Personal tools
You are here: Home Wildland Resources Wildland Fire Management and Planning Unit 2: Fuels Classification

Unit 2: Fuels Classification

Document Actions
  • Content View
  • Bookmarks
  • CourseFeed

Table of Contents

  1. Fuels Distribution and Fire Behavior
  2. Fuels Characteristics
  3. Timelag
  4. Stages In The Life Cycle Of Herbaceous Vegetation
  5. Fuels Availability
  6. Fuel Models
  7. Fire Behavior Fuel Model Key
  8. Exercises

Fuels Distribution and 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.

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.

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.


Fuel Characteristics

Our analysis of fuel complexes and their potential to support combustion and spread fire requires a more detailed study of individual fuel components. Here we introduce the principal characteristics of fuel components that can give us an indication of potential fire behavior within a fuels complex.

The principal fuel characteristics which affect fire behavior are:

  1. Loading
  2. Size and Shape
  3. Compactness
  4. Horizontal Continuity
  5. Vertical Arrangement
  6. Moisture Content
  7. Chemical Properties

Fuel characteristics chart

Each of these seven characteristics contributes to one or more fire behavior processes. In the figure below we have diagramed the primary relationships. Let's take a few minutes to study these relationships. First, we're concerned with whether ignition will result in a sustaining fire. There are five fuel characteristics that most affect ignition. These are compactness, loading, chemical content, size and shape, and moisture content.

Our next concern is how fast the fire will spread. Here six primary characteristics are involved. How hot or intense will the fire be? What are the possibilities of spotting, torching, or crowning? We can relate individual fuel characteristics to each of these. Since we have not given you the definition for each of the seven characteristics yet, we will move on at this point but return later to study the diagram in more detail.

Fuel Loading

The first principal characteristic is fuel loading. Loading is defined as the oven dry weight of fuels in a given area, usually expressed in tons per acre. Natural fuel loadings vary greatly by vegetative or fuel types. The pictures below give you some examples of total fuel loadings. Grassland areas may produce fuel loadings of 1 to 5 tons per acre. Brush species such as chaparral, may produce 20 to 40 tons per acre; logging slash, 30 to 200 tons per acre; and timber, 100 to 600 tons per acre. These are all typical ranges but will not fit every fuels complex. Often fuel loading refers only to surface fuels that are less than 3 inches in diameter. If this is the case, the loading for the timber stand in the above example would be 4 to 12 tons per acre.

Two very different fuel types with different fuel loadings

You can see that fuel loadings involve different size classes of fuel particles, various fuel arrangements, and particle distribution over a specific area. Fuel loading descriptions may not only state the total weight or mass per acre, but give weights by fuel size classes and describe their distribution vertically and horizontally. For example, it is important to know the amount of fuels in various size classes, whether the fuels are standing or lying on the ground, and whether the fuels are scattered or in piles.

Size and Shape


Size and Shape affect the surface area to volume ratio of fuels. Small fuels and flat fuels have a greater surface area to volume ratio than larger fuels.

Major Size Classes of Fuels

We've been using the terms large fuels versus small fuels in a relative sense. To be more specific for fuels analysis purposes, we normally break dead fuels into four size classes.

The Major Size Classes of Fuels Are:

  1. Grass, Litter, duff: less than 1/4 inch diameter
  2. Twigs and small stems: 1.4 inch to 1 inch diameter
  3. Branches: 1 to 3 inch diameter
  4. Large stems and branches: greater than 3 inch diameter



Compactness is the spacing between fuel particles and affects the rate of combustion.
The next principal fuel characteristic we need to discuss is compactness. Compactness affects the rate of combustion. The images below illustrate how the closeness and physical arrangement of the fuel particles affects both ignition and combustion. Those that are closely compacted have less surface area exposed and less air circulation between particles, thus requiring more heat or time for ignition.

Horizontal Continuity


Now let us look at horizontal continuity as a principal fuels characteristic. This characteristic influences where a fire will spread, how fast it will spread, and whether the fire travels through surface fuels, aerial fuels, or both.
Horizontal Continuity is the extent of horizontal distribution of fuels at various levels or planes.

Vertical Arrangement


We've discussed some aspects of surface fires versus torching out and crown fires. A very important fuels characteristic involved here is the vertical arrangement of fuels.
Vertical Arrangement : The relative heights of fuels above the ground and their verical continuity. This influences fire reaching various fuel levels or strata.

Fuel Moisture Content

A very important fuels characteristic is fuel moisture content. It can vary in different fuel levels and thus influence whether those levels become involved with fire. In nature, dead-fuel moisture very seldom gets below 3 or 4 percent. Dead fuel moisture fluctuates considerably over time due to several environmental factors. Live fuel moistures run much higher, perhaps 300 percent or more, but they change less rapidly than dead fuels. This is an interesting area of study that we will resume in Unit 5 of this course, which is entitled "Fuel Moisture."
Fuel moisture content is the amount of water in fuels expressed as a percent of the oven dry weight of that fuel.

Chemical Properties

Here we have another principal fuel characteristic for discussion, the chemical properties of fuels. There are certain fuels having rather high amounts of these volatile substances that can contribute to rapid rates of spread and high fire intensities. On the other hand, certain fuels may be high in mineral content, which can reduce fire spread and intensity. A firefighter is primarily concerned with the volatile substances that make his job more difficult.
Chemical properties include the presence of volatile substances such as oils, resins, wax, and pitch in the fuels, which affects rate of combustion.

