Introduction - Wildland Fuel Fundamentals and Applications - page 4

The fuelbed is vertically stratified into three fuel layers — ground, surface, and
canopy fuels. In this topic, surface fuels are all biomass within 2 m above the ground
surface (Fig. 1.1 ). This 2-m boundary is mostly arbitrary and was originally defined
by several heights depending on the fire application; Brown and Davis ( 1973 ), for
example, used a 4-ft height. Ground fuels are all organic matter below the ground
line. The position of the ground line is highly contentious; some put it below the
litter at the top of the duff because they feel that only the litter contributes to the
propagation of the flaming front, while others put the ground line below the duff
because it is incredibly difficult to distinguish between litter and duff in the field.
In this topic, litter is considered surface fuel while duff is considered ground fuel
(Chap. 2). Canopy fuels are the biomass above the surface fuel layer. Some define
the canopy as starting at 6 m (20 ft; NWCG 2006 ), while others define it as all tree
biomass no matter the height. To be consistent, canopy fuels are defined as all bio-
mass (e.g., shrub, moss, lichen, vine, dead material, and tree) that is higher than 2 m
above the ground surface (Fig. 1.1 ). The term aerial fuel is also used to describe
canopy fuel (Brown and Davis 1973 ).
Fuelbed layers are composed of finer-scale elements called fuel types and compo-
nents (Fig. 1.2 ). Fuel types are general descriptions of the kinds of fuels comprising
Fig. 1.2 Fuel types and fuel components. The quantitative description of these fuel types is called
a fuel component (e.g., shrub component is all shrub biomass with branch diameters less than
5 cm). Each fuel type or component is composed of a set of fuel particles, such as intact or frag-
mented twigs, needles, or leaves