Geoscience Reference
In-Depth Information
canopy, the normalization improved the comparability and accuracy of the generated light attenu-
ation profiles. In between these two extremes of vertical to lodged canopy orientations, improvement
of
Spartina alterniflora
aggregate attenuation profiles was somewhat similar to that associated with
the more vertical
improvement,
however, seemed dependent on the seasonal stage of canopy development. In the early stage of
regrowth and spring “green-up” the canopy is more vertical and therefore more conducive to
normalization. As the canopy transforms and becomes a dense mixture of leaf orientations, the
normalization tends to be less successful.
In all marshes, application of the sun zenith correction factor decreased the perceived light
falloff with canopy depth and increased the recorded light intensity reaching lower within the
canopy. After normalization the light attenuation profiles taken at different sun zeniths were more
closely aligned with each other and with the expected progression of canopy structure. A limitation
of the normalization seemed to be about a sun zenith of 60
Juncus roemerianus
canopy measurements.
Panicum hemitomon
∞
in vertical canopies and less than 49
∞
in more horizontal canopies; however, these normalizations used a spherical canopy orientation
parameter (
x
= 1,
r
= (x
/
(
x
+ tan
Q
)
). In most cases, preferred orientations deviate highly from
2
1/2
2
spherical in
Spartina patens
and, depending on the season,
Panicum hemitomon
canopies. Inclusion
of more appropriate values of
(0 = vertical, 1 = spherical, 1000 = horizontal) could improve the
reliability and accuracy of light attenuation profiles in more horizontal canopies such as
x
Spartina
patens
. Future analyses will examine inclusion of more appropriate orientation parameters.
It is difficult to relate canopy structure (as defined by light attenuation) to canopy reflectance
without further analysis, but a few observations are possible. In combination, the light attenuation
profiles show at least one major difference between marsh grass structures: the amount of vertical
to lodged grass. The relative amounts remain: (1) relatively stable throughout the year as shown
in the fairly vertical canopies of
Juncus roemerianus
and
Spartina alterniflora
; (2) relatively
transitional, as in
typifying a more vertical canopy in the winter and early
spring, a thicker, lodged canopy in summer, and a transition back to less lodged material through
fall; and (3) highly variable, as in
Panicum hemitomon,
which shows typically a highly lodged, hum-
mocky character. Even though local areas may show a fairly consistent trend or pattern in light
attenuation profiles, high variability in light attenuation seems more common within
Spartina patens,
Spartina patens
marshes than within other marsh types.
For remote sensing, structural influences would be the least variable in the
Juncus roemerianus
and
marshes, but background variability may be relatively higher because of
the higher base light levels throughout the year. More variable influence of canopy structure on
spectral reflectance may be expected in the
Spartina alterniflora
marsh, with possibly higher
influences of background in the winter and early spring. Structure in the
Panicum hemitomon
marsh
is closely related to the seasonal occurrences of “green-up” and senescence. Less light penetration
in the summer because of increased lodging would decrease spectral information from deeper
within the canopy, but in the winter dieback background influences may be higher through this
more open canopy. Further, without the ability to separate structure and leaf optical influences on
canopy reflectance in these marshes, it would be difficult or impossible to detect what canopy
property was changing, as both were dramatically varying during these periods. Higher structural
variability within
Panicum hemitomon
marshes would be expected to cause variability in canopy
reflectances, with reflectances least affected by structural variation in the summer and fall periods.
During winter and spring, however, increased high base light levels in
Spartina patens
Spartina patens
marshes
could further complicate interpretation of canopy reflectance variability.
5.5 SUMMARY
Light penetration field measurements were tested and described in terms of their completeness,
reliability or consistency and accuracy to characterize canopy structure. A ceptometer device