Volatile Substances

Some well known fuels in which volatile substances contribute greatly to fire intensity and fire spread are:

  1. Chaparral in the Southwest
  2. Gallberry bushes in the Southeast
  3. Sand Pine during varnish stage in Southeast
  4. Fountain Grass in Hawaii
  5. Pitchy stumps from some conifers



We also know that the moisture content in fine, dead fuels can change very rapidly, depending on the relative humidity of the air and precipitation. Moisture content changes in larger fuels, but at a much slower rate. How much slower? How do we predict what fuel moisture changes will occur in various fuels over periods of time? Well, fire scientists have determined drying times for different size fuels and have designed a system to determine and record fuel moisture percents. They use the term time lag and have placed various sizes of fuels into convenient time lag categories or classes.
Timelag is a measure of the rate at which a given dead fuel gains or loses moisture. The timelag categories are:

  1. 1 hour timelag fuels: less than 1/4 inch diameter
  2. 10 hour timelag fuels: 1/4 to 1 inch diameter
  3. 100 hour timelag fuels: 1 to 3 inch diameter
  4. 1000 hour timelag fuels: greater than 3 inch diameter

Do these fuel sizes look familiar? Our four size classes of fuels correspond to the four time lag categories for fuels. In Unit 5 of this course, we will study the time lag concept more and make estimates of fuel moisture percents from tables.


Stages in the Life Cycle of Herbaceous Vegetation

We noted earlier the wide range in fuel moisture percents from dead to live fuels. Since the fine fuels are the primary carrier of fire, we should be concerned with the amount of dead versus live fine fuels. We know that this proportion changes throughout the year with the seasonal growth and then with the curing of herbaceous vegetation.


Fuels Availibility

Available Fuels are those that will ignite and support combustion at the flaming front under specific burning conditions at a given time.

Consumption of Fuels by Fire


Do all the fuels burn during the passage of a fire? The answer is no. Ordinarily only a portion of them burn, depending on factors of fuels availability. In a cured grass stand, we might get nearly 100 percent of the fuels consumed by fire. These indeed have a very high degree of availability. A stand of brush is seldom completely burned, but perhaps 5 to 95 percent is consumed. The stumps, logs, and larger limbs of logging slash rarely are totally burned; thus, consumption in slash might be 10 to 70 percent. In timber, standing trees are only partially burned, and overall fire consumption might be 5 to 25 percent.


We'll discuss some other aspects of fuels availability. A major concern of the firefighter is; will fuels ignite if subjected to heat or burning firebrands? For example, he needs to be able to recognize fuel conditions that are receptive to spotting.
Fuels that ignite most readily from embers (spotting) are:

  1. Rotten wood in snags or on ground
  2. Dead foliage
  3. Moss and lichens in trees
  4. Slash compacted in a tight arrangement
  5. Needle or leaf accumulations on ground
  6. Cured grasses

Crown Fires

Before we leave fuels availability, we should examine the fuel conditions that influence the probability and character of crown fires.
Fuel conditions which influence the probability and character of crown fires are:

  1. Vertical positioning of fuels above surface
  2. Character and availability of surface fuels
  3. Presence of fine dry aerial fuels
  4. Continuity of aerial or canopy fuels


Fuel Models

Fuel model is a simulated fuel complex for which all the fuel descriptors required for the solution of a mathematical fire spread model have been specified.
Since the fire model only applies to surface fires, ground fuels and aerial fuels are not included in the fuel descriptors. The components that are described include needles or leaf litter, dead and down woody material, grasses and forbs, shrubs, and regeneration. Various combinations of these components make up the fuel models.
There are 13 stylized fuel models that are used to make fire behavior predictions. These must represent a wide variety of fuel conditions. Choosing the appropriate fuel model requires experience and personal judgment.
Four major fuel community groups are:

  1. Grass
  2. Shrub
  3. Timber
  4. Slash


Fire Behavior Fuel Model Key

Always check the selected model with fuel model description.

  1. Primary carrier of fire is grass. Expected rate of spread is moderate to high, with low to moderate intensity.
    1. Grass has a relatively fine structure, is generally below knee level and is easy to walk through. Model 1
    2. Grass has thick, coarse stems, is above knee level and is difficult to walk through. Model 3
    3. Mixture of grass and litter beneath open timber or brush overstory that does not burn. Model 2
  2. Primary carrier of fire is brush. Expected rate of spread and intensity are both moderate.
    1. Vegetation type is southern rough or low pocosin. Model 7
    2. Live fuels absent or sparse with no capability to reduce fire spread rate. Model 6
    3. Live fuel moisture can have a significant damping effect on the fire behavior.
      1. Brush is about knee deep with a ligh loading of 1-hour timelag fuels. Model 5
      2. Brush is close to head high with a heavy loading of 1-hour timelage fuels. Model 4
  3. Primary carrier of fire is debris beneath a timber stand.
    1. Live fuels are present in sufficient quantity to influence fire behavior. The load of 100-H TL fuels is heavy. Model 10
    2. Surface fuels are mostly foliage litter, with little or no live fuel.
      1. 1-H TL load strongly predominates; 10-H and 100-H TL fuels are sparse. Foliage litter is long needle pine or hardwood leaves, loosely compacted. Model 9
      2. 1-H and 10-H TL fuel load combined is about equal to 100-H TL load. Foliage litter is short needle coniferous or small hardwood leaves, tightly compacted. Model 8
  4. Primary carrier of fire is slash.
    1. Slash is not continuous. Other ground fuels must be present to help carry the fire. Average slash depth is about 1 foot. Model 11
    2. Slash is continuous or nearly so. Other surface fuels need not be present to carry the fire. Average slash depth is about 3 feet. Model 13
    3. Slash generally covers the ground, though there may be bare spots or areas of light coverage. Average slash depth is about 2 feet. Model 12


Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 05). 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