Fuel Concepts - Wildland Fuel Fundamentals and Applications - page 171

interpreted differently. Anderson ( 1966 ) characterized fuel flammability by three
criteria:
1. Ignitability. Ability to ignite at the lowest temperature with the least energy input
2. Sustainability. Ability to continue flaming combustion when heat source is
removed
3. Combustibility. Ability to facilitate combustion to have short burn times and the
highest proportion of consumption
These characteristics can be evaluated at particle, component, layer, and fuelbed
scales. However, the metrics used to rate these criteria can rarely be measured in
absolute terms (Mak 1988 ). Most assessments inevitably use the components of
combustion to rate flammability (Dimitrakopoulos and Papaioannou 2001 ), such as
measuring time to ignition at a standard temperature or heat flux. Dimitrakopoulos
( 2001 ), however, rated flammability based on five fuel characteristics including
heat content, ash content, silica-free ash content, surface area-to-volume ratios and
particle density. Liodakis et al. ( 2002 ) found that flammability (ease of ignition) was
insensitive to the inorganic concentration of the fuel, but rather was closely tied to
rates of cellulose decomposition. Dimitrakopoulos and Papaioannou ( 2001 ) found
moisture content was the overriding factor dictating the flammability of Mediterra-
nean fuels. Many believe that the chemical compositions of live and some dead fu-
els are important to flammability because of the presence of resins, crude fats, oils,
and other volatile substances might facilitate ignition and consumption (Jolly et al.
2012 ; Philpot 1969 ). In most cases, however, fuel differs in flammability because
of physical, morphological, and ecophysiological characteristics, and less because
of chemical characteristics. For example, some species (e.g., sagebrush, saltbrush)
have ecophysiological adaptations to allow them to survive when their foliar mois-
ture is low (< 50 %) thereby making them more flammable. Because flammability
cannot be objectively assessed using standardized tests, the concept is best used in
a relative, qualitative sense.
The problem with flammability is that it must be interpreted across many spa-
tial and temporal scales and across many combustion qualities to be useful for fire
management. Some shrub species may be highly flammable because of small, dense
foliage, but their foliage may be flammable for only a small portion of the year
because of high live fuel moistures. Grasses are highly flammable when cured,
but if they are not connected in space, or if they exist in communities that contain
abundant inflammable species, then their flammability at broader stand scales may
be lower. Some fuel types, such as resin-saturated wood, may have higher heat
contents contributing to high flammability, but they may also have a low SAVR that
serves to decrease flammability. Rotten wood has low particle densities that may
increase the potential for ignition at higher moistures over sound wood, but rotten
particles also have lower heat contents, that might decrease flammability at the
particle level, and the rotten particles may be widely scattered across the landscape
thereby decreasing flammability at coarser spatial scales.
A more current example of the challenges involved in evaluating flammability
concerns the extensive areas across North America that have been killed by pine