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Florida (Ramsey et al., 1992a,b, 1993). To provide a description of marsh characteristics, a few
data subsets were selected based on the presence of marsh grasses that dominate three of the gulf
coast wetland zones (Chabreck, 1970):
Juncus roemerianus
(Juncus R.) and
Spartina alterniflora
(Spartina A.) for saline marsh,
Spartina patens
(Spartina P.) for intermediate (brackish) marsh, and
Panicum hemitomon
(Panicum H.) for fresh marsh (Chabreck, 1970). Juncus R. dominates the
landscape and makes up the majority of biomass in marshes of the northeast gulf coast and Spartina
A. dominates the north-central gulf coast marshes (Stout, 1984). In these marshes, except for sites
recovering from recent burns, canopies usually contain a high proportion of dead canopy material
(Hopkinson et al., 1978; Ramsey et al., 1999).
After reaching maturity, turnover rates of both live and dead biomass can remain nearly constant,
showing no clear seasonal pattern. Although mostly vertical, Juncus R. and Spartina A. (relatively
less vertical and more leafy) canopy structures vary depending on local conditions (e.g., flushing
strength), and dominant leaf orientation can change from top to bottom. Spartina P. and Panicum
H. marshes dominate the interior marshes of Louisiana. Generally, Spartina P. canopies are hum-
mocky with vertical shoots rising above a layer of thick and logged dead material. As in Juncus
R. and Spartina A. canopies, Spartina P. canopies seem to have a low turnover with little seasonal
pattern in live and dead composition. Panicum H. canopies exhibit yearly turnover. Beginning with
nearly vertical shoots in the late winter to early spring, the canopy gains height and increasingly
adds mixed orientations until maturity in the late spring to summer, then senesces in winter.
Light measurements were collected along transects centered at flag markers, as were all mea-
surements describing the canopy characteristics. A 30-
30-m area was used to encompass the
spatial resolution of Landsat Thematic Mapper (TM) and similar Earth resource sensors. Additional
recordings and observations collected at each site included upwelling radiance from a helicopter
platform, canopy species type, percentage of cover, and height; photography; and estimates of live
and dead biomass percentages.
Field Collection Methods
Canopy light attenuation measurements were acquired using a Decagon Sunfleck Ceptometer
(Decagon Devices, 1991). The ceptometer measures both photosynthetically active radiation (PAR)
(400 to 700 nm) and the canopy gap fraction (sunflecks). Canopy light attenuation curves were
derived from PAR measurements. The ceptometer probe has 80 light sensors (calibrated to absolute
units) placed at equal intervals along an 80-cm probe covered with a diffuse plate. The narrow
probe (approximately 1.3
1.3 cm) is constructed with a hard and pointed plastic tip so that it can
be inserted horizontally with minimal disruption of the marsh canopy. After inserting the probe
into the canopy and obtaining a horizontal probe orientation relative to gravity (bubble level), the
80 sensors are scanned and an average light intensity value for the probe is calculated, displayed,
and recorded (Decagon Devices, 1991). At each site, measurements for estimating PAR canopy
reflectance and the fraction of direct beam PAR (1 - skylight/direct sun irradiance) were collected.
A correction for PAR canopy reflectance was not included in the calculations. Disregarding this
correction, in general, results in less than a 5% error in the intercepted radiation (Decagon Devices,
1991). The direct beam fraction was used to estimate the leaf area and angle distributions. Normally,
measurements were collected when clouds did not obstruct or influence (intensified by cloud
reflection) the downwelling sunlight; sky conditions were documented.
The depiction in Figure 5.1 presents our standard method of depicting light falloff with depth
in the canopy. Each point on the graph reflects the mean of all light measurements collected
throughout the site at the associated height above ground level. Error bars showing plus and minus
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