The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
472
2. METHODS
2.1 Study area and sampling
The study site is located in Majella National Park, Italy (latitude
41°52' to 42° 14' N, longitude 13°14' to 13° SOI). The park
covers an area of 74,095 ha and extends into the southern part
of Abruzzo, at a distance of 40 km from the Adriatic Sea. The
region is situated in the massifs of the Apennines. The park is
characterized by several mountain peaks, the highest being
Mount Amaro (2794 m). Coordinates (x y) were randomly
generated in a grassland stratum to select plots. A total of 45
plots (30 m x 30 m) were generated and a GPS (Global
Positioning System) was used to locate them in the field. To
increase the number of samples in the time available, four to
five randomly selected subplots were clustered within each plot.
This resulted in a total of 191 subplots being sampled. The 1 m
x 1 m subplots differed in species composition and relative
abundance while the within-subplot variability was small.
were randomly selected in each subplot, and their SPAD
readings were recorded. From the 30 individual SPAD
measurements, the average was calculated (Table 1). These
averaged SPAD readings were converted into leaf chlorophyll
content (units: pg cm" 2 ) by means of an empirical calibration
function provided by Markwell et al. (1995). The total canopy
chlorophyll content (CCC; units: g m" 2 ) for each subplot was
obtained by multiplying the leaf chlorophyll content by the
corresponding LAI.
Measured
variables
Min
Mean
Max
StDev
Range
LAI (m 2 m" 2 )
0.39
2.76
7.34
1.50
6.95
CCC (g m" 2 )
0.1
0.87
2.7
0.55
2.56
Table 1. Summary statistics of the measured biophysical and
biochemical variables of grassland sample subplots (n=191);
CCC is the canopy chlorophyll content.
2.2 Canopy spectral measurements
Fifteen replicates of canopy spectral measurements were taken
from each subplot, using a GER 3700 spectroradiometer
(Geophysical and Environmental Research Corporation,
Buffalo, New York).
The fiber optic, with a field view of 25°, was handheld
approximately 1 m above the ground at nadir position. The
ground area observed by the sensor of GER had a diameter of
45 cm and was large enough to cover the center of the subplots
without being influenced by the surroundings. The 15 replicate
spectral measurements taken from each subplot enabled to
suppress much of the measurement noise by averaging the
replicate measurements. Prior to each reflectance measurement,
the radiance of a white standard panel coated with BaS04 and
of known reflectivity was recorded for normalization of the
target measurements. The fieldwork was conducted between
June 15 and July 15 in 2005. To minimize atmospheric
perturbations and BRDF effects, spectral measurements were
made on clear sunny days between 11:30 a.m. and 2:00 p.m.
2.3 LAI measurements
In each subplot, LAI was non-destructively measured using a
widely used optical instrument, the Plant Canopy Analyzer
LAI-2000 (LICOR Inc., Lincoln, NE, USA). A detailed
description of this instrument is given by LI-COR (1992) and
Welles and Norman (1991). In this study, measurements were
taken either under clear skies with low solar elevation (i.e.,
within the two hours following sunrise or preceding sunset) or
under overcast conditions. The LAI measurements were taken
on the same day that the canopy spectral measurements were
made. To prevent direct sunlight on the sensor of LAI-2000,
samples of below- and above-canopy radiation were made in
the direction facing away from the sun (i.e., with the sun behind
the operator), using a view restrictor of 45°. For each subplot,
reference samples of above-canopy radiation were determined
by measuring incoming radiation above the grass subplot (in an
open area). Next, five below-canopy samples were collected
and used to calculate the average LAI (Table 1).
2.4 Chlorophyll measurements
A SPAD-502 Leaf Chlorophyll Meter (Minolta, Inc.) was used
to assess the leaf chlorophyll content (LCC) in each 1 m x 1 m
subplot. A total of 30 leaves representing the dominant species
2.5 Data analysis
We selected the normalized difference vegetation index (NDVI)
(Rouse et al., 1974) as a representative of ratio indices, and the
second soil-adjusted vegetation index (SAVI2) (Major et al.,
1990) as a representative of soil-based indices, for the analysis
in this study. The narrow band NDVI and SAVI2 indices were
systematically calculated for all possible (584 x 584 = 341,056)
band combinations between 400 nm and 2400 nm. The soil line
parameters were calculated from soil spectral measurement of
bare soils which were acquired from few subplots with no
vegetation. We assumed that the measured soil optical
properties were representative for the study area. Consequently,
the soil line parameters were considered constant for all 191
subplots.
For this study, we used two methods to calculate the red edge
inflection point (REIP). The linear interpolation method (Guyot
and Baret, 1988) assumes that the spectral reflectance at the red
edge can be simplified to a straight line centered around a
midpoint between (i) the reflectance in the NIR shoulder at
about 780 nm, and (ii) the reflectance minimum of the
chlorophyll absorption feature at about 670 nm. First, the
reflectance value is estimated at the inflection point. Then, a
linear interpolation procedure for the measurements at 700 nm
and 740 nm is applied to estimate the wavelength corresponding
to the estimated reflectance value at the inflection point:
^red-edge
REIP,
(^670 ^78o)/2
_ - 700 + 40
R _ D
red-edge lx j
- R-,
(1)
(2)
where the constants 700 and 40 result from interpolation
between the 700 nm to 740 nm intervals, and R^o, R?oo> R740
and R780 are, respectively, the reflectance values at 670 nm, 700
nm, 740 nm and 780 nm.
The linear extrapolation method (LEM) (Cho and Skidmore,
2006) is based on the linear extrapolation of two straight lines
(Eqs. 3 and 4) through two points on the far-red (680 nm to 700
nm) and two points on the NIR (725 nm to 760 nm) flanks of
the first derivative reflectance spectrum (D) of the red edge
region. The REIP is then defined by the wavelength value at the
intersection of the straight lines (Eq. 5).