Geoscience Reference
In-Depth Information
1.00
1.00
0.90
0.90
0.80
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0.70
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0.60
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A
0.10
B
0.00
0.00
0
20 40 60 80 100 120140
0
20 40 60 80 100 120140
Canopy Height (cm)
Canopy Height (cm)
Figure 5.11
Aggregate light attenuation profiles of a
Panicum hemitomon
marsh associated with PAR collections
at different sun zenith angles on [
] 7 February, [
✙
] 13 March, [
] 21 April, [
] 09 October, and
[
] 6 July (A) without
normalization and (B) with normalization of PAR recordings to a nadir collection.
] 11 December and the following year on [
✴
] 22 January, [
] 22 April, and [
—
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
B
A
0.00
0.00
0
20
40
60 80100 120 140
0
20
40
60 80100120 140
Canopy Height (cm)
Canopy Height (cm)
Figure 5.12
Aggregate light attenuation profiles of a
Spartina patens
marsh associated with PAR collections at
different sun zenith angles on [
] 5 August, [
] 21 October, and [
✴
] 30 December and the following
year on [
—
] 21 July at a sun zenith of 49
∞
, and [
] 21 July at 18
∞
(A) without normalization and
(B) with normalization of PAR recordings to a nadir collection.
PAR attenuation profiles over nearly a year and a half at a Panicum H. site highlight the
effectiveness of the sun zenith factor correction and depict typical seasonal changes and canopy
recovery following a burn (Figures 5.11A and B). Site occupations occurred chronologically begin-
ning in February and subsequently occurred in March, April, September, December, and the fol-
lowing year in January, April, and July. Although differences between the corrected and uncorrected
profiles were subtler than in the Juncus R. marshes, the sun zenith-corrected profile sequencing is
a more convincing depiction of expected canopy structure changes over the selected time period.
From the earliest occupation in February, PAR attenuation increased and the profiles deepened until
October, after which the December profile shows the expected decrease in PAR attenuation. After
the December occupation, the site was burned, as is confirmed in the February and subsequent April
profiles. The final July profile shows the canopy recovering to the late summer and early fall profile.
A final series of profiles associated with a Spartina P. site illustrate the increased consistency
in the sun zenith-corrected vs. -uncorrected profiles (Figures 5.12A and B). The final two profiles
in these series again show the zenith angle limitation of the correction technique in more vertical
canopies. Canopy PAR penetration measurements collected at very low sun zeniths are not com-
parable to those collected at sun zenith angles at least < 60
. The movement of the PAR attenuation
profile from shallower to deeper with decreasing sun zenith angle in the Spartina P. canopy was
similar to what occurred in the Juncus R. marsh canopy (Figure 5.9A and Figure 5.9B).
∞
5.4 DISCUSSION
Light penetration field measurements were described and discussed in terms of their complete-
ness, reliability or consistency and accuracy to characterize canopy structure. Marsh types discussed
