SPATIAL HETEROGENEITY IN VEGETATION CANOPIES AND REMOTE-SENSING OF ABSORBED PHOTOSYNTHETICALLY ACTIVE RADIATION - A MODELING STUDY

A large number of previous studies have investigated the possibility of estimating absorbed photosynthetically active radiation (PAR) from spectral reflectance of plant canopies. An important factor not considered in these studies is the case of horizontally heterogeneous plant stands where the ground cover is usually less than 100%. We investigated the relationship between spectral indices and fraction of absorbed PAR in horizontally heterogeneous vegetation canopies with the aid of a three-dimensional radiative transfer model. Canopy reflection at optical wavelengths and PAR absorption was simulated using this model. The errors incurred in using a ID model for calculating the radiation regime of spatially heterogeneous canopies are shown to be significant. Our analysis indicates that the leaf area index of a canopy is less of an instructive parameter than ground cover and clump leaf area index for these canopies. The relationship between normalized difference vegetation index and fraction of absorbed PAR is found to be almost linear and independent of spatial heterogeneity. However, this relationship is sensitive to the reflectance of the soil or background.