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The use of a constant LAD does not fit the study objective of
producing both orientation and density structure indicators,
however, constant extinction coefficients are commonly em-
ployed to simplify calculation of LAI so the comparison is use-
ful. Equation 4 was used as before, however, pre-calculation
of an optimized LAD based on the measured PAR profile was
bypassed and a constant spherical distribution (LAD = 0.5) was
used. The predicted and observed PAR transmittance exhibited
high correspondence with a slope and bias of 1.00 and -0.0273,
respectively, and R2 of 0.99 (n = 105). The resulting correspon-
dence was similar to that based on optimized LAD values.
LAI calculated by using a constant LAD value and LAI calcu-
lated with optimized LAD values were directly compared. The
LAI based on a constant (LAD = 0.5) explained 92 percent of
the LAI variance based on the optimized LAD (Figure 5a). The
relationship had a 0.70 slope and 0.067 intercept. Differences
in the two LAI calculations, however, increased with increasing
LAI. The strength of covariance with measured total biomass
was also used as an indirect measure of which of the two LAI
better represented marsh canopy structure. A simple regression
found LAI calculated with a constant LAD was a poor predictor
(slope = 111.2 ±80.4 gr/m2, p = 0.19, bias = 524.8 ±349.3 gr/m2,
p = 0.15, and R2 = 0.11) of total biomass (RMSE= ±506 gr/m 2).
Although still low, LAI calculated with an optimized LAD was a
better predictor (slope = 240.5 ±124.0 gr/m2, p = 0.071, bias =
239.9 ±398.9 gr/m2, p = 0.56, and R2 = 0.20) with a somewhat
lower total biomass prediction error (RMSE= ±480 gr/m2). The
results of these regressions are depicted in plots of the observed
versus predicted total biomass (Figures 5b and 5c). As shown in
Figure5a, LAI based on a constant LAD dominantly fell in a nar-
row range between 3 and 5 while LAI based on the optimized
LAD had a more uniform distribution extending from 0 to 5.
Description of Calculated KM and LAD and Associated LAI
Whether based on KM or LAD measures, canopy LAI varied
widely over time and from site to site (Table 1, Figure A1).
LAD tended to be slightly lower than KM magnitudes due to its
adjustment by the sun zenith. Other than a single peak near
0.4, LAD values were nearly evenly distributed from just over
0.3 to a little over 1. The KM distribution exhibited a more
normal distribution ranging from 0.4 to 1.1. In contrast to KM
and LAD relative magnitudes, LAD-LAI tended to have slightly
higher magnitudes than KM-LAI . Both exhibited fairly uniform
distributions. The bulk of KM-LAI values ranged from just
under 2 to a little below 5 with a single low value at 1.1. LAD-
LAI ranged a bit higher from just over 2 to 5 with a low value
at 1.2. As in the measured biomass quantity and composition
distributions, the canopy structure parameterized as LAI and
KM and LAD exhibited wide ranges and fairly uniform distri-
butions well suited for accomplishing our objectives.
Discussion
We developed a methodology for producing three-dimension-
al marsh canopy vertical density and orientation structure di-
rectly from field measurements of PAR transmittance without
reliance on operator inputs. The data driven parameterization
of the PAR profiles as standard canopy structural measures,
LAI and LAD, provides both non-bias estimates and a stan-
dard calculation method based on accepted relationships
and published field protocols. The direct calculation of LAI
profiles and a LAD average in these complex wetland sys-
tems offers a more meaningful representation of the marsh
that should improve the understanding and representation of
biophysical function, and importantly, provide variables more
amenable to remote sensing mapping (Zheng and Moskal,
2009), particularly polarimetric radar (Pairman et al. , 1999).
The final assessment of the methodology and produced den-
sity and orientation variable, LAI and KM (or LAD), from the
PAR measurements is whether they are of reasonable magni-
tudes and distribution and better represent the marsh canopy
structure as compared to simpler approximations.
LAI and LAD Compared to Published Values
Combining LAI ranges from seven grassland, prairie, and pasture
references produces an aggregate LAI range of 1.14 ±1.40 to 5.22
Figure 3. Total biomass dry weights listed in Table 1 plotted
against the site-averaged bottom PAR measured.
Figure 4. PAR transmittance predicted with the implemented
methodology and the site-averaged PAR transmittance profile
measured at all sites and times listed in Table 1 (note: only bot-
tom PAR is listed in Table 1). Slight clustering near 1 is a result of
the standard above-canopy reading at all sites.
812 October 2015 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING