Fuels can be classified into four broad categories based on their vertical distribution:

• Ground fuels: organic soils, forest floor duff, stumps, dead roots, and buried fuels
• Surface fuels: litter layer, downed woody materials, and dead and live plants to 2 m (6 ft.) height
• Ladder fuels: vine or liana fuels and draped foliage fuels
• Canopy fuels: tree crowns

These fuel categories are not to be confused with the fuel types used in fuel models (such as grasses, brush, timber litter, and logging slash). Fuel models are more specific classes of fuels used in fire behavior modeling.

Ground fuels are those forest fuels that lie below the litter layer or within the soil, including organic soils, forest floor duff, stumps and dead roots, and buried fuels. Ground fuels can ignite and smolder for days to months following flaming front passage. Ground fires produce persistent and harmful smoke and can re-ignite surface fuels making them a bane for fire managers.

The forest floor is the layer of organic matter overlying the mineral soil and has both surface and ground fuel components. The forest floor fuel complex contains distinct horizons, each with different moisture relationships, particle sizes, chemical composition, densities, and depths. The surface fuel component of the forest floor is the litter (Oi) horizon. The ground fuel component, duff, is beneath the litter horizon. It is comprised of the fermentation (Oe) and humic (Oa) horizons. In long-fire interval ecosystems the duff layer can become well-developed, however in frequently burned systems it may be intermittent or nonexistent. Duff is created by litter decomposition, so many volatile compounds are lost, particle sizes are reduced, and it is shaded by the overlying litter horizon. Similar to 1,000-hour timelag fuels, duff is slow to absorb moisture. Therefore, when duff moisture is low, smoldering phase combustion often consumes this horizon, resulting in high fire severity and copious amounts of smoke.

Organic soils are important forest fuels in several southeastern ecosystems. Organic soils contain the duff layer overlying a variety of soils (see earlier discussion) and true histosol organic soils. Histosols are dark-colored soils consisting of large amounts of organic peat and muck, underlying poorly-drained forested and nonforested wetlands (e.g., cypress domes, pitcher plant bogs, and bay swamps). Available fuel in organic soils is defined by three factors: moisture, packing, and mineral soil content (Frandsen 1987). Increases in any of these factors decreases flammability and retards combustion. However, following extended droughts, organic soils can ignite and burn for days to months, often smoldering beneath the surface (so called “muck fires”). Organic soil fires are serious concerns in many southeastern wetland communities; they are difficult to control, and have serious ecosystem effects (see: Prescribed Burning in Organic Soils).

See also: Moisture Content of Ground Fuels.

Surface fuels are the primary fuel of interest for fire behavior in most southeastern ecosystems (Wade et al. 2000). Surface fuels include understory plants < 2 m (6 ft.) tall (dead and alive), the litter layer, downed woody materials, and often midstory tree and shrub fuels. Surface fuel availability for consumption is determined by moisture content, particle size, horizontal continuity, compactness, and fuel type (particularly fuels with high volatile compounds). Under most burning conditions in most southeastern ecosystems these fuels carry surface fires.

The understory is the layer of living and dead vegetation from the soil surface to 2 m (6 ft.) tall. Many southeastern ecosystems (e.g., open pine savannas and forests, freshwater marshes, pitcher plant bogs, prairies) contain a dominant understory with abundant grass, forb, small woody shrub and litter fuels. Both grasses and their allies (sedges and rushes) and forbs have high surface area-to-volume ratio, low fuel moisture, are within the flaming zone of most surface fires, and retain abundant dead leaves making them ignite and combust rapidly (exceptions to this are succulents and large-leaved species). Understory fuel availability in southeastern ecosystems is controlled by fuel moisture, horizontal fuel continuity, and fuel loading.

Small woody shrubs can be important understory surface fuels (Blackmarr and Flanner 1975, Hough and Albini 1978). Pocosins, flatwoods, sand pine scrubs, and bogs contain large loadings of shrubby fuels. Many southeastern shrubs have high surface area-to-volume (e.g., saw-palmetto, Serenoa repens), high volatile contents (e.g., gallberry, Ilex glabra), grow within the flaming zone of surface fires, and are highly flammable. In some ecosystems, shrubs and small trees grow into the midstory (between 2 and 5 m; 6 and 16 ft.) and carry surface fires into lower canopy fuels. Midstory fuel availability is regulated by vertical fuel continuity, fuel moisture, and fire behavior. Low-intensity fires with low flame lengths often don’t ignite midstory shrub fuels.

