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

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In all forests-both conifer and hardwood-fires start and buildup in the ground level fuels. They then develop into
one of the forest fire types from a creeping ground fire to a crown fire or a firestorm. For a given weather condition, fire
intensity will depend to a large extent on the amount and arrangement of litter and woody material on the ground-as in
shrub types. In contrast to both grass types and-most shrub types, ground fuel temperatures and moisture contents in
forests depend on the microclimates developed under the tree canopies. Kind and amount of ground fuel and its moisture
content differ greatly among the various forest types.
Crown fire behavior, and firestorm potential are more directly related to the trees themselves, as distinct from the
forest litter and undergrowth, and the four forest types are largely distinguishable on the basis of their ability to achieve
and sustain crown fires. On this basis, the conifer type can be subdivided into two subtypes: Pines, and Other Conifers.
CourP•r Forest - Pine
Pine types have two distinctive characteristics when compared to other conifers: they generally grow on drier sites
and their crowns are more open (less space is occupied by foliage and branches). Consequently, soil moistures are lower,
and the forest floor is more exposed to sunshine and wind. Both litter fuels and shrubs have lower moisture contents under
pine stands than under stands of other conifers given the same weather conditions. Both ground and running surface
fires burn more fiercely, and spread more rapidly in pine forests.
Pine forests will also sustain crown fires when conditions are still too damp for them to occur in forests of other
species. However, pine forests have less total fuel near the ground than do most conifers. Thus, crown fires in pine are
not as intense or destructive as those in other conifers.
Conifer Forest -
Othe- Than Pine
In addition to growing in dense stands on comparatively nrost sites, other conifers have branches and foliage on a
greater proportion of the tree trunk than do pines of the same height and age. As a result, crown fuel weights are heavier
and, equally important, small sized crown fuels are closer to the surface litter and underbrush which provides the heat to
initiate crown fires. When the litter and underbrush have dried sufficiently to burn well (which usually takes at least 6 to
8 weeks of drought) all conifers will crown easily and burn extremely hot. Because of their heavier fuel weights, firestorms
are more common in conifers other than pine, than in any other fuel type.
Hardwood Forest
The hardwood forest type includes all areas where more than half the ground surface, as viewed from the air, is tree
covered, and more than half the trees are broadleaf species such as oak and beech-with lowest foliage-bearing branches
more than 2 feet above the ground.
Hardwood types are extremely variable, and there are about as many exceptions as there are rules governing fire
behavior in hardwoods. In spite of this, the following generalization is important: With the sole exception of Eucalyptus,
11_ pure stands of hardwoods will not support sustained crown fires. If sufficient litter and dry undergorwth is
!_ Fire behavior in Eucalyptus has been the subject of several monographs, the best of which is McArthur, A. G. Fire
Behavior in Eucalypt Forests. Commonwealth of Australia, Department of National Development, Forestry and Timber
Bureau Leaflet No. 107, 36 pp., illus. 1967.
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