This appendix is an excerpt from the National Fire Danger Rating (NFDR) System, 1978, United States Department of Agriculture Forest Service, general technical report INT-39.
    This appendix is for information purposes and is not intended for adoption.
    The fuel models that follow are only general descriptions because they represent all wildfire fuels from Florida to Alaska and from the East Coast to California.


    1. I.Mosses, lichens and low shrubs predominate ground fuels.

      1. A.An overstory of conifers occupies more than one-third of the site: MODEL Q

      2. B.There is no overstory, or it occupies less than one-third of the site (tundra): MODEL S

    2. II.Marsh grasses and/or reeds predominate: MODEL N

    3. III.Grasses and/or forbs predominate.

      1. A.There is an open overstory of conifer and/or hardwood trees: MODEL C

      2. B.There is no overstory.

        1. 1.Woody shrubs occupy more than one-third, but less than two-thirds of the site: MODEL T

        2. 2.Woody shrubs occupy less than one-third of the site.

          1. a.The grasses and forbs are primarily annuals: MODEL A

          2. b.The grasses and forbs are primarily perennials: MODEL L

    4. IV.Brush, shrubs, tree reproduction or dwarf tree species predominate.

      1. A.Average height of woody plants is 6 feet or greater.

        1. 1.Woody plants occupy two-thirds or more of the site.

          1. a.One-fourth or more of the woody foliage is dead.

            1. (1)Mixed California chaparral: MODEL B

            2. (2)Other types of brush: MODEL F

          2. b.Up to one-fourth of the woody foliage is dead: MODEL Q

          3. c.Little dead foliage: MODEL O

        2. 2.Woody plants occupy less than two-thirds of the site: MODEL F

      2. B.Average height of woody plants is less than 6 feet.

        1. 1.Woody plants occupy two-thirds or more of the site.

          1. a.Western United States: MODEL F

          2. b.Eastern United States: MODEL O

        2. 2.Woody plants occupy less than two-thirds but more than one-third of the site.

          1. a.Western United States: MODEL T

          2. b.Eastern United States: MODEL D

        3. 3.Woody plants occupy less than one-third of the site.

          1. a.The grasses and forbs are primarily annuals: MODEL A

          2. b.The grasses and forbs are primarily perennials: MODEL L

    5. V.Trees predominate.

      1. A.Deciduous broadleaf species predominate.

        1. 1.The area has been thinned or partially cut, leaving slash as the major fuel component: MODEL K

        2. 2.The area has not been thinned or partially cut.

          1. a.The overstory is dormant; the leaves have fallen: MODEL E

          2. b.The overstory is in full leaf: MODEL R

      2. B.Conifer species predominate.

        1. 1.Lichens, mosses, and low shrubs dominate as understory fuels: MODEL Q

        2. 2.Grasses and forbs are the primary ground fuels: MODEL C

        3. 3.Woody shrubs and/or reproduction dominate as understory fuels.

          1. a.The understory burns readily.

            1. (1)Western United States: MODEL T

            2. (2)Eastern United States:

              1. (a)The understory is more than 6 feet tall: MODEL O

              2. (b)The understory is less than 6 feet tall: MODEL D

          2. b.The understory seldom burns: MODEL H

        4. 4.Duff and litter, branchwood, and tree boles are the primary ground fuels.

          1. a.The overstory is overmature and decadent; there is a heavy accumulation of dead tree debris: MODEL G

          2. b.The overstory is not decadent; there is only a nominal accumulation of debris.

            1. (1)The needles are 2 inches (51 mm) or more in length (most pines).

