Investigations on the biochemical components NIR absorption features in AVIRIS and laboratory reflectance spectra of vegetation

The reflectance spectrum of green vegetation is mainly determined by the leaf content in chlorophyll and water, the respective spectral signatures being modulated by the structural characteristics of the leaf and the canopy. However, in the near infrared, additional absorption features are present which are attributed to other components such as lignin, cellulose, proteins, etc. Of particular interest is an absorption centred around 1.7 μm; it is indeed placed in an atmospheric window and therefore accessible to remote sensing, in particular to imaging spectrometry. This paper presents a procedure, based on a very simple radiative transfer model, which evaluates the amplitude of this residual absorption. The technique has been applied to several AVIRIS scenes acquired during the MAC-Europe campaign to produce "residual absorption amplitude maps". Comparing with maps of chlorophyll indices and water indices, it is clear that the residual absorption contains different information which is probably related to the biochemical content. Several investigations are now pursued to document the link between this new parameter and the plant/canopy characteristics. The work on the Mac-Europe scenes has shown that the residual exhibits systematic behaviours with respect to the plant species: it is markedly higher on forests than on agricultural fields and, inside the forest, higher for the conifers than for the deciduous trees. These variations have also been found from laboratory spectra of leaves and needles. On the Black Forest, which is an almost homogeneous spruce forest, the residual shows some correlation with the forest age. The procedure has now also been applied on the 1992 AVIRIS scene over Blackhawk Island and the residual image shows similarities with the published lignin map, obtained by regression analysis.