Surface and Ground Fuels - Wildland Fuel Fundamentals and Applications - page 51

it is nearly impossible to consistently identify where the upper duff ends and lower
duff starts. Here, duff is considered one layer fully recognizing the great amount of
variability in fuel properties across the thickness of this component.
Duff properties are especially difficult to define and measure because of other
factors besides the decomposition gradient. The novel fuel beds created by new
fuel treatments provide an example. Mastication, the modification of the fuel bed
through mechanical means, often creates fuel particles whose origins are also dif-
ficult to identify but they are in the early stages of decomposition. So should these
masticated materials be considered part of the duff if they are distributed through-
out the duff profile? Another aspect of duff that is difficult to evaluate in the field
is whether a decomposed piece of wood is considered duff or woody fuels. Some
logs are so decomposed that the remaining biomass often burns more like duff
than woody material, yet the features of the log are still fully recognizable. When
you kick these logs, they often break apart with little effort. In this topic, rotten
logs are duff only if the central axis of the log (parallel to log length) is below the
top of the litter (Lutes et al. 2006 ). This physical definition is a typical example of
the differences between biomass and fuel, and the difficulty in operationally de-
fining biomass as fuel. Sound woody fuels often protrude into the duff and are
easily recognizable in the duff profile, yet operationally, these woody fuels are of-
ten considered part of the duff; only those woody particles above the duff and lit-
ter layers are considered woody fuels. These examples illustrate how difficult it is
to measure and understand duff dynamics. There is considerable overlap between
fuel types, and then, once fuel types are defined as components, there is a smaller,
but still considerable, overlap between fuel components. It is nearly impossible to
stratify fuel components so that they are always easily and accurately identified in
the field.
Duff is important to fire behavior and effects in a way that is quite different from
surface fuels. Duff necromass rarely contributes to fire spread because, in general, it
is densely packed so it dries slowly and is often moister than other fuel components
making it difficult to ignite and burn. It also consists of highly decomposed material
just above the mineral soil so it usually has a high mineral content that dampens
fire spread. As a result, duff layers burn mostly under smoldering rather than flam-
ing combustion, although the upper duff may contribute to a surface fire if it is dry
enough and there are sufficient litter fuels.
Duff has major implications for fire management because most of the dead nec-
romass in a fuel bed is usually contained in the duff and logs (Table 3.2 ). It is
common for logs and duff to contain over 60 % of the surface fuel loading in most
forested ecosystems (Brown and Bevins 1986 ), and duff often comprises the major-
ity of that 60 % because of its high bulk density values that range from 12 kg m −3
in deciduous ecosystems to over 150 kg m −3 in decomposed logs in duff of conifer
ecosystems (Brown 1981 ; Stephens et al. 2004 ). And, because of high moisture
and mineral content, duff biomass burns produces great amounts of smoke during
smoldering combustion that can reduce visibility, lower air quality, and adversely
impact the health of people.