Surface 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).