The forest floor is the layer of organic matter overlying the mineral soil and has both surface and ground fuel components. The surface fuel component of the forest floor is the litter (Oi) horizon. The ground fuel component of the forest floor is the duff layer. Litter horizons are fuels in almost all forested southeastern ecosystems, and are therefore somewhat diverse in their composition and structure. Most litter horizons contain recently deposited litter, small woody fuels (10-, 100-, and few 1,000 hour timelag fuels), cones, and other dead plant parts. Litter fuels have reduced volatile content, low fuel moisture content (often 5 to 15%), and are usually loosely packed. Surface fires can be carried solely by litter fuels. Litter fuels may also ignite live understory fuels, pre-heat larger woody fuels, and initiate smoldering of underlying ground fuels, if present. 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.

Understory and shrub fuels are measured using quadrat, point-quarter center, and line transect sampling methods (see Measuring Fuel Loads). Loads (measured in dry kg/m² or lb/acre) are calculated and extrapolated to larger areas or can be input into fire behavior models (e.g., BEHAVE). Forest floor surface fuels are measured by harvesting small quadrats (in kg/m² or lb/acre, and drying for moisture content) and by determining fuelbed bulk density (in kg/cm² or lb/ ft³).

Ladder fuels are those that provide vertical continuity between understory or midstory surface fuels and canopy fuels. Ladder fuels consist of vine or liana fuels, draped foliage fuels, and hanging broken branches. Most surface fires in southeastern ecosystems involve isolated ladder fuels, though in particular circumstances ladder fuels can accumulate and lead to high severity fires.

Vine fuels include several southeastern species that are important in surface fires, such as yellow jessamine (Gelsemium sempervirens), greenbriers (Smilax spp.) and wild grape (Vitus spp.). Vines ascend trees and shrubs creating vertically continuous fuels. Dead and live foliage, stems, and flower structures have low fuel moisture, are bathed in convective heat, and contain volatile compoundscrown fires. Vine fuel availability is governed by fuel moisture, flame height, windspeed, and the live-to-dead ratio. Vine fuels are usually measured as presence/absence, height in canopy, vine loads, and live to dead ratios.

Draped dead foliage (especially pine needle litter) on vines and living or dead shrubs is another important ladder fuel type. Draped fuels have very low fuel moisture (wind, sun, and humidity effects are increased) and are highly flammable. Draped fuels increase the height of the combustion zone, linking understory and midstory fuels to canopy fuels. Southeastern pine plantations and long-unburned forested ecosystems with well-developed vine and/or shrub layers are especially prone to draping.

Hanging broken branches become important ladder fuels in forests following hurricanes, tornadoes, ice storms, and other disturbances.

Canopy fuels are the crowns of trees that form the overstory. The receptivity of the canopy fuels to crown fire is based primarily on three factors: canopy base height, canopy bulk density, and, to a lesser degree, foliar moisture content (Fieldhouse and Dickinson 2003). Canopy base height relates the bottom of the overstory tree crowns to the top of the understory fuel bed and ladder fuels. Canopy bulk density is a measure of the amount of fuel contained in a unit volume of the canopy. High bulk densities present large fuel loads for a fire.

Canopy or crown fuels are typically not consumed during fires in the southeastern US except in isolated cases of "torching" which affect individual trees. Crown foliage is commonly scorched, but rarely is it consumed (i.e., combusted) in crown fires. Particular exceptions are the stand-replacing fires common in sand pine scrub forests in central Florida, in stand-replacing fires in non-indigenous melaleuca forests in south Florida, and limitedly in Table mountain pine forests in the southern Appalachians. General exceptions to this statement occur in fires, either prescribed or wildfire, with extreme fire behavior (caused by low moisture levels, erratic winds, or high fuel loadings).