ASD FieldSpec 3 User Manual page 68

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ASD.Document 600540 Rev. C
Only by controlling the viewing and illumination geometry can changes be
detected in canopy reflectance. True differences in canopy reflectance
between several canopies may be due to either differences in canopy structure
or in leaf/needle optical properties (Curtiss and Maecher, 1991).
For example, a conifer canopy will appear darker in the infrared than the
broad leaf canopy even though the reflectance of the individual needles and
leaves may be almost identical for almost all viewing geometries.
Observed differences in canopy level reflectance may be attributed to
differences in either leaf level optical properties or other, larger scale,
structural properties of the canopy. Important structural properties include:
• leaf/needle size
• leaf/needle density at the branch level
• number of years of needles retained (conifers)
• branching angles
When differences are observed between canopies of the same species, it is
almost always due to differences in canopy or branch level structure.
The absorption features seen in vegetation spectra are all related to organic
compounds common to the majority of plant species (Peterson et al., 1988;
Gao and Goetz, 1992).
Thus, the information about a plant canopy is contained in the relative
intensity of the various absorption features rather than in the presence or
absence of a specific absorption feature.
The major spectral absorption features can be attributed to:
• water
• plant pigments
» chlorophyll
» zanthophyll
» carotenoids
Other, minor, absorption features are attributable to other chemical
components; these include:
• cellulose
• lignin
• proteins
• starches
• sugars
Non-photosynthetic components of the canopy have spectra which are
dominated by absorption features attributed to lignin and cellulose.
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Chapter 7 Field Measurements
FieldSpec® 3 User Manual