Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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Consequently, fire behavior in grass types is particularly affected by the weather of the moment. A pasture that cannot be
made to burn at dawn may nearly explode from a single spark at 3 p.m., and this fire once started may go out by itself
shortly after sundown.
Some exceptions to normal fire behavior have been noted in some grass fuels. For example, many species of bamboo
cor.tain an abnormally high proportion of inorganic salts, particularly silicates, and burn much more slowly than would be
predicted from considerations of weight and form alone. Several species of elephant grass (Neyraudia) and sword grass
(Imperata) retain their green color when curing, and burn well even though they would be considered nonflammable on
the basis of color. Abou* the only way to predict fire behavior in these special fuels is to set test fires in representative
stands and observe the results.
Shrub
The shrub type includes all areas where more than 50 percent of the ground surface, as viewed from the air, is
covered with perennial woody vegetation whose lower leafy branches are within 2 feet of the ground (within the flame
zone of the average ground fire). Locally, shrub types are known by such names as brushlands, scrub, chaparral, and bush,
although scrub and bush are also used occasionally to refer to non-commercial forests and woodlands, and to savanna
types with short trees. For forest fire rating purposes a stand of young trees whose lowest branches are still close to the
ground would be classed as shrub rather than forest-the governing criteria is the availability of foliage within the ground
fire flame zone.
Shrub types vary widely in height, form, and fuel weight. Semi-desert species (sagebrush, for example) are usually
fine stemmed, and either low growing or thinly foliaged in order to reduce transpirational water losses. Fuel weights are
usually less than 0.7 pounds pr square foot. At the other extreme, brush thickets in cooler temperate climates may stand
over 30 feet high and have more than 2-1/2 pounds of burnable material per square foot.
Evergreen shrubs-those that have green leaves yearlong-usually have a hard waxy leaf surface which serves to reduce
moisture loss during dry periods. These leaf coatings are especially heavy in arid climates or those with long dry seasons.
Since the coatings contain gums and resins they burn readily. The effects of such coatings on fuel flammability are least
under climates with adequate yearlong precipitation.
Leaves of deciduous shrubs are newly formed each year; they lack a protective coating, and tend to drop from the
branches during periods of moisture stress. Their normal flammability is low compared to evergreen leaves.
Despite this diversity, shrub fuels all have the following common characteristics:
I. Foliage extending into the surface fire flame zone so that the shrub will be automatically ignitied if foliage
moisture is sufficiently low.
2. A large proportion of leaves and small twigs in comparison with stemwood and larger branches. Since burning
rate varies inversely with fuel size, shrub fuels bum rapidly.
In shrub fuels-as with all woody vegetation-the moisture content of dead material responds to changes in atmos.
pheric moisture, while that of living material is governed more by soil moisture. Depending on the climpte, moisture
contents of living plant parts also are governed by changes in the life processes within the plant. Consequently, moisture
contents of green leaves, twigs, and small stems vary slowly over time, and during the year they fit orderly patterns
S.within each cimate .........
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