Fresh Marsh Canopy
100 120 140 160
Canopy Height (cm)
Left: A Ceptometer showing the sunflect (22.5) and PAR (299) readings. Middle: Ruth Spell is
shown measuring the above PAR intensity after collecting readings with canopy depth at one point
along one of the two transect directions (Figure 5.2). The above canopy PAR intensity (shown) is
used to normalize PAR measurements at each canopy depth. Right: The resulting summer and
winter profiles of a fresh marsh canopy site showing the light penetration (PAR at each depth/above-
canopy PAR) with depth averaged over the 22 measurement points along the two transect directions.
The standard errors of the 22 measurements at each canopy depth also are shown as horizontal
bars attached to each symbol.
one standard error (65% confidence interval) depict the variance about the mean at each canopy
height. The light attenuation curve represents the percentage of above-canopy PAR sunlight
(abscissa) reaching varying depths in the canopy (ordinate) throughout the site area. The curve
typifies light attenuation in an undisturbed and fully formed Panicum H. marsh.
Area Frequency Sampling
The three considerations combined to set the distance between profile collections included: (1)
the estimated canopy spatial variability, (2) the decision to sample 30-m transects in two cardinal
directions, and (3) the necessity to restrict the site occupation time. Early analyses of the collected
light attenuation data suggested profiles collected every 3 m and averaged over the 30-m transects
provided the best compromise to all sampling considerations (Figure 5.2).
Vertical Frequency Sampling
Two vertical sampling distances were compared to assess the accurate and reliable portrayal of
canopy light attenuation. The earliest measurements were collected relative to the canopy height,
not at a constant height above the ground level. In a few of these early site occupations the relative
top, middle, and bottom measurements collected every 3 m along the 30-m transects were supple-
mented with light attenuation profiles (every 20 cm) collected at three to four transect locations.
Atypical Canopy Structures
At each profile location, sky condition and canopy structure were recorded. Indicator flags were
inserted into the database to indicate whether (1) sunny or (2) cloudy sky existed and whether the
profile location was (1) undisturbed, (2) a partial gap or hole, or (3) completely lodged. These flags
could be used during the data processing to exclude or include any combination of sky and canopy
conditions. In almost all cases only sunny sky conditions were processed. Similarly, undisturbed
canopy was most often solely processed for generation of PAR light attenuation profiles typifying
each site. In relation to remotely sensed data, however, all canopy conditions will be incorporated