Spectral Reflectance and Leaf Chemistry

Introduction

Quantification of biochemical concentrations in vegetation is a goal in the study of biotic processes occurring at many scales, from small ecosystems to large regions of the earth's surface. The influence of non-pigment chemicals on canopy spectral reflectance is at an early stage of research. But these biochemicals are critical at the ecosystem scale; amino acids and proteins, sugars and carbohydrates, cellulose, tannins and lignin are related to growth, allocation, plant defense, decomposition and other ecosystem processes. These compounds have primary absorptions in the middle infrared region with overtones and combinations appearing in the near infrared. This phenomenon has allowed the development of practical, operational methods of near infrared spectroscopy in which biochemicals are quantified in dried, ground organic material.

We carried out an experiment to examine this phenomenon in living, whole canopies. We used Douglas fir grown under different fertilization treatments designed to induce differences in foliar nitrogen, incorporated mainly into non-pigment compounds, and the pigment chlorophyll, while minimizing changes in leaf structure, biomass, moisture and other biochemicals.

The work was supported by NASA under the Accelerated Canopy Chemistry Program (ACCP). The ACCP was designed to to research extent to which infrared spectroscopy for dried, ground plant materials could be extended to fresh leaves, whole canopies in laboratory and field settings, and entire ecosystems.

Approach

Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) seedlings were grown with different fertilization treatments in an experiment designed to investigate the effects of foliar biochemistry on needle and canopy reflectance measurements. Potentially confounding effects of the covariance of canopy structure with foliar biochemical concentration were minimized by fertilizing after leaf expansion. Seedlings showed no significant differences in specific leaf area, biomass or LAI, but showed significantly different total nitrogen concentrations, and some differences in chlorophyll concentrations. Measurements were made of needle optical properties and bidirectional reflectance was obtained of needles and of canopies. Canopy reflectance was acquired under sky illumination using two field spectroradiometers (photograph of experimental set-up).

Conclusions

Needle and canopy treatment differences were highly significant in the visible region.
Large treatment differences were also found throughout the infrared in the canopy reflectances.
Narrow band infrared differences were detected in needle first derivative spectra, and coincided with known protein absorption features.
Positions of red edge inflection points and NDVI values showed no significant treatment effects.
Needle and canopy bidirectional reflectances were similar, but narrow-band features found in the needle infrared spectra were not found in the canopy data, most likely because of low signal-to-noise characteristics of the field spectroradiometers relative to the laboratory instrument.

This data set represents one of the few available in which needle and canopy reflectance measurements were acquired from the same trees. (To get the raw reflectance data, click here). These data, and the accompanying data on chlorophyll and nitrogen concentrations, tree shape, and structural variables, have the potential to be used to parameterize current canopy reflectance models which use leaf optical properties as a basis.

Results of this work can be read in two recent publications:

Dungan, J.L., L.F. Johnson, C.R. Billow, P.A. Matson, J. Mazzurco, J. Moen, and V.C. Vanderbilt (1996) High spectral resolution reflectance of Douglas fir grown under different fertilization treatments: Experiment design and treatment effects. Remote Sensing of Environment. 55:217-228.

and

Johnson, L.F. and C.R. Billow (1996) Spectrometric estimation of total nitrogen concentration in Douglas-fir foliage. International Journal of Remote Sensing. 17:489-500.

Further Information

Contacts:
Jennifer Dungan, JCWS, Inc.
Email: jdungan@gaia.arc.nasa.gov
USmail: Mail Stop 242-4
NASA Ames Research Center
Moffett Field, CA 94035-1000
Phone: (650) 604-3618
FAX: (650) 604-4680 Information last updated: 4/27/95