Surface and Ground Fuels - Wildland Fuel Fundamentals and Applications - page 50

used to flail, chip, chop, and crush live and dead biomass to create a compact mas-
ticated fuel bed that many believe will burn slower and with lower intensities than
pre-masticated fuel beds. However, current research has shown that mastication
treatments do not always achieve fire management objectives (Ulanova 2000 ) and
fires that burn in masticated fuels may be more intense, kill more trees, and cause
more damage to soil biota than fires that burn in pre-masticated fuels.
Another similar special fuel type is stumps (Fig. 3.3e ). Stumps are now a com-
mon occurrence on mechanically treated lands (Fule et al. 2001 ) and the biomass in
stumps may contribute to many ecological and fire management concerns, such as
smoke, wildlife habitat, and soil heating. Tree-cutting treatments, for example, may
leave an abundance of stumps of various sizes, shapes, and species. Stumps, like
snags, rarely burn in prescribed fires or wildfires unless they are thoroughly decom-
posed, yet they play important ecological roles such as stabilizing soil, providing
long-term nutrient retention, and fostering biodiversity.

3.3 Ground Fuel Layer

The most common ground fuel type is duff and it is usually present as a result of
the decay of the surface fuels. Most non-duff ground fuel layers are found in special
locations where organic material may accumulate because of retarded decomposi-
tion or heavy biomass deposition.

3.3.1 Duff

Duff is the layer of decomposing organic materials lying just below the litter layer
and immediately above the mineral soil (Fig. 3.1a ). However, this simple definition
is difficult to employ in the field because of the pronounced gradient of decomposi-
tion from the top of the litter layer (freshly fallen material) to the mineral soil (most
decomposed material). Therefore, a better, operational definition of duff is the de-
composing layer of biomass where the original source of fuel particles is no longer
identifiable above mineral soil and below the litter layer. In reality, most dead bio-
mass in the duff is in some stage of decomposition and this contributes to a corre-
sponding wide range of duff fuel properties, such as moisture, mineral content, heat
content, combustion efficiency, and smoldering duration across the duff profile. To
remedy this, some have divided the duff layer into the upper and lower duff (Brown
et al. 1991 ). The upper duff is usually referred to as the fermentation layer (F layer)
where some fuel particle origins are still somewhat identifiable. The lower duff is
often called the humus layer (H layer) where the particles are too decomposed to be
identified (Soil Classification Working Group 1998 ). The Soil Survey Staff ( 2006 )
have defined the upper duff as the O e layer and lower duff as the O a layer. However,
this upper and lower duff differentiation is also difficult to employ in the field in that