Co-Principal Investigators:
Lee Johnson1, Barry Ganapol2
Co-Investigators and Collaborators:
Christine Hlavka3, Philip Hammer3, Jennifer Dungan3, David Peterson3, Barbara Bond4
1994-96. A within-leaf radiative transfer model (Leaf Experimental Abosrptivity Feasibility Model, or LEAFMOD) was constructed and tested for future use in evaluating sensitivity of leaf reflectance to leaf chemistry, and for investigating the influence of leaf chemistry and leaf amount on vegetation canopy reflectance. The radiative transfer model is based on solving the one-dimentional radiative transfer equation in a slab of leaf material with homogeneous optical properties. When run in the forward mode, LEAFMOD generates the leaf bi-directional reflectance and transmittance given the leaf thickness and optical characteristics of the leaf material (ie, the absorption and scattering coefficients based on material concentrations). In the exact inverse mode, LEAFMOD computes the total within-leaf absorption and scattering coefficients from the measured thickness, reflectance and transmittance. Inversion experiments with real and simulated data demonstrate that the model appropriately decouples scattering and absorption within the leaf. The model produces absorption profiles with peaks at locations corresponding to the major absorption features for water and chlorophyll. Using available spectral data for fresh grape leaves, the magnitude of the absorption coefficient profile in the visible region is strongly correlated to chlorophyll concentration determined from wet chemical analysis. Data from a leaf-stacking and drying experiment demonstrate that absorption features related to various biochemical constituents can be identified in the dry-leaf absorption profile.
1997-99. LEAFMOD was linked with a vegetation canopy radiative transfer model (CANMOD). The integrated model (LCM2) generates canopy reflectance as a function of leaf chemistry, leaf scattering properties, leaf thickness, canopy architecture (leaf area index, leaf angle distribution) and soil reflectance. The model features inter-leaf scattering with the radiance obtained using an adaptation of the analytically-based FN method. The current version assumes bi-Lambertian leaf scattering phase function. Multiple leaf species may be represented in a single canopy. LCM2 predictions were compared with measurements of leaf reflectance and transmittance (rms errors <5%), field measurements of monospecific broadleaf canopy reflectance (rmse 2.4-6.2%), and field measurements of monospecific needle-leaf canopy reflectance (rmse 4.4-11.7%).
1999-2000. Simulations and measurements were used derive information on the form and strength of the nitrogen (N) "signal" in near-infrared spectra of fresh leaves. Simulations across multiple species indicated that in total, protein absorption decreased near-infrared reflectance by up to 1.8% absolute, and transmittance by up to 3.7% absolute, all other inputs held equal. Associated changes in spectral slope were generally of range ±0.02% nm-1 absolute. Spectral effects were about an order of magnitude more subtle for a smaller, though potentially ecologically significant, change in N concentration of 0.5% over measured. Nitrogen influence on spectral slope was fairly consistent across four empirical datasets as judged by wavelength dependence of N correlation, and there was reasonable agreement of observed and modeled slope perturbations with locations of known protein absorption features. Improved understanding of the form and strength of the N signal under differing conditions will support continued development of laboratory-based spectral measurement and analysis strategies for direct N estimation in individual fresh leaves. A pragmatic approach for canopy level estimation by remote sensing, however, might additionally consider surrogate measures such as chlorophyll concentration or canopy biophysical properties.
Publications:
Johnson, L. (2001). Nitrogen Influence on Fresh-Leaf NIR Spectra.Remote Sensing of Environment 78:314-320 download pdf [180k]
Ganapol, B., L. Johnson, C. Hlavka, D. Peterson, and B. Bond (1999). LCM2: A Coupled Leaf/Canopy Radiative Transfer Model. Remote Sensing of Environment 70:153-164 download pdf [550k]
Ganapol, B., L. Johnson, P. Hammer, C. Hlavka and D. Peterson (1998). LEAFMOD: A New Within-Leaf Radiative Transfer Model. Remote Sensing of Environment 63:182-193 download pdf [990k]
Hlavka, C., D. Peterson, L. Johnson and B. Ganapol (1997). Analysis of Forest Foliage Spectra Using a Multivariate Mixture Model. J. Near Infrared Spectroscopy 5:167-173.
Contacts:
Lee F. Johnson
Ljohnson@mail.arc.nasa.gov
tel: (650) 604-3331
Barry D. Ganapol
ganapol@cowboy.nee.arizona.edu
tel: (520) 621-4728
Sponsors: NASA/Terrestrial Ecology Program, National Academy of Science
1 California State University, Monterey Bay
2 University Arizona; NASA ASEE Fellow
3 NASA Ames Research Center
4 Oregon State University
last update: 26 FEB 2002
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