Introduction - Wildland Fuel Fundamentals and Applications - page 6

1.2.2 General Fuel Descriptions

Many general terms are also used to describe characteristics of a fuelbed. Ladder
fuel is a term used to describe a vertical continuous layer of fuel that, when burned,
can transport a surface fire into tree crowns to become a crown fire. Ladder fuel
can be any live or dead biomass but most of the time it is tree and shrub foliage and
small branches that extend down into the surface fuel layer or extend upwards into
the canopy layer. Flashy fuel is a term often used to describe the finest fuel types
that are most easily ignited and combust quickly, such as grasses, small twigs, and
litter. Sound and rotten are two terms that are used to stratify the degree of decay in
woody fuel particles although there isn't really a metric that can be used to deter-
mine what is a sound log and what is a rotten log. Sound woody fuels usually have
greater particle densities, higher heat contents, and lower surface area to volume
ratios than rotten fuels (Chap. 3).
Fuel availability is a term often used to describe the potential for biomass to burn.
For fuel to be available, it must be dry enough to ignite and there must be enough
of it to burn. In fact, some older fuel studies have often referred to wildland fuel
as only the biomass that is available to burn. A similar term is fuel condition often
defined as relative flammability of fuel based on type, environment, and mostly fuel
moisture (NWCG 2006 ). Fuel flammability is defined as the relative ease at which
fuels will burn regardless of amount (NWCG 2006 ; see Chap. 10). The problem
with these three terms is that they are ambiguous, scale-dependent, and difficult to
quantify, and therefore, they are often only used to qualitatively describe fuel types
and fuelbeds. Some examples of the problems with these terms are illustrated with
these questions: Is the fuel available if it only burns in smoldering combustion?
Does fuel condition include continuity? Are fuels highly flammable if they produce
high intensities? This ambiguity is partially a result of dynamic and complex eco-
logical entities (biomass) being described in a combustion science context.
Fuelbeds are often given names that describe the factors involved in their cre-
ation. Natural fuelbeds are those fuelbeds created by vegetation development in
the absence of disturbance. Endemic (within stand) disturbances may act on the
vegetation to also create natural fuelbeds. However, the term “natural” is somewhat
ambiguous and open to wide interpretation, so, in this topic, the term undisturbed
fuelbed is used to describe fuelbeds that haven't been affected by major distur-
bances. Major exogenous disturbances often create their own unique set of fuel-
beds that are usually named for the disturbance that created them. Activity fuels are
those fuelbeds that have been altered by mechanical treatments, such as thinning,
timber harvest, and mastication (Hirsch et al. 1979 ; Fig. 1.3a ). The cut or fallen
woody fuel particles, such as limbed branches, destroyed tree seedlings, and aban-
doned tree tops, that are left on the ground after fuel treatments are often referred
to as slash in activity fuels . Blowdown fuelbeds are created by localized high-wind
events that topple trees en masse (Woodall and Nagel 2007 ), while hurricane fuel-
beds are caused by regional storm events (Busing et al. 2009 ; Fig. 1.3b ). Insect and
disease outbreaks often create fuelbeds that many consider hazardous (Jenkins et al.