              1. (a)Eastern United States: MODEL P

              2. (b)Western United States: MODEL U

            2. (2)The needles are less than 2 inches (51 mm) long: MODEL H

    6. VI.Slash is the predominant fuel.

      1. A.The foliage is still attached; there has been little settling.

        1. 1.The loading is 25 tons/acre (56.1 tons/ha) or greater: MODEL I

        2. 2.The loading is less than 25 tons/acre (56.1 tons/ha) but more than 15 tons/acre (33.7 tons/ha): MODEL J

        3. 3.The loading is less than 15 tons/acre (33.7 tons/ha): MODEL K

      2. B.Settling is evident; the foliage is falling off; grasses, forbs, and shrubs are invading the area.

        1. 1.The loading is 25 tons/acre (56.1 tons/ha) or greater: MODEL J

        2. 2.The loading is less than 25 tons/acre (56.1 tons/ha): MODEL K


    This fuel model represents western grasslands vegetated by annual grasses and forbs. Brush or trees may be present but are very sparse, occupying less than a third of the area. Examples of types where Fuel Model A should be used are cheatgrass and medusahead. Open pinyon-juniper, sagebrush-grass, and desert shrub associations may appropriately be assigned this fuel model if the woody plants meet the density criteria. The quantity and continuity of the ground fuels vary greatly with rainfall from year to year.


    Mature, dense fields of brush 6 feet (1829 mm) or more in height are represented by this fuel model. One-fourth or more of the aerial fuel in such stands is dead. Foliage burns readily. Model B fuels are potentially very dangerous, fostering intense, fast-spreading fires. This model is for California mixed chaparral generally 30 years or older. The F model is more appropriate for pure chamise stands. The B model may also be used for the New Jersey pine barrens.


    Open pine stands typify Model C fuels. Perennial grasses and forbs are the primary ground fuel but there is enough needle litter and branchwood present to contribute significantly to the fuel loading. Some brush and shrubs may be present but they are of little consequence. Situations covered by Fuel Model C are open, longleaf, slash, ponderosa, Jeffrey, and sugar pine stands. Some pinyon-juniper stands may qualify.


    This fuel model is specifically for the palmetto-gallberry understory-pine overstory association of the southeast coastal plains. It can also be used for the so-called “low pocosins” where Fuel Model O might be too severe. This model should only be used in the Southeast, because of a high moisture of extinction.


    Use this model after leaf fall for hardwood and mixed hardwood-conifer types where the hardwoods dominate. The fuel is primarily hardwood leaf litter. The oat-hickory types are best represented by Fuel Model E, but E is an acceptable choice for northern hardwoods and mixed forests of the Southeast. In high winds, the fire danger may be underrated because rolling and blowing leaves are not accounted for. In the summer after the trees have leafed out, Fuel Model E should be replaced by Fuel Model R.


    Fuel Model F is the only one of the 1972 NFDR System Fuel Models whose application has changed. Model F now represents mature closed chamise stands and oakbrush fields of Arizona, Utah and Colorado. It also applies to young, closed stands and mature, open stands of California mixed chaparral. Open stands of pinyon-juniper are represented; however, fire activity will be overrated at low wind speeds and where there is sparse ground fuels.


    Fuel Model G is used for dense conifer stands where there is a heavy accumulation of litter and downed woody material. Such stands are typically overmature and may also be suffering insect, disease, wind or ice damage-natural events that create a very heavy buildup of dead material on the forest floor. The duff and litter are deep, and much of the woody material is more than 3 inches (76 mm) in diameter. The undergrowth is variable, but shrubs are usually restricted to openings. Types meant to be represented by Fuel Model G are hemlock-Sitka spruce, Coast Douglas-fir, and wind-thrown or bug-killed stands of lodgepole pine and spruce.


    The short-needled conifers (white pines, spruces, larches and firs) are represented by Fuel Model H. In contrast to Model G fuels, Fuel Model H describes a healthy stand with sparse undergrowth and a thin layer of ground fuels. Fires in H fuels are typically slow spreading and are dangerous only in scattered areas where the downed woody material is concentrated.


