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Unit 2: Fuels Classification

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Distribution & Behavior  ::   Characteristics   ::   Timelag & Life Cycle   ::   Availability   ::   Models   ::   Exercises


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.
Upon completion of this unit you will be expected to:

  • State how each of the three components making up the fire environment can vary over time and space to produce changes in the behavior of a wildfire.
  • List and give examples of the three methods of heat transfer, and give three methods of mass transport of firebrands on fires.
  • Give four primary environmental factors affecting each: ignition, fire intensity, and rate of spread of wildfires.
  • Discuss the relationship of fires of differing intensities to their environments.
  • Describe the behavior of fires in standard fire behavior terminology.
  • Give four necessary input values and five output values of a fire behavior prediction model.
  • Give five reasons why the results of a fire behavior prediction model may differ from the observed fire behavior.


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.

Fuels Distribution & Fire Behavior

A systematic approach to looking at the fuel complex is to divide it into three broad groups or levels - aerial, surface, and ground fuels.

Aerial Fuels


All green and dead materials located in the upper forest canopy including tree branches and crowns, snags, moss, and high brush.

Surface Fuels


All materials lying on or immediately above the ground including needles or leaves, duff, grass, small dead wood, downed logs, stumps, large limbs, low brush, and reproduction.

Ground Fuels


All combustible materials lying beneath the surface including deep duff, roots, rotten buried logs, and other woody fuels.

Fuel Components and Levels

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.

Typical Fire Behavior in Fuels

Now 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.

Typical fire Behavior

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.

Crowning in Closed Stand

Copyright 2008, Michael Jenkins. Cite/attribute Resource . admin. (2005, October 20). Unit 2: Fuels Classification. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License