BIOPHYSICAL AND BIOCHEMICAL SOURCES OF VARIABILITY IN CANOPY REFLECTANCE

Analyses of various biophysical and biochemical factors affecting plan t canopy reflectance have been carried out over the past few decades, yet the relative importance of these factors has not been adequately a ddressed. A combination of field and modeling techniques wee used to q uantify the relative contribution of leaf, stem, and litter optical pr operties (incorporating known variation in foliar biochemical properti es) and canopy structural attributes to nadir-viewed vegetation reflec tance data. Variability in tissue optical properties was wavelength-de pendent. For green foliage, the lowest variation was in the visible (V IS) spectral region and the highest in the near-infrared (NIR). For st anding litter material, minimum variation occurred in the VIS/NIR, whi le the largest differences were observed in the shortwave-IR (SWIR). W oody stem material slowed opposite trends, with lowest variation in th e SWIR and highest in the NIR. Leaf area index (LAI) and leaf angle di stribution (LAD) were the dominant controls on canopy reflectance data with the exception of soil reflectance and vegetation cover in sparse canopies. Leaf optical properties (and thus foliar chemistry) were ex pressed most directly at the canopy level in the NIR, but LAI and LAD strongly controlled the relationship between leaf and canopy spectral characteristics. Stem material played a small but significant role in determining canopy reflectance in woody plant canopies, especially tho se with LAI<5.0. Standing litter significantly affected the reflectanc e characteristics of grassland canopies; small increases in the percen tage of standing litter had a disproportionately large affect on canop y determine the relative contribution of tissue, canopy, and landscape factors that drive variation in a reflectance signal. Deconvolution o f these factors requires an understanding of the sources of variance a t each scale (which is ecosystem dependent) as well as an adequate sam pling (spectral, angular, and temporal) of the shortwave (400-2500 nm) spectrum. (C)Elsevier Science, Inc. 1998.