TheImprovement ofTropical and Subtropical Rangelands

REHABlLlTATION TEOHNIQUES 147
Topography
Aa the steepness of the slopes increases, the rate of fire spread
increasel!l. The upslope rate is favored because of convection (the
rising of hot air currents) and the more intense radiation that occurs
on the upslope side of the fire. Winds tend to move upslope during
the day and downslope during the night. Vegetation in depressions
is more readily burned because the depressions are subject to wind
tunnel effects.
Fuel
Sufficient fuel must be prelflnt to carry a ground fire, and under
ideal conditions this should be at least 700 kg per ha. The surfac~
~volume ratio affects the rate of combustion. A high surface area
of small-sized fuel releases heat quickly; that is, the area has a high
heat intensity. Larger fuels burn with less intensity but for a longer
time. Intensity and duration are the controlling influences on plants
and animals. Fire duration is related directly to total heat yield
and fuel quantity, and indirectly to intensity. Heat penetration into
soil, bark, and other plant tissue increalflS as fire duration increases.
Penetration is by conduction-the slowest rate of heat transfer. In
addition to size and amount offuel, environmental temperatures and
moisture levels need to be prescribed within rather narrow limits
(Jordan, 1981).
Weather
Weather patterns before, during, and after a burn must be considered
in developing the prescribed control bum management plan.
Of the three elements necessary for combustion-oxygen, fuel, and
heat-fuel and heat can be manipulated by prescribing the weather
conditions to be met during the bum. Before combustion can occur,
the fuel must be raised to the appropriate temperature. All moisture
must be removed before this temperature is reached, or else temperatures
remain near the boiling point of water. Thus, fuel moisture,
relative humidity, and temperatures are critical elements of weather
that must be within narrow limits preceding and during a prescribed
bum. While the cooling effect of water or moisture on a fire is elementary
knowledge, its overall effect on a large fire is much more
difficult to predict.