    Fuel Model I was designed for clearcut conifer slash where the total loading of materials less than 6 inches (152 mm) in diameter exceeds 25 tons/acre (56.1 metric tons/ha). After settling and the fines (needles and twigs) fall from the branches, Fuel Model I will overrate the fire potential. For lighter loadings of clearcut conifer slash, use Fuel Model J, and for light thinnings and partial cuts where the slash is scattered under a residual overstory, use Fuel Model K.


    This model is complementary to Fuel Model I. It is for clearcuts and heavily thinned conifer stands where the total loading of materials less than 6 inches (152 mm) in diameter is less than 25 tons/acre (56.1 metric tons/ha). Again, as the slash ages, the fire potential will be overrated.


    Slash fuels from light thinnings and partial cuts in conifer stands are represented by Fuel Model K. Typically, the slash is scattered about under an open overstory. This model applies to hardwood slash and to southern pine clearcuts where the loading of all fuels is less than 15 tons/acre (33.7 tons/ha).


    This fuel model is meant to represent western grasslands vegetated by perennial grasses. The principal species are coarser and the loadings heavier than those in Model A fuels. Otherwise, the situations are very similar; shrubs and trees occupy less than one-third of the area. The quantity of fuel in these areas is more stable from year to year. In sagebrush areas, Fuel Model T may be more appropriate.


    This fuel model was constructed specifically for the saw-grass prairies of south Florida. It may be useful in other marsh situations where the fuel is coarse and reedlike. This model assumes that one-third of the aerial portion of the plants is dead. Fast-spreading, intense fires can occur even over standing water.


    The O fuel model applies to dense, brushlike fuels of the Southeast. O fuels, except for a deep litter layer, are almost entirely living, in contrast to B fuels. The foliage burns readily, except during the active growing season. The plants are typically over 6 feet (1829 mm) tall and are often found under an open stand of pine. The high pocosins of the Virginia, North and South Carolina coasts are the ideal of Fuel Model O. If the plants do not meet the 6-foot (1829 mm) criterion in those areas, Fuel Model D should be used.


    Closed, thrifty stands of long-needled southern pines are characteristic of P fuels. A 2- to 4-inch (51 to 102 mm) layer of lightly compacted needle litter is the primary fuel. Some small-diameter branchwood is present, but the density of the canopy precludes more than a scattering of shrubs and grass. Fuel Model P has the high moisture of extinction characteristic of the Southeast. The corresponding model for other long-needled pines is U.


    Upland Alaskan black spruce is represented by Fuel Model Q. The stands are dense but have frequent openings filled with usually flammable shrub species. The forest floor is a deep layer of moss and lichens, but there is some needle litter and small-diameter branchwood. The branches are persistent on the trees, and ground fires easily reach into the tree crowns. This fuel model may be useful for jack pine stands in the Lake States. Ground fires are typically slow spreading, but a dangerous crowning potential exists.


    This fuel model represents the hardwood areas after the canopies leaf out in the spring. It is provided as the off-season substitute for E. It should be used during the summer in all hardwood and mixed conifer-hardwood stands where more than half of the overstory is deciduous.


    Alaskan or alpine tundra on relatively well-drained sites is the S fuel. Grass and low shrubs are often present, but the principal fuel is a deep layer of lichens and moss. Fires in these fuels are not fast spreading or intense, but are difficult to extinguish.


    The bothersome sagebrush-grass types of the Great Basin and the Intermountain West are characteristic of T fuels. The shrubs burn easily and are not dense enough to shade out grass and other herbaceous plants. The shrubs must occupy at least one-third of the site or the A or L fuel models should be used. Fuel Model T might be used for immature scrub oak and desert shrub associations in the West, and the scrub oak-wire grass type in the Southeast.


    Closed stands of western long-needled pines are covered by this model. The ground fuels are primarily litter and small branchwood. Grass and shrubs are precluded by the dense canopy but occur in the occasional natural opening. Fuel Model U should be used for ponderosa, Jeffrey, sugar pine, and red pine stands of the Lake States. Fuel Model P is the corresponding model for southern pine plantations.