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

SECRET
present, individual trees, of even groups of trees may burn. However, fires in the tree crowns damp out very rapidly because
living hardwood foliage requires.more heat for ignition than it liberates in combustion.
The achievement of a crown fire or a firestorm in any hardwood forest requires external measures which will lower
the moisture content of the crown foliage and branches and/or increase the amount of dead surface fuels that can transfer
heat into the tree crowns.
Ground fires and running surface fires do 6ccur naturally in hardwood forests, and we can distinguish between two
hardwood subtypes.
Deciduous Hardwool Forest
Hardwoods that periodically lose all their leaves are much more subjec: to forest fires during the season when the
leaves are off the trees. The dead leaves themselves add considerable fuel to the normal ground litter (a single large oak
tree will drop about 900 pounds of leaves in the fall). The bare branches expose the fallen leaves to the full drying effects
of sun and wind. In temperate climates, such as the Northern US and USSR, where leaf fall occurs in the autumn, surface
fires burn the deciduous hardwood forests before the winter precipitation begins, and again in early spring. Later, new
plant growth adds too much moisture to the ground fuels, and new tree foliage provides too much shade.
where leaf fall is brought on by drought rather than cold, surface fires burn during the entire dry period.
In climates
Surface fire behavior in deciduous hardwood types not in leaf is fairly well represented by the shrub burning indexes
(Tables 1, 2, and 3).
Evergreen Hardwood Forests
Evergreen hardwoods fcrests retain green leaves yearlong. Virtually all the tropical forests loosely classed as "jungle"
belong to this subclass. Such forests are generally found on moist sites to begin with, and the perpetual canopy of foliage
serves to retain ground moisture through occasional short dry spells. Consequently, fires of any kind are rare in evergreen
hardwoods (Eucalyptus excepted).
Extensive fires occur only after unusual and prolonged periods of drought. Under
normal circumstances, artificial defoliation is required to expose the surface litter to sun and wind before even the lightest
ground fire will spread. To achieve a crown fire requires both partial defoliation and desiccation of the smaller twigs
and branches in the tree and shrub canopy. After defoliation, evergreen hardwoods bum almost as well as deciduous hardwoods
not in leaf. Without defoliation, evergreen hardwoods do not burn at all.
CLIMATE
Climate, or the normal annual weather pattern, has a profound influence on planning forest incendiary operations.
The kind of vegetation at any location, and the ease with which it can be ignited and burned at any time of year is largely
the result of interacting climatic and vegetative factors. Specifically, climate governs:
I. The rate of accumulation and amount of litter and pound fuels needed for forest fire initiation.
2. The moisture content of both dead and living vegetation.
3. The physiological processes that determine when, or whether, herbicide applications can be used to desiccate
living fuels.
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