MULTISPECTRAL BIDIRECTIONAL REFLECTANCE OF NORTHERN FOREST CANOPIES WITH THE ADVANCED SOLID-STATE ARRAY SPECTRORADIOMETER (ASAS)

Understanding the bidirectional reflectance characteristics of forest canopies is important for relating remote sensing observations to biomass, species, stand structure, and albedo. studies of the directional scattering of forest canopies have been limited in the past because of difficulties in suspending instruments over tall canopies or by being restricted to airborne scanning systems. In this article we discuss the use of the Advanced Solid-State Array Spectroradiometer (ASAS) to measure the bidirectional reflectance of several diverse forest canopies. ASAS data were acquired in 1989 and 1990 as part Of an extensive field measurement campaign at International Paper's Northern Experimental Forest (NEF) located near Howland, Maine. Multiangle data sets were acquired for several sites under clear sky conditions and at view azimuths parallel, oblique, and perpendicular to the principal plane of the sun. The sensor radiance data for several surface types including forest stands and openings were converted to bidirectional reflectance using an atmospheric correction algorithm and optical thickness measurements. In addition, photosynthetically active radiation (PAR) hemispherical reflectance (albedo) was calculated from the corrected ASAS data and compared to a vegetation index. Highest observed reflectance factors were recorded in or near the solar principal plane at viewing geometries approaching the antisolar direction. Minimum reflectances were also observed in the solar principal plane, but in the forward scattered direction. Bidirectional reflectance factors (BRFs) acquired in the backscatter direction at visible wavelengths showed larger percent differences between viewing angles of 45-degrees in the forward and backscattered direction (up to 95%) than near-IR data (up to 60%). The normalized vegetation index (NDVI) also varied with view angle but to a lesser degree than single-band BRFs. Also in contrast to single-band BRFs, maximum NDVI was recorded in the forward scattered direction and minimum NDVI was observed in the backscattered direction. Estimated fraction of absorbed photosynthetically active radiation F(APAR) was determined from the hemispherical PAR reflectance for several canopy types within the forest area. Examining the relationships with NDVI revealed a strong dependence of NDVI on F(APAR) for nadir and 450 forward scattered data. A poor relationship was observed for data acquired at 45-degrees in the backscattered direction and for NDVI derived from hemispherical reflectance. From these results, it is apparent that the sensor viewing geometry that minimizes the contribution of reflectance from branches and the ground will yield higher relationships between F(APAR) and NDVI.