THEORETICAL LIMITS TO THE ESTIMATION OF THE LEAF-AREA INDEX ON THE BASIS OF VISIBLE AND NEAR-INFRARED REMOTE-SENSING DATA

The Leaf Area Index (LAI) of a plant canopy is an important environmen tal parameter required by various applications, It would be highly des irable to be able to estimate this parameter on the basis of satellite remote sensing data in the optical spectral range, However, LAI affec ts the propagation of light in a plant canopy (and therefore its measu rable reflectance factor) exclusively through a boundary condition of the equation of radiation transfer. It is shown that LAI may be retrie vable accurately and reliably only when the canopy is optically thin e nough to allow a significant illumination of the underlying soil, and when the optical properties of this soil are such that the radiance fi eld emerging from this level is sufficiently different from that which would be exhibited by a deeper canopy, The combinations of radiative conditions (soil and plant properties) necessary for the reliable and accurate retrieval of the LAI on the basis of remote sensing reflectan ce data acquired above the canopy in the red and near-infrared spectra l regions are, investigated and documented with the help of simulation studies. These results show the retrievability of LAI from remote sen sing data in optimal situations, however.