WAVELET DECOMPOSITION OF HYPERSPECTRAL REFLECTANCE DATA FOR
QUANTIFYING PHOTOSYNTHETIC PIGMENT CONCENTRATIONS IN
VEGETATION.
G. A. Blackburn
Dept. of Geography, Lancaster University, Lancaster, LAI 4YB UK —alan.blackburn@lancaster.ac.uk
KEY WORDS: Remote Sensing, Vegetation, Hyper spectral, Analytical, Experiment
ABSTRACT:
The dynamics of pigment concentrations are related to vegetation photosynthetic potential and hence primary production, nutrient
status, stress physiology and plant-environment interactions. Therefore, information about the spatial and temporal dynamics of
pigments can provide important contributions to a range of scientific disciplines and environmental management endeavours, yet our
current our capabilities for providing this information are limited. With the advent of airborne and spaceborne imaging
spectrometers, there are now enhanced opportunities to acquire vegetation reflectance spectra and therefore quantify pigments over a
wide range of spatial scales, repeatedly. However, of the spectral approaches that exist, none are sufficiently robust and remain
sensitive to confounding factors and lack generality and extendibility. The present study examines the potential of wavelet
decomposition for quantifying vegetation pigment concentrations from hyperspectral remotely-sensed data, using reflectance spectra
and pigment data collected for a range of plant species at leaf and canopy scales. The research indicates that wavelet analysis holds
promise for the accurate determination of chlorophyll a and b and the carotenoids, but work is needed to further test and refine the
approach.
1. INTRODUCTION
Antenna pigments in leaf’ chloroplasts absorb solar radiation
and the energy is transferred to the reaction centre pigments,
which initiates the process of photosynthesis (Richardson ef af.,
2002). Chlorophylls (Chls; chlorophyll a and b) are the most
important of these pigments, a physiological parameter of
significant interest. However, from an applied perspective, Chls
concentration is important for several reasons: The amount of
solar radiation absorbed by a leaf is largely a function of the
Chls and low concentrations can directly limit photosynthetic
potential and hence primary production; much of leaf nitrogen
is incorporated in Chls, so quantifying their concentration gives
an indirect measure of nutrient status; pigmentation can be
directly related to stress physiology, Chls generally decrease
under stress and during senescence; relative concentrations of
Chl a and b change with abiotic factors such as light (e.g. sun
leaves have a higher Chl a:b ratio) so quantifying these
proportions gives information about plant-environment
interactions (Gross, 1991).
Carotenoids (Cars) are the second major group of plant
pigments, composed of carotenes and xanthophylls. Cars can
absorb incident radiation and contribute energy to
photosynthesis. The fraction of photosynthetically active
radiation absorbed by a plant canopy (APAR) has been related
to net primary productivity as a function of a light use
efficiency (LUE) coefficient defining the carbon fixed per unit
radiation intercepted. Such studies assume that the contribution
of each pigment to the energetics of photosynthesis is equal, but
this is an insufficient interpretation, as the concentration of Ch/
a is the limiting factor in the utilisation of light for
photosynthesis, because it receives energy absorbed by Chl b
and Cars (Kim et al., 1994). Thus, the photosynthetic potential
of two plants may differ even though their APAR is equal,
depending upon the concentrations of individual pigments.
Furthermore, when incident radiation exceeds that needed for
photosynthesis, Cars that compose the xanthophyll cycle
dissipate excess energy and protect the reaction centres. Thus,
while changes in Chis are indicative of stress and phenological
stage, Cars concentration provides much complementary
information on vegetation physiological status (Young and
Britton, 1990).
Information about the spatial and temporal dynamics of plant
pigments can, therefore, provide important contributions to
scientific investigations and applied environmental /
agricultural management, yet our current our capabilities for
providing this information are limited. Traditional techniques
for measuring foliar pigment concentrations involve extraction
with a solvent and spectrophotometric analysis using standard
procedures. This is possible because pigments have differing
spectral absorption properties and even though the absorption
features overlap, simple combinations of absorbance values at a
number of wavelengths can be used to accurately determine
individual pigment concentrations from mixed extracts.
However, these wet laboratory techniques are time and labour-
intensive thus for whole canopies pigments must be quantified
by extrapolation from a limited number of samples, which
introduces inaccuracies. Spectral absorbance properties of
pigments are manifest in the reflectance spectra of leaves and
this offers the opportunity of using measurements of reflected
radiation as a non-destructive method for quantifying pigments.
Moreover, with the advent of airborne (e.g. AVIRIS, CASI)
and, more recently spaceborne imaging spectrometers (e.g.
HYPERION), with high spectral and radiometric resolutions
and signal:noise ratios, there are now enhanced opportunities to
acquire vegetation reflectance spectra and therefore quantify
pigments over a wide range of spatial scales, repeatedly.
1.1 Evaluation of previous spectral approaches
To extract pigment information we must first account for the
range of other factors which also influence vegetation
reflectance spectra. The internal structure of leaves, with large
numbers of refractive discontinuities between cell walls and
intercellular air spaces, scatters incident radiation and allows à
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