Surface and Ground Fuels - Wildland Fuel Fundamentals and Applications - page 41

be unattached. Surface woody fuels must also be dead, not living. This is important
because dead woody fuel's physical properties, such as moisture, react differently
to exogenous factors, such as temperature, precipitation, and relative humidity, than
live fuels (Chap. 5). It is sometimes difficult to determine whether woody fuels are
live or dead, especially when live branches are partially covered by the duff and lit-
ter layers, so when there is a question, it is often necessary to break the particle in
half and ensure living tissue is present to verify that it is in fact live. And last, the
particles should be woody . This is often difficult to tell in the field because some
forbs produce structural plant stalks that look quite similar in appearance to woody
fuels. However, you can tell if particles are woody simply by picking them up and
breaking them to ensure that there is a woody center. Nondendritic plants will often
be hollow or contain a nonwoody center.
In most fire management applications, woody fuel types are divided into fuel
components based on their size as determined by their diameter (Table 3.1 ). His-
torically, ranges of woody fuel particle diameters were based on their rate of drying
rather than some other ecological characteristic (Fosberg et al. 1970 ). So, to under-
stand conventional stratifications of woody fuels, one has to understand the concept
of time lag in dead fuels. Time lag is the time it takes for a fuel particle to lose 63 %
of the difference between its initial moisture content and its equilibrium moisture
content under specific environmental conditions (80 o F, 20 % relative humidity) as-
suming an exponential drying function (see Chap. 5; Fosberg et al. 1970 ). There-
fore, based on time lags, woody fuel particles were stratified into four standardized
size classes; the 1-h woody fuels are those fuel particles that lose approximately
two thirds of their moisture in 60 min and this corresponds to particle diameters
that are less than 0.6 cm (0.25 in) in diameter (Fig. 3.1c ). The 10-h woody fuels are
particles with diameters between 0.6 and 2.5 cm (0.25-1 in) and 100-h woody fuels
are 2.5-8.0 cm (1-3 in) in particle diameter (Fig. 3.1d , e ). Woody particles with
diameters greater than 8 cm are called 1000-h fuels (Fig. 3.1f ).
One consequence of defining woody fuel components by rate of moisture loss is
that the particle diameter size classes are inappropriately designed to get accurate
assessments of fuel properties and characteristics. Woody fuel properties can vary
widely across the particle sizes comprising a woody fuel component. Fuel moistures
can also be highly variable within a size class; smaller particles within a size class
can be much drier on any given day. This also matters in the calculation of loading.
The 100-h time lag woody fuel component, for example, has large differences in
fuel biomass between a 3.5-cm diameter branch and an 8-cm branch because par-
ticle mass is often computed as the product of particle volume and density, and since
volume increases with the square of the diameter, small particles in the 100-h class
can have nearly nine times more volume than the larger particles of the same length
(2.5-8.0 cm). This makes the accurate determination of loading difficult because
100-h fuel (lodgepole pine forest in central Montana, USA), e 100-h woody fuel at 5.0 kg m −2 load-
ing (Keane and Dickenson 2007 ), f logs or 1000-h fuel (lodgepole pine forest central Idaho, USA),
g shrub (snowberry in southwestern Montana, USA), and h herbaceous (eastern Washington USA
grassland, photo courtesy of US Forest Service FERA)