Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 60

4.3 Canopy Fuel Characteristics

Given the simplistic empirical approach in which canopy fuels are represented in
the equations above, canopy fuels did not need the complex array of fuel com-
ponents used for surface fire simulation. A problem arose, however, when many
observed that only the smaller canopy fuels, such as needles and small branches,
actually burned in crown fires, and that inclusion of the larger canopy fuel (branch-
es, tree boles) was perhaps inappropriate for crown fire modeling (Call and Albini
1997 ). Indeed, loadings of these smaller canopy materials are significantly less than
unconsumed loadings of the larger branches and boles (Table 4.1 ). So, most canopy
fuel descriptions today are quantified using only “burnable” canopy biomass or
canopy fuel available for combustion.
The canopy fuel particle size range that defines burnable canopy biomass has
been described in many studies. Brown and Reinhardt ( 1991 ) suggested estimat-
ing burnable canopy fuels as 50 % of the crown branchwood that is less than 6 mm
in diameter and all of foliar material. Reinhardt et al. ( 2006b ) specified burnable
canopy biomass as all foliage and live branchwood less than 3 mm diameter, and
dead branchwood less than 6 mm, while Keane et al. ( 2005 ) defined canopy fuels
as all crown fuels less than 3 mm diameter. Call and Albini ( 1997 ) estimated that
65 % of the canopy biomass less than 6 mm burned in a crown fire. Stocks et al.
( 2004 ) conducted one of the few studies that actually measured canopy fuel con-
sumption and found nearly all canopy material less than 1.0 cm were consumed in
a boreal crown fires. Obviously, the amount of burnable canopy biomass differs by
ecosystem, time of year, weather, and fire, so defining the size for burnable canopy
material would depend on the analysis objectives. For most applications, the three
canopy fuel characteristics needed for crown fire behavior prediction ( CBD, CH,
and CBH ) are calculated using only burnable canopy biomass, mainly so that CFB
equals CFL in Eq. 4.7. However, to fully understand how the three canopy fuel
characteristics are quantified, it is important to know how canopy fuel is arrayed
above the ground using vertical distributions of canopy fuels called canopy profiles.

4.3.1 The Canopy Fuel Profile

The canopy fuel profile is the vertical distribution of burnable canopy biomass
above the ground (Fig. 4.2 ). For operational uses, this can be envisioned by slicing
the canopy into a numerous horizontal layers, and for each layer, summing all the
canopy fuel weights within each layer and dividing by the volume of that layer to
estimate a CBD for each layer. Plotting the CBD for each layer by the height of each
layer gives the canopy fuel profile in Fig. 4.2 . Canopy fuel profiles can be measured
directly using destructive sampling or they can be modeled using allometric tree
biomass estimates and the tree dimensions in Fig. 4.1 (Reinhardt et al. 2006b ).
Canopy fuel profiles can be generated for both loading ( CFL ) and bulk density
( CBD ), but here CBD profiles are used because they better represent inputs into the
fire models.