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

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Aerial observers have a natural tendency to overestimate shrub cover because they are misled by looking at conditions
along the edges of roads and clearings where shrubs are always unusally dense. Observers should be instructed to try
to look through small gaps in the tree crowns to check the portion of the ground occupied by low-growing vegetation.
Pilots should fly into the shadows (generally north to south), and keep ground speeds as low as practical. An altitude of
about 1500 feet is optimum unless aircraft speeds are so high that altitude must be increased to allow the observer enough
time to scan each spot. The maximum altitude from which accurate understory determinations can be made without binoculars
is about 3500 feet.
For aerial photographic interpretation of understory shrub density, Ektachrome film Type 2448 at a scale of 1:8000
and flown at right angles to the shadows (generally east to west) is optimum, but photos at any scale between 1:2500 and
1:18,000 are usable. When using aerial photos, the interpreter should pick out the shrub color and texture characteristics
by looking at clearing edges, and then estimate understory shrub density by slowly moving his view into the tree stand. 15J
WEATHER CHARACTERISTICS
Since the moisture content of the fuel is the overriding determinant of forest fire behavior, it is not surprising that
weather variables are primarily important for the manner in which they affect fuel moisture. Temperature, humidity, wind
and precipitation all affect the moisture content of woody materials-acting directly on dead wood, and indirectly off living
plants by influencing the physiological moisture regulating processes. 16 The moisture content of living plannrvts.
changes relatively slowly. The effect of weather on living fuel moisture has already been discussed in the section on climate.
But dead twigs, leaves and other litter depends more on the weather of the moment than it does on the seasonal climate.
The following weather variables have a direct effect on the drying of dead forest fuels.
Precipitation:
Obviously, rain or snowfall will greatly increase the moisture content of forest litter. Since water
holding capacity differs among various forest fuels, the amount of rainfall required to saturate forest litter varies with both
3 the amount and type of litter. Table 7 shows the rainfall requirements for each of the standard vegetation types.
Not all rainfall reaches the forest floor. A substantial amount is caught on the twigs and foliage of the upper canopy,
and evaporates without ever reaching the ground. The amount thus lost depends on the amount of surface area in the
foliage and on the amount of rain in the storm. Table 7 also shows the interception losses that can be expected in each of
the vegetation types, and the amount of rain needed to saturate both light and heavy litter accumulations. As shown in
the last column, it takes nearly twice as much rain to saturate a dense accumulation of shrub or hardwood litter as it does
for an equal weight of grass or conifers.
15/ More complete guidelines and photo interpreter's keys can be found in Lund, H. G. and G. R. Fahnestock. Color
aerial photography for interpreting understory vegetation. USDA-Forest Service. Final Progress Report PNW-2103:
FA-2.2. /9 pp., illus. July 1968
1N The detailed mechanics of wood-moisture relations are beyond the scope of this paper. Three excellent references are
available:
For dead wood, see Simard, A. J., The moisture content of forest fuels - I: A review of basic concepts. Forest Fire
Research Institute, Ottawa, Ontario. Information Report FF-X-14. 47 pp. July 1968.
For living vegetation, see Kozlowski, T. T., Water metabolism in plants. Harper and Row, N.Y. 227 pp. 1964
For living plants under drought stress, see Zgurovskaya, L N. and Yu. L. Tsel'niker., The influence of irrigation after
a prolonged drought on transpiration and on the condition of the absorbing roots of arboreal species in the Derkul'
Steppe. (Translated from Russian). Office of Tech. Svcs., U.S. Department of Commere., PSTCat. No. 311. 10 pp.
1961.
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