Ground Fuels

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.