Surface and Ground Fuels - Wildland Fuel Fundamentals and Applications - page 44

3.2.3 Shrubs

The shrub fuel type consists of nontree, woody, and foliage vegetation biomass
(Fig. 3.1g ). A shrub fuel type can dominate a fuel bed where the loading of shrub
biomass overwhelms the loading of all other components to become a shrub fuel
complex (Fig. 3.2c ), or shrubs can be a minor component in the surface fuel layer
(Fig. 3.1g ). Shrub fuel types are often stratified by live and dead components, but
most fire applications consider shrubs mostly live fuels, even though there are many
shrub species of the world that retain their dead leaves after heavy frosts (e.g., tur-
key oak). Like litter, shrubby fuels are also diverse in terms of size, type, and dis-
tribution of fuel particles, but most fire applications put all shrub material (foliage,
twigs, branches) into only one component. The woody material in shrub stems is
rarely stratified into components based on diameter size classes, and foliage parti-
cles are rarely separated into finer species, size, or shape components. Some studies
have suggested dividing shrub fuels into two components based on size (above and
below 5 cm (2 in) in diameter, Brown et al. 1982 ), but few fire models accept these
components. Frandsen ( 1983 ) divided live and dead sagebrush fuels into size class-
es that correspond to fire behavior inputs: 0-0.6, 0.6-2.0, and 2.0-7.0 cm diameter.
Some of the most important fuel beds in the world are shrub fuel complexes
(Fig. 3.2c ). Shrub-dominated communities are often stratified into several structural
types: scrub (dense foliar cover > 30 %, 2-8 m tall), tall shrub (sparse foliar cover
< 30 %, 2-8 m tall), heath (dense foliage with cover > 30 %, 0-2 m tall), and low
shrub (sparse foliage with cover < 30 %, 0-2 m tall). Sclerophyllous shrublands,
such as the chaparral ecosystems of western North America, the bushlands of Aus-
tralia, the garrigue and maquis communities of the Mediterranean, and the fynbos
of southern Africa, are some of the most interesting shrub ecosystems because they
have several characteristics in common. First, they all have a Mediterranean cli-
mate that has a pronounced summer dry season and winter rains. The vegetation is
dominated by shrubs with hard leaves that have short internodes (sclerophyll) and
these ecosystems also are diverse in species composition. Soils are often poor and
productivity is commonly low in most of these communities. But most importantly,
these sclerophyll shrublands have a history of frequent burning, either by aborigi-
nal populations or by lightning, and most of the shrub species has evolved diverse
adaptations for surviving fire.

3.2.4 Herbaceous

Herbaceous fuel types are probably the most interesting of all the fuels because they
consist of biomass from nonwoody plants such as grasses, forbs, and ferns, and not
trees, shrubs, mosses, and lichens. This diverse group includes many species of
grasses, sedges, rushes, forbs, and ferns; an eclectic group of life-forms and species
that span a wide variety of ecological characteristics and combustion properties
(Fig. 3.1h ). Grasses, sedges, and rushes, for example, have leaves that are long and