Four factors that control the severity of forest fires are:
■ Weather
■ An abundance of fuel (combustible forest materials)
■ Lack of moisture
■ Terrain characteristics
Of these four factors, only an abundance of fuel can be directly influenced by
human intervention; however, the treatments to reduce fuels can significantly
modify fire behavior and severity and reduce the environmental damage caused
by the fire.
Current wildfire frequency in the United States is about 4 times the average
of 1970–1986, resulting in about 7 times more acres burned. This change in
wildfire frequency appears to be linked to annual spring and summer
temperatures. Longer, warmer, and drier summers are documented with an
increase in the number of forest fires. A correlation exists between early arrivals
of the spring snowmelt in the mountainous regions and the incidence of large
forest fires. An earlier snowmelt can lead to an earlier and longer dry season,
which provides greater opportunities for larger fires. Another reason for the
increase in forest fires is a change in fire management philosophy—any
naturally started fire on federal land that is not threatening resources (homes or
commercial structures) is allowed to burn.
Types of Wildfires
CROWN FIRES
Crown fires occur in forests that have not had surface fires for a long time. They
are extremely hot. They burn entire trees and leap from treetop to treetop. They
kill wildlife, increase soil erosion, and destroy structures. The most effective
method for reducing the incidence of crown fires is through understory cutting,
where only small trees are cut, and overstory thinning, which leaves a semi-open
stand in a fire break.
GROUND FIRES
Ground fires are fires that occur underground, burn partially decayed leaves, and
are common in peat bogs. They are difficult to detect and extinguish.
SURFACE FIRES
Surface fires burn undergrowth and leaf litter. They kill seedlings and small
trees. They often spare older trees and allow many wild animals to escape. The