NDVI Directionality in Boreal Forests: A Model Interpretation of Measurements

SUMMARYTwo-band (red and near-infrared) vegetation indices are often used in remote sensing for estimating biophysical properties of vegetated surfaces. Although many considerations have been made in formulating vegetation indices, the directionality dependency has not been comprehensively investigated. Many space-borne and air-borne remote sensing sensors acquire data in large ranges of solar and view angles, thus the angular dependence of vegetation indices is of particular concern in remote sensing product validation. The 4-Scale model (Chen and Leblanc, 1997), based on detailed consideration of canopy architecture, is used to investigate the implications of the bidirectional reflectance distribution function (BRDF) on vegetation indices for four boreal forest canopies (old black spruce, young and old jack pine, and old aspen). The model is used to systematically simulate the effects on the Normalised Difference Vegetation Index (NDVI) of different viewing and illumination geometries and canopy architecture (tree crown volume, reflectivity of the soil and foliage, non-random distribution of the trees, tree density and LAI). The model reproduces closely the angular distributions of NDVI measured in the four stands. It is shown that the directionality of NDVI depends on many canopy architectural parameters and the differences in the foliage and background optical properties, indicating considerable uncertainties in deriving biophysical parameters from two-band vegetation indices when the directionality information is not used and canopy architectural parameters are unknown.