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
