Full text: Proceedings of the Symposium on Global and Environmental Monitoring (Part 1)

479 
Figures 3 and 4 show the development of leaf re 
flectance from the early beginning in May when the 
buds are broken and the leaves are light-green. 
With the increasing pigment concentrations from May 
15 to June 1 the reflectance spectrum changes dra 
matically. In the blue region of the spectrum the 
flattening of the reflectance spectrum is observed 
due to the saturation of the absorption. The chloro- 
plasts are no more transparent in the blue absorbing 
all incoming light. In the red part of the spectrum a 
linear decrease of the reflectance is seen associated 
with the shift of the red edge towards longer wave 
lengths. When the leaf is dark green at July 1 the 
reflectance spectrum does not change remarkable in 
the blue. In contrast the green to red domain of the 
reflectance decreases. At about 680 nm, the maxi 
mum of the chlorophyll a absorption, the reflectance 
flattens due to saturation and absorption line broa 
dening resulting in an additional shift of the red 
edge. 
Figure 3: Saisonal cycle of the reflectance spectra of 
the model leaf 
1: reflectance, when buds are broken, May, 15 
2: reflectance at June,l 
3 reflectance at September, 1 4 
Figure 4: Saisonal cycle of the reflectance spectra of 
the model leaf 
4. reflectance at September, 15 
5: reflectance at October, 1 
During summer, the pigment concentration remains 
relativ constant. As seen in figure 4, the reflectance 
spectrum for September 15 is comparable with the 
spectrum from July 1. During the next two weeks, 
the chlorophyll concentration breaks down accompa 
nied by an increase of a new pigment, the anthocya- 
nin. This pigment occurs during the autumn and is 
localized in the vacuoles of the epidermis. Due to 
the strong absorption of anthocyanin in the 500nm 
to 600nm region the green reflectance decreases 
while the reflectance in the domain of the red edge 
increases leading to the typical red coloring of lea 
ves during autumn. 
A comparison with the measured reflectance spectra 
during autumnal leaf coloring (Boyer et al. 1988) 
shows good agreement with our calculated spectra. 
The only major difference is seen again in the near- 
infrared region due to the constancy of the scatte 
ring coefficient used in the model. 
4 CONCLUSION 
Based on the experimental findings and the different 
interpretations of several investigators (Boyer et al. 
1988, Buschmann and Lichtenthaler 1988, Collins et 
al. 1983, Horler et al. 1980, Horler et al. 1983, Rock 
et al. 1988, Schutt et al. 1984, Singhroy et al. 1985), 
that the blue shift of the red edge of the reflectan 
ce spectrum of vegetation may be correlated with 
the vegetative chlorophyll status, the species, the 
developmental stage, the leaf surface exposed to the 
sensor, the water content and the chlorophyll fluo 
rescence, the stochastic model of Tucker and Garratt 
(1977) was reinvestigated in order to find a theoreti 
cal, bio-optical approach for a comprehensive inter 
pretation of all the different explanations. 
In a revised version the stochastic leaf model de 
monstrated its potential to calculated the reflectan 
ce spectrum of vegetation. Optical, geometrical and 
physiological parameters as e.g. the specific absorp 
tion coefficient of chlorophyll a,, the pigment con 
centration and the leaf thickness are used as input 
parameters. The revised model takes into account 
different mechanisms as absorption, reflection and 
Mie scattering at the chloroplasts and the leaf cells. 
According to our first results showing good agree 
ment with data from the literature, the model pre 
sents two ways for interpreting the blue shift. First, 
by reducing the light absorption in the leaf, a blue 
shift of the red edge can be produced because the 
absorption line in the red part of the spectrum 
narrows. In turn a reduced light absorption can be 
modeled twofold by reducing the pigment concen 
tration of the leaf or by reducing the thickness of 
the leaf assuming constancy of the pigment concen 
tration. Thinner leaves have in general a shorter 
light path resulting in an overall less absorption.
	        
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