LEAF REFLECTANCE 
A STOCHASTIC MODEL FOR ANALYSING THE BLUE SHIFT 
W. Lüdeker, K.P. Günther 
DLR - Oberpfaffenhofen, Institute of Optoelectronics, D- 8031 Wessling , West-Germany 
ABSTRACT 
In order to investigate the blue shift of the red edge in the reflectance spectra theoretically, the stocha 
stic leaf model of Tucker and Garratt (1977) was used. This approach treats the radiative transfer of dif 
fuse light as transitions of light with weighted probabilities according to the theory of homogeneous 
Markov processes with a finite number of states. The transition probabilities are determined by the opti 
cal parameters (e.g. specific absorption coefficients of different pigments, scattering coefficients and 
refractive index of cell membranes), the pigment concentrations and the geometrical dimensions of the 
leaf (e.g. total leaf thickness, thickness of palisade and spongy parenchyma and the geometrical cross 
section of chloroplasts and cells). 
Assuming realistic leaf parameters our results showed that the modeled reflectance spectra were not in 
agreement with measured data and the spectra presented by Tucker and Garratt (1977). 
Based on these findings the model was reinvestigated. By introducing a new radiation state and by revi 
sing the transition probabilities of Tucker and Garratt a successful modeling of reflectance spectra is 
possible assuming realistic leaf parameters. 
In order to investigate the blue shift of the red edge the model parameters were varied systematically. It 
was shown that 
- a reduced light absorption and / or 
- a shift in the absorption spectra of chlorophyll a. and b 
are responsible for the observed blue shift of the red edge. 
A reduced light absorption is not only caused by a reduced pigment concentration but also by a reduced 
light path. In turn the light path may be reduced by a decrease of the thickness of the palisade and / or 
spongy parenchyma or may be reduced by a decrease of the scattering coefficients. 
The shift of the absorption bands of chlorophyll a or b may occur to a minor extent when the chloro 
phyll-protein aggregates make a phase transition from polymeres to dimers or monomers due to environ 
mental stress. By traeting leaves with the herbicide DCMU in the laboratory the observed blue shift may 
be attributed to a shift of the absorption bands by partially extracting the chlorophyll pigments from the 
proteins. 
Key Words: Leaf reflectance, stochastic model, Markov chain, blue shift, red edge 
1 INTRODUCTION 
Observing the reflectance spectra of vegetation in 
the visible domain a variation of the steep rise of 
the near-infrared reflection edge at about 690 nm 
can be seen. In the last few years the application of 
high resolution imaging spectroscopy stimulated the 
remote detection of this phenomenon, called the 
blue shift of the red edge, in order to monitor vege 
tative chlorophyll status and leaf area index inde 
pendently (Horler et al. 1983) and plant stress indu 
ced e.g. by geochemical metal deposition (Collins et 
al. 1983). The trace metal-induced stress retards the 
pigment synthesis and in turn decreases the pigment 
absorption leading to higher reflectance. Since the 
spectral shift is related not only to soil geochemi 
stry but also to tree species ( Singhroy et al. 1985) a 
uniform forest stand is best suited for studying the 
blue shift. A comparison of in situ and airborne 
spectral measurements of the blue shift in Vermont 
and West-Germany over homogeneous forest stands 
(Rock et al. 1988) showed that the blue shift is 
associated with damage in spruce needles. The au 
thors suggested that the shift of the red edge to 
shorter wavelengths may be due to an absorption li 
ne narrowing when the chlorophyll a concentration 
decreases in contrast to the explanation of a shift in 
the position of the maximum of the chlorophyll 
absorption. Also Buschmann and Lichtenthaler (1988) 
correlated the observed blue shift of the red edge