Grass Group:
Fuel Model 1 (1 foot deep) Fire spread is governed by the fine herbaceous fuels that have cured or are nearly cured. Fires are surface fires that move rapidly through cured grass and associated material. Very little shrub or timber is present, generally less than one-third of the area.
Grasslands and savanna are represented along with stubble, grass-tundra and grass-shrub combinations that meet the above area constraint. Annual and perennial grasses are included in this fuel model.
Fuel Model 2 (1 foot deep) Fire spread is primarily through the fine herbaceous fuels, wither curing or dead. These are surface fires where the herbaceous material, besides litter and dead-down stemwood from the open shrub or times overstory, contributes to the fire intensity. Open shrub lands and pine stands or scrub oak stands that cover 1/3 to 2/3 of the area may generally fit this model but may include clumps of fuels that generate higher intensities and may produce firebrands. Some pinyon-juniper may be in this model.
Fuel Model 3 (2.5 feet deep) Fires in this fuel are the intense of the grass group and display high rates of spread under the influence of the wind. The fire may be driven into the upper heights of the grass stand by the wind and cross over standing water. Stands are tall, averaging about 3 feet, but considerable variation may occur. Approximately one-third or more of the stand is considered dead or cured and maintains the fire.
Shrub Group:
Fuel Model 4 (6 feet deep) Fire intensity and fast spreading fires involve the foliage and live and dead fine woody materials in the crown of a nearby continuous secondary over story. Examples are stands of mature shrub, 6 or more feet tall, such as California mixed chaparral, the high pocosins along the east coast, the pine barrens of New Jersey of the closed jack pine stands of the north-central states. Besides flammable foliage, there is a dead woody material in the stand that significantly contributes to the fire intensity. Height of stands qualifying for the model varies with local conditions. There may be also deep litter layer that confounds suppression efforts.
Fuel Model 5 (2 feet deep) Fire is generally carried in the surface fuels made up of little cast by the shrubs and the grasses or forbs in the under story. Fires are generally not very intense as surface fuel loads are light, the shrubs are young with very little dead material, and foliage contains little volatile material. Shrubs are generally not tall, but nearly cover the entire area. Young, green stands with little or no deadwood such as laurel, vine, maple, alder, or even chaparral, manzanita, or chamise are examples. As the shrub fuel moisture drop, consider using a Fuel Model 6.
Fuel Model 6 (2.5 feet deep) Fires carry through the shrub layer where the foliage is more flammable than Fuel Model 5, but require moderate winds (<8mi/h) at midflame height. Fire will drop to the ground at low wind speeds or openings in the stand. Shrubs are older, but not as tall as shrub types of Model 4, nor do they contain as much fuel as Model 4, a broad range of shrub conditions is covered by this model. Typical examples include intermediate stands of chamise, chaparral, oak brush, low pocosin, Alaskan spruce taiga, and shrub tundra. Cured hardwood stash can be considered. Pinyon-juniper shrub lands may fit, but may over predict rate of spread except at high winds: e.g. 20 mi/h at the 20 foot level,
Fuel Model 7 (2.5 feet deep) Fire burns through the surface and shrub strata equally. Fire can occur at higher dead fuel moisture contents due to the flammable nature of live foliage. Shrubs are generally 2 to 6 feet high. Examples are Palmetto-gail berry under story pine with over story sites, low pocosins, and Alaskan Black Spruce shrub considerations.
Timber Little Group
Fuel Model 8 (0.2 feet deep) slow burning ground fires with low flame heights are generally the case, although an occasional “jackpot” or heavy fuel concentration may cause a flare up. Only under severe weather conditions do these fuels pose fire problems. Closed-canopy stands of short needle conifers or hardwoods that have leafed out support fire in the compact litter layer. This layer is mainly needles, leaves, and some twigs since little undergrowth is present in the stand. Representative conifer types are white pine, lodgepole pine, spruces, true firs, and larches.
Fuel Model 9 (0.2 feet deep) Fire runs through the surface little faster than Model 8 and have higher flame height. Both long-needle conifer and hardwood stands, especially the oak-hickory types, are typical. Fall fires in hardwoods are representative, but high winds will actually cause higher rates of spread than predicted because of spotting caused by rolling and blowing leaves. Closed stands of long-needle like ponderosa, Jeffrey, and red pines or southern pine plantations are grouped in this model. Concentrations of dead-down woody material will contribute to possible torching out of trees, spotting, and crowning activity.
Fuel Model 10 (1 foot deep) the fire burns in the surface and ground fuels with greater fire intensity that other timber litter models. Dead-down fuels include greater quantities of 3-inch or larger limb wood resulting from over-maturity or natural events that create a large load of dead material on the forest floor. Crowning out, spotting and torching of individual trees are more frequent in this fuel situation leading to potential fire control difficulties. Any forest type may be considered when heavy down materials are present, examples are insect or diseased stands, wind-thrown stands, over mature situations with deadfall, and cured light thinning or partial-cut slash.
Logging Slash Group
Fuel Model 11* (1 foot deep) Fires are fairly active in the slash and herbaceous material intermixed with the slash. The spacing of the rather light fuel load, shading from overstory, or the aging of the fine fuels can contribute to limiting the fire potential. Light partial cuts or thinning operations in mixed conifer stands, hardwood stands, and southern pine harvests are considered. Clear-cut operations generally produce more slash than represented here. The <3 inch material load is less than 12 tons per acre. The >3 inch material is represented by not more than 10 pieces, 4 inches in diameter along a 50 foot transect.
Fuel Model 12 – round fuel** (2.3 feet deep) rapidly spreading fires with high intensities
*When working in Fuel Model 11 or 12 with significant “red” needles attached to limbs, consider using the heavier model. For example Fuel Model 11 with “red” needles, use Fuel Model 12.
**Anderson, Hal E. Aids to Determining Fuel Models for Estimating Fire Behavior. Gen. Tech Report INT-122. 1982