3.2 Reflectance data 
In order to characterize more fully and to compare 
more easily seasonal(phenological) variations of 
agro-ecologmcal units, we used reflectance data. 
A PCA performed on the ISCO measurements showed 
that the more useful wavelengths were: 550 nm (green) 
675 nm (red), 850 and 1400 nm (near infra-red). 
The first two wavelengths are inversely correlated 
with chlorophyll a, and chlorophyll (a+b)/ carote 
noid respectively (Gates and all 1965, Knipling 1970) 
while the third is correlated with internal struc 
ture of leaves, and the last to water content(Gates 
1970) . 
Variations of reflectance in red and near infra-red 
are linked with phenological and coverage variations 
of plants(Figure 2). 
Red low 
near IR high 
Plants with high chlorophyll content 
Red high 
near IR low 
Plants with less chlorophyll: dying or maturing 
A R% 
Red low 
near IR low 
* * 
R nIR 
Plants with high chlorophyll content but low 
coverage: soil showing 
Red high 
near IR high 
Plants with high chlorophyll and other pigment 
content: flowering. 
Figure 2. Variations of reflectance in red and near 
infra-red in relation with phenological and coverage 
varia tions of plants 
The reflectance values at each period, for each wave 
length of each agro-ecological unit was obtained by 
Calculating the mean of reflectance values of the 
different stations belonging to this agro-ecological 
unit. 
A PCA of the reflectance values in the 4 wavelengths 
at the 4 periods, for the 8 agro-ecological units 
enabled to draw a model of spectral behaviour through 
time (Figure 3). 
unit 1 : heliophilous and calcicolous grasslands 
and grazed rangelands on shallow soils with exces 
sive drainage. 
unit 2:grazed grasslands on steep èlope, shallow soil 
dry. 
unit 3 : grazed grasslands on slope, more or less shal 
low soil, dry. 
unit 4:grazed grasslands on middle or (bottom of slope 
deep, moist soil. 
unit 5:grazed grasslands on middle or bottom of slope 
deep, humid soil. 
unit 6:grazed grasslands on bottom of slope, deep, 
very humid soil. 
unit 7: grazed grasslands on bottom of valleys, or 
slope bogs on deep muddy soils. 
unit 8: grazed grasslands on bottom of valleys, or 
slope bogs on very muddy soils. 
Figure 3.Radiometric behaviour model for the 8 agro- 
ecological units according to red and near infra-red 
from PCA. 
This spectral behaviour of each unit varies accor 
ding to the nature, percent cover and phenological 
stages of the 26 more abundant species. According 
to its agro-ecological characteristics, each unit 
shows a more or less important ! .chlorophvll content 
and a long or short chlorophvllian activitv period. 
For example, chlorophyIlian activity period of helio 
philous and calcicolous grasslands and grazed range- 
lands on shallow soils with excessive drainage is 
short (May, June), while it is long(April till June) 
for grazed grasslands on middle or bottom of slope 
on deep, moist soils. 
This model has been tested with the three stations 
already used to validate the model for phytosociolo- 
gical relevés analysis. 
These stations are clustered respectively to units