Fuel Availability and Consumption

Often, total fuel loads do not equal fuel available for consumption. Fuel availability is dependent on:

• fuel moisture content
• fuel location (ground, surface, ladder, canopy),
• fuel physical properties (continuity, compactness), and
• fuel chemical properties (volatile compounds).

While dead fuels are generally the most important source of available fuels, in some areas of the southeastern US live fuels can comprise a very significant portion of available fuels. Fuel consumption models are available for predicting fuel consumption for hazard reduction, smoke management, and ecological restoration.

Consumption of dead fuels

In terms of fuel consumption, dead fuels comprise the dominant portion of available fuel loads. Dead fuels generally have low moisture contents, are close to flaming and smoldering zones, have smaller particle sizes and altered chemical content than they had when living. Consumption of dead fuels is a major goal for forest managers attempting hazard reduction and ecological restoration throughout southeastern forests.

In most southeastern communities, dead standing herbaceous fuels comprise the most important component of the fuel matrix (see fuel models 1-3). The dead component of bunch grasses (notably wiregrass (Aristida stricta) and bluestems (Schizachyrium and Andropogon spp.)) can be the largest source of fine fuels in pine savannas. Dead standing forbs are also an important source of fine dead fuels, but generally comprise less of the fuel load than dead grasses. Fuel moisture in standing dead fuels is based on short and long-term weather changes, topography, and fuel bulk density (see: Moisture Content of Dead Fuels).

Dead woody fuels are divided into 4 broad categories based on fuel particle size and related drying times: 1-, 10-, 100-, and 1000-hour fuels (see Timelag classes). Fuel moisture in dead fuels ranges widely based on these particle sizes, short and long-term weather changes, topography, decay class, and fuel composition (see: Moisture Content of Dead Fuels). Standing dead hardwood sprouts and saplings killed in previous burns can be an important component of dead woody fuels. In the southeastern U.S., broadscale herbicide application results in a large amount of dead standing 1-hour and 10-hour fuels.

The litter layer is a third source of dead fuel available for consumption. Litter (Oi) horizons contain primarily 1-hour litter fuels that dry quickly and burn with high intensity and short duration, with complete consumption except under elevated fuel moistures. Within or residing on litter fuels is a patchy complex of downed woody 10-, 100-, and 1000-hour fuels which can also alter consumption patterns (see Surface Fuels).

Litter forms a part of the forest floor complex, a matrix of litter, duff, and down woody debris above the mineral soil. Fuel availablity of the forest floor is complex. Each forest floor horizons has different moisture relationships, particle sizes, chemical composition, densities, and depths. Forest floor fuel availability is determined primarily by fuel moisture content and fuelbed bulk density. Separation of available and unavailable fuel is made on depth to moisture, with all dry fuel included as surface fuel and the remaining wet included as ground fuel.

Beneath the litter horizon in many unburned stands is a well-developed duff layer comprised of the fermentation (Oe) and humic (Oa) horizons. Due to decomposition processes, the shade provided by the overlying litter, and its greater packing, the fermentation horizon often retains moisture and resists consumption except during smoldering-phase combustion. Similarly, the humic horizon often has greater moisture content than the overlying litter, but can undergo drying from the mineral soil beneath. In areas where duff has accumulated, duff consumption can be of major importance in southern forests (see Ground Fuels). Duff, when sufficiently dried, can smolder for hours to days post-ignition. Duff smoldering kills surficial tree and plant roots, can kill tree vascular tissues, and with optimal conditions can re-ignite unburned areas (Frandsen 1987, Miyanishi 2001).

Dead standing trees, also called snags, also represent a major dead fuel available for consumption. Snags can ignite during both surface and ground fires and flame or smolder long after the ground or surface fires are extinct. Pine snags often contain resinous heartwood cores that maintain structural integrity even when large portions of the stem are consumed by fire. Standing, burning snags are a source of firebrands if unburned fuels are nearby (e.g., along a fire break). When burning snags fall, they can re-ignite forest floor fuels and contribute to fuel consumption, re-burn hazards, and generate smoke that is a source of both safety and health concerns. Low stumps can also ignite when low moisture conditions prevail. Stumps can smolder both above and below ground, igniting dead roots and posing both re-burn and smoke hazards. Hurricanes, beetle infestations, drought, and intense wildfire can all dramatically increase the number of standing snags and down woody debris in southeastern forests.

Consumption of live fuels

Live fuels differ from dead fuels primarily in their high and stable moisture content, elevation above the surface, and protective structures. In many situations, these factors make live fuels less available for consumption than dead fuels except under extreme events (erratic fire behavior, extended droughts). However in the southeast U.S., live fuels make up an important part of the fuel complex and in fact, can comprise the majority of total available fuel (e.g., in pine flatwoods, herbaceous wetlands, Florida scrub).

The primary determinants of live fuel moisture content are: internal factors that regulate diurnal and seasonal changes, climate, site factors that affect the fuel environment, phylogenetic differences among species groups (evergreen vs. deciduous), and differences among plant tissues (leaves vs. stems). How these factors regulate live fuel moisture is briefly explained in Live Fuel Moisture.

Live fuel types can be divided into plant leaves, stems, twigs, branches, cones, flowers, and roots. Most important for consumption are generally leaves, small woody fuels (twigs and branches), and roots. Plant leaves have high surface area-to-volume ratios, lower fuel moisture contents, and are often consumed in surface fires. Many leaves contain volatile compounds that enhance flammability and often carry surface fires. Examples of this phenomenon are prominent in gallberry (Ilex glabra), wax-myrtle (Myrica cerifera) and saw palmetto (Serenoa repens) of pine flatwoods ecosystems. Small woody structures (twigs and branches) can be consumed when located on the forest floor or in shrubs. Small-diameter living plant roots are consumed in ground fires in organic soil and forest floor. Ladder fuels can be consumed and encourage consumption in lower branches and tree canopies. Limited consumption occurs in crown fires; when this does occur, fires are extremely severe, with extreme fire behavior and excessive tree mortality resulting.

For a discussion of the species-specific factors that influence flammability at an individual plant level, see Plant Flammability.