Geoscience Reference
In-Depth Information
r
= (1.0
/
(1.0 + tan
2
Q
)
1/2
(5.4)
2
where normalization of PAR measurements to a sun nadir zenith (= 0.0) was estimated to be:
PAR(
Q
) = PAR(
Q
)
(5.5)
r
1
2
5.3 RESULTS
5.3.1
Vertical Frequency Sampling
Two examples were selected to illustrate noticeable differences between samples taken only
at the top, bottom, and middle canopy positions and at every 20 cm above the bottom (Figure
5.3). These early measurements were collected during July and August when the canopies reached
full growth. In both the Spartina P. (more hummocky and logged) and Panicum H. (more vertical
and leafy) marsh sites, similar differences were revealed between curves associated with the higher
and lower (relative) frequency depth sampling. Light attenuation was overpredicted nearer the top
of the canopy and underpredicted in the lower canopy. Even though measurement techniques were
further refined following these early collections to expressly test field sampling techniques, these
and similar results laid the basis for the vertical sampling frequency used throughout field collec-
tions in all marsh types. The choice of vertical sampling frequency relied on what was necessary
to obtain our primary purpose: to detect and monitor canopy structure differences between and
within wetland types that might influence variability in the remote sensing image data. An
additional consideration was our goal to use the data we collected to estimate what influence these
structural differences have on the canopy reflectance and whether these differences could be
detected at some level with remote sensing data. We felt light penetration collections limited to
a few positions in the canopy profile would severely jeopardize our ability to fulfill this purpose
and to reach our goal.
Following these initial tests, our standard collection technique was to profile light intensities
from the ground level to above the canopy in 20-cm increments at 3-m intervals along each transect.
To ensure proper measurement height at each profile, a pole marked in 20-cm increments was
driven into the ground until the zero mark was at ground-surface level (flood or nonflood). In
Spartina P. canopies the pole was placed between grass clumps. Profile measurements were collected
perpendicular to the transect direction and toward the hemisphere containing the sun. At each site
occupation, either 11 or 22 (most often) PAR recordings (one or two transects) were taken at each
profile height. The above-canopy PAR measured at the associated profile location normalized each
recorded PAR.
1.00
1.00
0.90
0.90
0.80
0.80
0.70
0.70
0.60
0.60
0.50
0.50
0.40
0.40
0.30
0.30
0.20
0.20
0.10
0.10
Spartina Patens
Panicum hemitomon
0.00
0.00
0
20
40 60 80 100 120 140
Canopy Height (cm)
0
20
40 60 80 100 120 140
Canopy Height (c m)
Figure 5.3
Aggregate site profiles (
Spartina patens
and
Panicum hemitomon
) associated with PAR intensity
collections at top, middle, and bottom canopy depths (shown as
with a dashed line) and at every
20 cm (shown as
●
with a solid line).
