflectance tends to be decreased relatively more than'the visible reflectance because of a reduction in the infrared enhancement due to fewer multiple leaf layers and because of an increase in background exposure.