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

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The most critical aspect of climate is its influence on development and accumulation of plant litter on the soil surface.
This fuel layer-required for ignition and buildup of fire-is present under many dry orcold climates. It is virtually
absent, except for current year leaf drop, in hot, humid climates where there is a continuous breakdown of woody material
by chemical and biological agents.
4 Within any given fuel type, the profound local effects of climatic factors must be recognized and evaluated. The
climate, modified by other factors such as geology, topography, soil type and drainage, determines density and total biomass
of the existing vegetative cover that serves as fuel for burning. The yearly weather pattern also determines which
species are present and how they grow and mature seasonally-whether or not they ever become dry fuels that can be
burned without desiccation treatment.
The moisture content of living plants changes relatively slowly in response to weather. Just as each climate has a
characteristic annual cycle of temperature. humidity and precipitation, so each type of vegetation has a fixed season:;
pattern of moisture :ontent.
Grasses, being shallow rooted, respond rapidly to climatic change. Annual grasses mature rapidly and dry soon after
the onset of moisture stress in the upper soil layers. Perennial grasses stay green longer, but are usually dead and dry by
the end of summer. In climates with cold winter's dry grass is burnable from late summer or early fall until new growth
appears in the spring. In mild winter climates, new growth begins early, and the grassburning season is restricted to summer
and early fall.
Perennial shrubs and trees follow a more complex seasonal pattern of growth, maturity and drying. The flush of
new growth, and high moisture content, always occurs at a time when adequate soil moisture coirncides with daily mean
temperatures above 60 degrees Fahrenheit. The new leaves of shrubs and hardwoods have moisture contents above 200
percent at this time, and even the persistent leaves of evergreen species contain 120-I50 percent moisture. Within a few
weeks, moisture contents begin to drop-very rapidly for the succulent new foliage, and more s:owly for evergreen
leaves-and reach levels of 100-120 percent after about a month. Foliage moisture will stabilize at about this level in
warm, humid climates. In climates with dry summers or cold winters, moisture contents will continue to decline. The
minimum moisture will depend on the length and severity of cold or drought, but 50-70 percent is the absolute minimum
that plants can reach and still recover. Since dead fuel moisture is invariably lower than that of living plants even
under the most extreme drought conditions, external measures that kill forest vegetation will always result in increased
flammability.
Systemic herbicides aimed at killing and drying vegetation before burning must be applied during seasons favorable
for entry and movement of the herbicides within the plants. Consequently, yearly weather patterns have great importance
in planning desiccation treatments. The seasonal influence is of less importance in applying contact desiccants aimed at
drying only the foliage. Desiccation treatments such as slashing or crushing of vegetation can be done at any time of
year, but must be timed to prevent regrowth of succulent vegetation before burning.
Thus, climate to a large extent determines fuel characteristics of the natural vegetation, and it dictates the planning
and execution of the various phases of the burning operation-from prefire desiccation treatment to the final ignition and
burning. The first step in planning an incendiary operation is to study climatic records and decide whether odds are
favorable for successful burning of dry vegetation during a certain month, or months. The next step is to plan the
date for fuel desiccation treatment to provide the necessary quantity of dead, dry vegetation.
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