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

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The figures illustrate the fuel and weather situations under which a creeping ground fire will be generated, the point
at which the fire can develop into a running ground fire, and the point at which fire intensity is sufficiently great to develop
a crown fire. The lightest fuel loading will not burn at all under marginal conditions, and crown fires can only be obtained
under near optimum conditions of fuel and weather.
Only two classes of grounid level woody fuels have been shown, each wath a full range of ground litter. Hundreds of
intermediate claSses occur-with various arrangements of dead shrubby plants having different sizes and shapes. Fuel
moisture content at the time of burning can be expected to vary on either side of the 15 percent level assumed here.
Numerous other combinations of air temperature and relative humidity will be encountered, and their effects on fire
intensity will be modified by wind and terrain. Obviously, the thousands of different combinations of fuel moisture
content and weather can not be illustrated. Instead, the expected fire intensity for a specific situation must be determined
from data on fuel and clima.ic conditions and knowledge of how these conditions affect fire behavior. The remainder of
this report discusses forest fire behavior as determined by specific fuel, weather and ignition system factors, and shows how
these factors can be utilized in considering and planning forest incendiary operations.
VEGETATION TYPES
The most obvious requirement for forest burning is the presence of fuel to burn. The amount and arrangement of
potential forest fire fuels depends on the type of vegetation and the climate. In considering susceptibility to forest fires,
we can distinguish four primary vegetation types and four subtypes:
I. Grass
I. Shrub
Ill.
IV.
Conifer Forest
A. Pine
B. Other conifers
Hardwood Forest
A. Deciduous
B. Evergreen
Grass
The grass type includes all areas where more than 50 percent of the ground surface, as viewed from the air, is covered
with grasses, forbs, sedges, reeds, canes or mosses. This includes such vegetative cover as tundra, steppe, savanna, prairie,
meadow, marsh, bamboo, anid open woodland.
The grass type varies from cover a few inches high with a fuel weight less than 0.04 pounos per square foot in the
tundra to 20 foot tall stands of elephant grass that weight over I pound per square foot in parts of the Congo. Yet all grass
type fuels have certain characteristics in common that produce a profound effect on fire behavior.
1. Individwal plants with very thin leaves provide an extremely fast burning fuel that ignities easily when dry.
2. A moisture content that is well indicated by color. Green grass won't burn, yellow grass will.
3. A uniform distribution of fuel, both horizontally and vertically, produces fires that stabilize quickly and
spread evenly and predictable.
4. Fuel surfaces that are completely exposed to rain, sunshine and the full sweep of the wind.
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