Introduction - Wildland Fuel Fundamentals and Applications - page 5

the fuelbed, whereas fuel components are fuel types that are qualitatively and quan-
titatively defined for specific purposes, mostly for fire behavior prediction. A fuel
type might be “woody fuel,” while a woody fuel component might be defined as
woody fuel of a certain diameter size range (Chap. 3). Many fire practitioners refer
to a fuel type as a general term for the dominant fuel of a fuelbed, such as a shrub fuel
type describing a fuelbed where the loading is mainly shrubs. In this topic, however,
a fuel type is specific to the kind of fuel in a fuelbed independent of its loading, and
the dominant fuel of a fuelbed is called a fuel complex (Bebi et al. 2003 ). A shrub
fuel type would indicate that a fuelbed has some shrub biomass, while a shrub fuel
complex would refer to a fuelbed that is dominated by shrubs. Similar to fuelbeds,
fuel types and components also have specific properties, such as bulk density, load-
ing (mass per area), and surface area, which are important inputs to fire behavior
and effects models and important descriptors of fuel characteristics.
The finest scale of fuelbed description is the fuel particle, which is a general
term that defines a specific piece of fuel that is part of a fuel type or component
of a fuelbed (Fig. 1.2 ). For example, a fuel particle can be an intact or fragmented
stick, grass blade, shrub leaf, or pine needle. Fuel particles have the widest diver-
sity of properties, such as specific gravity, heat content, and shape (Chap. 2), and
the properties of fuel components and fuelbeds are often quantified from statistical
summaries of the properties of the particles that comprise them. For example, heat
content of the herbaceous fuel component may be quantified by averaged heat con-
tent estimates across all particles (leaf blades) from all plant species that compose
the herbaceous fuel type.
Wildland fuels are also defined as dead or live within any given fuel type, com-
ponent, or particle. Dead fuel is suspended and downed dead biomass, often called
necromass by ecologists, while, live fuel is the biomass of living organisms, mostly
vascular plants (trees, shrubs, herbs), but also of mosses, lichens, and many oth-
er living organisms. The principle reason for this dichotomous stratification is to
distinguish between two completely different mechanisms that control both fuel
moisture (Chap. 5) and fuel dynamics (Chap. 6). Live fuel moistures, for example,
are controlled by ecophysiological processes, such as transpiration, evaporation,
and soil water, that vary greatly between species and climates, whereas dead fuels
moistures are dictated by the interactions of the physical properties of the fuel (e.g.,
size, density, surface area) and exogenous factors, such as climate, topography, and
shading vegetation. Some live fuels may contain dead fuels; trees, for example, may
have live wood surrounding dead wood, such as in a healing fire scar. And, most
fuelbeds consist of a complex distribution of live and dead fuels so determining live
versus dead fuel in field situations can sometimes be difficult, often because some
dead fuels may appear as live or they may be attached to live fuels. Mosses and
lichens, for example, occur as complexes of live and dead fuels distributed through-
out the surface and canopy fuel layers. In another example, dead branches can be
attached to live trees, and live branches can be embedded in the litter. Besides mois-
ture dynamics, live fuels also have significantly different physical properties than
dead fuels with different particle size distribution, heat content, and mineral content
(Chap. 3).