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ers ignores the role that other canopy layers play in crown fire spread (Keane et al.
2005
) and may overestimate crown fire behavior in some canopies that have unique
profiles because it may be incompatible with the original van Wagner (
1977
) model
assumptions (Fig.
4.2a
). Early attempts at rectifying this problem calculated
CBD
using a running mean of
CBD
across all canopy layers with the number of layers
included in the running mean ranging from 3 (one below and one above) to 11 (five
layers below and five layers above the evaluated layer) (Reinhardt et al.
2006a
,
b
).
A limitation of these averaging techniques is that the mean
CBD
value often
masked the great variability of canopy fuel over horizontal space (Keane et al.
2012b
). Most
CBD
estimates are derived from tree inventories summarized from
plots sampled across large areas disregarding the actual locations of trees that con-
tribute to the heterogeneous distribution of canopy fuels (a summary of all canopy
fuel sampling techniques are detailed in Chap. 7). A stand-level summary of
CBD
may be too coarse for accurately representing the spatial distribution of
CBD
at the
scales that fires spread. On the other hand, plot-level summaries of
CBD
may match
the scale of crown fire behavior, but assigning one plot to represent canopy fuel con-
ditions across an entire stand also ignores the high spatial variability of canopy fuels.
There may be canopy patches that are unable to sustain crown fire spread, and the
height of the densest
CBD
layers often varies across heterogeneous forests.
An accurate, scale-appropriate, and physically credible estimate of
CBD
is prob-
ably not a great concern right now given the coarse resolution of the current crown
fire modeling approaches (Eq. 4.1). Users of the FARSITE model and its derivatives
(Finney
1998
) often adjust the carefully field-calculated estimates of
CBD
inputs to
more realistically simulate crown fire spread due to limitations in the design of the
fire model (Keane et al.
2006
; Reeves et al.
2006
). And since
CBD
can be estimated
using any number of techniques, its quantification can be modified to fit a particular
application. The
CBD
variable was initially selected by van Wagner (
1977
,
1993
) to
represent “canopy opaqueness” and perhaps there are more ecologically appropriate
and easily measured variables that better represent this vague physical concept at
the appropriate scale and resolution, such as Leaf Area Index (LAI). Future crown
fire simulation systems may also find that the canopy fuel characteristic
CBD
could
be replaced, modified, or redefined to fit more comprehensive spatial simulations
(Parsons et al.
2010
).
4.3.3
Canopy Base Height and Canopy Height
CBH
is most important in estimating the potential of surface fires to transition to
crown fires (Eqs. 4.1, 4.7).
CH
is less important because it is only used to cal-
culate crown fire intensity (Eq. 4.7). Similar to
CBD,
both
CBH
and
CH
are dif-
ficult canopy characteristics to describe and measure because of major conflicts
in spatial scales.
CBH
is defined as the lowest height above the ground at which
there is sufficient canopy fuel to propagate fire vertically through the canopy (Scott
and Reinhardt
2002
). The problem is that a single sapling may provide the vertical
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