Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 57

called “crown” fires, not canopy fires. And to add to the confusion, when crown
fires occur, they can burn at various scales; passive crown fires often burn at the
tree- and branch-scale, whereas active crown fires may burn at the canopy-scale or
maybe even coarser scales of hillsides or topographic settings. Canopy fuels have
their own intrinsic scale for description and measurement, and this scale is coarser
than surface fuels and finer than active crown fires (Keane et al. 2012a ). This be-
comes important when surface fuels are sampled at the same time as canopy fuels
(Chap. 8).
Canopy fuels are probably the most misunderstood and misrepresented fuel
type in wildland fuel science. The reason for this is that most fire behavior models
simulate crown fire behavior for operational fire management using a simplistic
approach (Rothermel 1991 ; Finney 1998 ). As a result, there really hasn't been the
need to stratify the complex forest canopy fuel properties by various fuel particles
and components. Therefore, to fully understand the contemporary characterizations
of canopy fuels, it is important to understand how crown fires are currently being
simulated.

4.2 Crown Fire Simulation

In a first attempt to describe canopy fuels for operational use for rating fire hazard,
Fahnestock ( 1970 ) created a key to rate crowning potential based on canopy clo-
sure, crown density, and ladder fuels. While effective, a more mechanistic, detailed
approach was needed for robust applications at diverse local conditions. The first
operational attempt at quantitative crown fire modeling was by van Wagner ( 1977 )
who developed a mathematical classification system to determine if a surface fire
transitions to a crown fire, and then, once the fire was in the canopy, whether the fire
actively spread through the crown or the crown fire was passive and merely torched
individual trees. This basic model has since been modified (van Wagner 1993 ) and
it has also been merged with other models (Scott and Reinhardt 2001 ) for imple-
mentation in various fire behavior prediction systems, such as FARSITE (Finney
1998 ). Material presented here were synthesized from several sources (Cruz and
Alexander 2010 ; Finney 1998 ; Alexander 1998 ; Scott and Reinhardt 2001 ).
In general, crown fire behavior is simulated by comparing fire intensity ( I vari-
ables below) and spread rates ( R variables) to critical crown fire thresholds. The van
Wagner ( 1993 ) crown fire model simulates the intensity threshold for transition to
crown fire I o (kW m −1 ) from crown foliar moisture content ( FMC, % dry weight)
and canopy base height ( CBH, m) (van Wagner 1977 ; van Wagner 1993 ; Nabel
et al. 2014 ) using the following empirical relationship:

I

o =

001 . CBH ( 460 25 9 + . FMC ) .

15

(4.1)