LEAF OPTICAL-PROPERTIES WITH EXPLICIT DESCRIPTION OF ITS BIOCHEMICAL-COMPOSITION - DIRECT AND INVERSE PROBLEMS

This study presents a methodology to estimate the leaf biochemical com pounds specific absorption coefficients and to use them to predict lea f biochemistry. A wide range of leaves was collected including variati ons in species and leaf status. All the leaves were dried out. The bio chemical composition was measured rising classical wet chemistry techn iques to determine lignin, cellulose, hemicellulose, starch, and prote in contents. Concurrently, leaf reflectance and transmittance were mea sured with a high spectral resolution spectrophotometer in the 800-250 0 nm range with approximately 1 nm spectral resolution and sampling in terval. In addition, infinite reflectance achieved by stacking leaves was also measured. The PROSPECT leaf optical properties model was firs t inverted over a selection of wavebands in the 800-2400 nm domain to provide estimates of the scattering characteristics using leaf reflect ance, transmittance, and infinite reflectance data. Then, the model wa s inverted again over all the wavelengths to estimate the global absor ption coefficient, using the previously estimated scattering propertie s. The global absorption coefficient was eventually explained using th e measured biochemical composition by fitting the corresponding specif ic absorption coefficients after substraction of the measured contribu tion of the residual structural water absorption. Results show that th e derived specific absorption coefficients are quite robustly estimate d. Further, they are in good agreement with known absorption features of each biochemical compound. The average contribution of each biochem ical compound to leaf absorption feature is also evaluated. Sugar, cel lulose, and hemicellulose are the main compounds that contribute to ab sorption. Results demonstrate the possibility of modeling leaf optical properties of dry leaves with explicit description of leaf biochemist ry. Estimates of the detailed biochemical composition obtained by mode l inversion over the 1300-2400 nm spectral domain show poor predictive performances. In particular, the protein content is very poorly retri eved. The retrieval performances of several combinations of the bioche mical compounds are investigated. Results show that the total amount o f dry matter per unit leaf area is the only variable to be accurately retrieved. Possible improvements of these results are discussed.