Fuel Components or Levels

Most of the fuel in the forest stand with which we are concerned is on or near the ground.  It consists of needles, leaves, grass, pine cones, brush, stumps and reproduction.  We call this “Surface Fuel.”  It is most important because it is involved in wildfires and prescribed fires. 

One of the other components of fuel are the "Aerial Fuels".  Aerial fuels are all the green and dead material located in the upper canopy.  It includes tree branches and crowns, snags, vines, moss and high brush or reproduction.  High intensity fires may “crown out” especially in young plantations where an abundance of fine fuel is available. Such fires can also become crown fires in large, sawtimber size stands if it is a 'hot' fire and there is "ladder-fuel" to carry the fire into the crown.

The third component is "ground fuel". It is composted of all combustible material beneath the surface. It includes deep duff, roots, buried logs, coal and organic soil. It seldom becomes part of the problem except where organic soil is located and it is dry. Such fires are difficult to control and will burn for days. During severe droughts, dead roots may become involved because of the fire burning deeper. These become a "mop-up" problem because dead roots below the fire line may carry fire to the other side of the line.

Common Fuel Types of the Southeast

Grasses 

Grasses are generally associated with weeds, ferns and other seasonal plants.  During the growing season, they are green with high moisture content.  They act as barriers to fire when green rather than a carrier of fire.  As the season advances, they cure and when fully mature, all but the roots will die and dry out.  When dry, they have the fastest rate of spread of any fuel.  The loading, however, is low and the fire will not be as intense. 

The intensity of these fires will be closely associated with the rate of spread.  Slow moving fires in grass fuel, will have very low intensity but high winds will change it to a very fast moving fire of moderate intensity.  Moisture content closely follows daily weather changes.  It is very sensitive to changes in relative humidity and wind. 

Young Pine Plantation

The fuel in pine plantations will change according to the size of the pines.  When first planted, the crowns of seedlings are close to the ground.  Even though green, the pine needles will dry out and burn with little heat due to their small size and high rosin content.  Fire behavior will be about the same as grass fuel.  The fuel between the seedlings will also be grass for the first few years. 

As the seedlings get larger, more fuel will be available. The grass and other weeds have also grown increasing the quantity of fuel. Most will still be readily available fuel to a fire.  Needle drape will add to the fuel loading and will tend to carry the fire into the crowns.  All but the stems of the trees will readily burn.  Fires will generally be intense and fast spreading. 

The volume of available fuel will continue to increase until the crowns begin to close, shading out the weeds and grasses.  As this occurs, a smaller percentage of the total fuel loading becomes available to most fires due to the height of the crowns and less “ladder” fuel to carry the fire into them.  The fuel available to most fires will generally be the understory fuels that are on the surface.

Light Brush and Pine Needle Litter 

The fuel under most forest stands consists of light to moderate loading of fuel, most of which is compacted on the ground.  Fuels of this type are found throughout the Piedmont and Upper Coastal Plain regions of the Southeast.  In dense pine stands, the predominant fuel is the matted pine needles.  In upland hardwoods, it is compacted hardwood leaves.  The amount of brush will vary from almost non-existent to almost solid brush, especially if there is little overstory.  This type fuel will generally consist of grasses, pine needles, deciduous shrubs, small saplings, pine cones, twigs and branches. 

Fires in this type fuel will generally be of low intensity and slow spreading.  The surface fuel is compacted and dries out very slowly.  Consequently, much of it will not be available.  Shrubs and small saplings tend to be more readily available and will add to the intensity where they are present.  Most fires will be of rather low intensity and easy to control except during droughts when a larger percent of the fuel will be available.  Firefighters can be surprised when this happens if they are not alert because of the increased intensity and rapid spread of the fire. 

Palmetto-Gallberry 

Palmetto and gallberry is found throughout the lower Coastal Plain of the southeast.  This fuel type is usually found under an overstory of longleaf and slash pine.  Pine needles, grass and other vegetative debris will be in the ground litter.  Fuel loading can range from light to heavy with an accumulation of dead material.  Both palmetto and gallberry are low shrubs with very high amounts of waxes that make them very volatile.  This fuel is loosely arranged and a large percent will be available fuel.  Fire behavior (both intensity and rate of spread) will range from moderate to extreme.  The pocosins of the Atlantic Seaboard are similar to this fuel type except the loading can be even higher and fires more intense. 

Logging Debris 

Logging debris is generally burned after drying out so it generally does not present a wildfire problem.  The loading is moderate to heavy but it is compacted and moisture remains high for long periods. Fires in this type can burn intensely if dry.  Suppression is difficult, especially if windrowed. Firebrands can carry fire into adjacent stands.

Hardwood Stands 

Hardwood stands are usually found along streams and other areas of better sites.  In solid hardwood stands, the fuel will consist primarily of matted hardwoods leaves.  Few fires occur in this fuel type due to moist conditions and compaction of the fuel.  The fires that do burn into this fuel type will be low intensity and slow spreading.  In some areas, understory brush is found and will contribute to the fire’s intensity.  Some stands will be intermixed with pine.  Fire behavior in these stands will occur more often and be more intense due to the pine needles, light brush and dryer conditions found in such stands. 

Turkey Oak and some other hardy species of oak will grow in dry areas of deep, sandy soils.  The fuel will be oak leaves and grass.  Fire behavior will be similar to grass fuel except the large leaves will not be compacted.  Being loosely arranged and dry, they will  readily ignite.  Light winds will tend to blow them across fire lines. 

Swamps

Hardwood swamps are usually too wet to burn and are not a problem except during long periods of drought when they dry out. Fuel loading is high and line construction is extremely difficult to impossible. Fires should be kept out of swamps when they are dry. If fire does get into one, firelines should be located on the other side.