This unit is about fuels in the fire environment. In order to make reliable estimates of fire
behavior, we must understand the relationship of fuels to the fire environment and be able to
recognize the variations in these fuels. Before starting this unit, read the instructions to the
student on page 1 of your workbook. On page 2, you will find the objectives on which you will
be tested at the end of this unit. Please study these objectives. When you have finished, return to
In fire control language, fuel is any organic material--living or dead, in the ground, on the
ground, or in the air--that will ignite and burn. Fuels are found in almost infinite combinations of
kind, amount, size, shape, position, and arrangement. The fuel on a given acre may vary from a
few hundred pounds of sparse grass to 100 or more tons of large and small logging slash. It may
consist of dense conifer crowns, heavy and deep litter and duff, or underground peat. Any one
composite fuel system is referred to as a fuel complex and has built-in flammability potential.
We can predict fire behavior to a large extent by analyzing the physical properties and
characteristics of fuels. Topographic and weather factors must also be considered before rate of
spread and general behavior of fires can be determined.
A systematic approach to looking at the fuel complex is to divide it into three broad groups or
levels--ground, surface, and aerial fuels. Look at figure 1 on page 3, Fuel Components and
Since most wildfires are carried by the surface fuels, this fuel level receives the most emphasis.
Aerial fuels must also be considered because they may be consumed by fire under certain
conditions and can contribute to extreme fire behavior. Ground fuels are important in relation to
line construction and mop-up operations. Each level must be evaluated according to
characteristics that affect ignition and combustion.
On page 4, figure 2, we take our discussion of fuel groups or levels a step further and generalize
on typical fire behavior under normal fire season conditions. Ground fuels will usually be
compacted, and fire spread will be slowest, typically smoldering or creeping.
Surface fuels will be less compacted with other characteristics more favorable for faster rates of
spread. If no aerial fuels are present, we essentially have an open environment subject to stronger
winds and more heating and drying by solar radiation. Thus, fires often run through this fuel
complex with higher rates of spread than if aerial fuels were present.
If aerial fuels are present, we should be concerned with crown or canopy closure. Timber stands
with an open canopy will probably have a faster spreading surface fire than closed canopy
stands, and torching of individual trees with possible spotting could occur. Unless very strong
winds are present, crowning is unlikely without a closed canopy. Closed canopy stands, whether
timber or tall shrubs, offer the best opportunity for a running crown fire.
Now please do question 1, mark your choice or choices, then return to the text.