Surface and Ground Fuels - Wildland Fuel Fundamentals and Applications - page 39

different physical, chemical, and combustion properties (Table 3.1 ). Litter particles
are often small, light, and thin, such as needles, leaves, and grass blades, which
mean they have high SAVRs, and the particles are often loosely distributed above
the duff (high packing ratios and low bulk densities). Therefore, litter material is
able to dry quickly and is easily ignited to spread fire, and it burns mostly in the
flaming combustion phase (Chap. 2).
The high diversity of litter fuel particles makes the differentiation and measure-
ment of the litter layer properties difficult. In fact, many litter particle types could
be their own fuel component in more refined fire modeling applications, but these
particles are often included in the litter component for ease of operational measure-
ment and simulation modeling. Cones, for example, are often included in the litter
layer for simplicity (Fig. 3.1b ), yet cones smolder for long times and could impart
significantly different thermal effects on the ground than many of the other litter
particles. One consistent quality of litter is that fallen litter particles are often small
and evenly distributed across the fuel bed. However, some fallen plant parts, such
as cones, buds, and most importantly bark, are large and, if they are included in the
litter, they often have a more heterogeneous spatial distribution and more disparate
fuel properties (Keane 2008 ). To include these odd particles in the litter component
for a specific fire application may add to greater uncertainty into any sampling ef-
fort or inventory protocol used to estimate loading. Yet, these particles contribute to
fire spread and may affect other fire behavior attributes, such as spread rate, inten-
sity, and residence time. Another major problem is identifying where the litter layer
stops and the duff layer begins because of the gradient of decomposition from the
litter to mineral soil (Fig. 3.2a ) (see Sect. 3.3.1 for more detail). In the end, the set
of elements and particles that define a litter fuel component must be decided based
on the objective of the particular fuel application and compromises must be made to
facilitate cost-effective sampling (Lutes et al. 2006 ; Chap. 8).

3.2.2 Woody Fuels

All forest, woodland, and shrub ecosystems have dead woody fuels in the surface
fuel layer (Fig. 3.1c , d ). Woody fuels are those twigs, branches, and boles of woody
plants (mostly trees, shrubs, and lianas) that are dislodged and drop to the ground.
These woody particles are mostly composed of cellulose and lignin. Woody fuel
types are important to fire science in that they can foster intense wildland surface
fires, so their study has dominated much of fire and fuels research. In an operational
sense, a surface fuel particle is classified as woody fuel if it meets three criteria—
particles must be down, dead, and woody—hence the name “down dead woody
fuels” used throughout the wildland fuel literature. Particles are downed if they are
detached from their parent plant and are below the 2-m surface fuel layer (Fig. 1.2,
Fig. 3.1c , d ). Some studies considered attached dead branches below 2 m as part of
the fuel bed for certain measurement objectives, but generally, downed fuels must