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

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5. Full Bed Characteristics. In the previous example, the combustion process was started by exposing a few twigs
and needles to the heat from a burning candle. This may result in a few burned twigs and needles-or it may result in a
totally destroyed house. The outcome depends not so much on the properties of the individual fuel particles as it does on
how those particles are arranged together to form an overall accumulation of fuel, or fuel bed. For once fire has burned a
short distance from the external heat source (in this case the Christmas candle), the forest fuels themselves become both
heat source and heat receiver. The important properties of the fuel bed are those that affect heat transmission. These are
a. Amount. The total weight of fuel available for burning determines the total potential heat output of the fire. Since
not all of the vegetation is burnable (living stems and branches larger than an inch or so in diameter never burn completely
under amy circumstances, for example), fuel weight is not the same thing as biomass, a term frequently used
in forestry and ecology to express the total weight of organic material. Although the weight of available fire fuels
actually varies with weather conditions, the term is commonly used to mean the weight of living and dead material
one-half inch in diameter or less. Under average forest fire conditions in the United States, this is the fuel that contributes
to firhe spread. Figure 29 shows the weight of available fuel for several common tree species of the United
States. The values refer to foliage and branches of the trees themselves. Weights of ground litter and understory
shrubs or grass are not included.
b. Vertical Distribution. Given an adequate amount of available fuel, thevertical distribution of particles has the greatest
influence on fire spread through a fuel bed. Figure 30 illustrates various natural distributions. In a typical forest or
bru&Lfield, 60-80 precent of the potentially available fuel is aerial, above the surface litter. If there is an empty space
greater than about twice the flame length between the surface litter and these aerial fuels, they will not ignite. This
means that if the average height of the lowest branches in a forest is 10 feet above the ground, a crown fire will not
occur until burning conditions are such that the understory litter and shrubs will burn with flames at least 5 feet
high. Figure 31 shows the relationship between flame height, fuel weight, and the shrub burning index (Tables I, 2.
and 3, page 20).
Appendix B lists typical litter and shrub fuel weights by climatic and vegetation type. These values should be used
when no other data is available. If direct observations of the target area are possible, fuel weight can be calculated by
assuming that each inch of conifer litter depth represents 0.2 pounds per square foot, each inch of hardwood litter represents
0.07 pounds per'square foot, and dead shrub cover represents an additional 0.125 pounds per square foot for each
foot of shrub height when the ground is completely shrub covered. Thus a forest with I inch of dry conifer litter and a 30
percent cover of dead shrubs 4 feet high will have about 0.35 pounds of ground fuels per square foot
[(I X 0.2) + (0.125 X 4 X 0.3)]
Flames will be 6 feet high when the burning index ik '6
I
Knowledge of understory shrub cover is vital to the prediction of fire behavior in forest. When ground access to the
target area is denied, shrub estimates must be made from aerial observations or interpretation of aerial photographs.
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