
Environmental Encyclopedia 3
Wildfire
sion
and in sustaining certain types of plant communities.
Fires can help maintain seral successional stages, prolonging
the time for the community to reach climax stage. Some
ecosystems depend on recurring fire for their sustainability;
these include many prairies, the
chaparral
of mediterranean
climatic regions, pine savannas of the Southeastern United
States, and long-needle pine forests of the American West.
Fire controls competing vegetation, such as brush in
grass-
lands
, prepares new seed beds, and kills harmful insects and
disease organisms. Nonetheless, diseases sometimes increase
after fire because of increased susceptibility of partially
burned, weakened trees.
The effects of fire on an
ecosystem
are highly variable,
and they depend on the nature of the ecosystem, the fire
and its fuel, and weather conditions. In climatic regions
where natural fires occur seasonally, grasslands usually are
the first ecosystems to burn because the fuel they supply has
a large surface area to volume ratio, which allows it to dry
rapidly and ignite easily. Grassland fires tend to burn quickly,
but they release little energy compared to fires with heavier
fuel types. As a result, the effect on soil properties normally
is minor and short-lived. The intense greening of a grassland
as it recovers from a burn is due largely to the flush of
nutrients released from mature and dead plants and made
available to new growth. Grassland fires have been set inten-
tionally for many generations in the name of forage im-
provement.
Naturally caused brushland fires, including the infa-
mous chaparral fires of the southwestern United States, usu-
ally start somewhat later in the season than the first grass
fires, and they normally have more intense, longer lasting
impacts. These fires burn very rapidly but with far more
thermal output than grassland fires, because fuel
loading
is
five to fifty times greater.
The season for forest fires normally begins somewhat
later than that for grass or brush fires. The fuel in a mature
forest, which may be 100 times greater than that of a medium
density brushland, requires more time to dry and become
available for
combustion
. When the forest burns, the ground
may or may not be intensely heated, depending on the ar-
rangement of fuels from the ground to the forest canopy.
Under a hot burn with the heavy ground fuel found in some
forests, heat can penetrate mineral soil to a depth of 12 in
(30 cm) or more, significantly altering the physical, chemical,
and biological properties of the soil. When the soil is heated,
water is driven out; soil structure, which is the small aggrega-
tions of sand,
silt
, clay, and organic matter, may be destroyed,
leaving a massive soil condition to a depth of several inches.
Forest and brushland soils often become hydrophobic,
or water repellent. A hydrophobic layer a few inches thick
commonly develops just below the burned surface. This con-
dition is created when the fire’s heat turns organic matter
1516
into gas and drives it deeper into the soil, where it then
condenses on cooler particle surfaces. Under severe condi-
tions, water simply beads and runs off this layer, like water
applied to a freshly waxed car. Soil above the hydrophobic
layer is highly susceptible to sheet and rill
erosion
during
the first rains after a fire. Fortunately, it is soon broken up
by insects and burrowing animals, which have survived the
fire by going underground; they penetrate the layer, allowing
water to soak through it.
Forest fires decrease soil acidity, often causing
pH
to
increase by three units (e.g., from 5 to 8) before and after
the fire. Normally, conditions return to prefire levels in
less than a decade. Fires also transform soil nutrients, most
notably converting nutritive
nitrogen
into gaseous forms
that go up in the smoke. Some of the first plants to recolonize
a hotly burned area are those whose roots support specialized
bacteria that replenish the nutritive nitrogen through a pro-
cess called
nitrogen fixation
. Large amounts of other nutri-
ents, including
phosphorus
, potassium, and calcium, re-
main on the site, contributing to the so-called “ashbed
effect.” Plants that colonize these fertile ashbeds tend to be
more vigorous than those growing outside of them. When
heating has been prolonged and intense in areas such as
those under burning logs, stumps, or debris piles, soil color
can change from brown to reddish. Fires hot enough to
cause these color changes are hot enough to sterilize the
soil, prolonging the time to recovery.
In the absence of heavy ground fuels, so much of the
energy of an intense forest fire may be released directly to
the
atmosphere
that soils will be only moderately affected.
This was the case in the great fires at
Yellowstone National
Park
in 1988, after which soil scientists mapped the entire
burn area as low or medium intensity with respect to soil
effects. Although soils on some sites did suffer intense heat-
ing, these were too small and localized to be mapped or to
be of substantial ecological consequence, and most of the
areas recovered quickly after the fires.
Although fire is vital to the long-term health and
sustainability of many ecosystems, wildfires take numerous
human lives and destroy millions of dollars of property each
year. Controlling these destructive fires means fighting them
aggressively. Fire suppression efforts are based on the fact
that any fire requires three things: heat, fuel, and oxygen.
Together, these make up the three legs of the so–called “fire
triangle” known to all fire fighters. The strategy in all fire
fighting is to extinguish the blaze by breaking one of the
legs of this triangle. An entire science has developed around
fire behavior and the effects of changing weather,
topogra-
phy
, and fuels on that behavior.
Competing conceptions of the costs and benefits of
wildfires have led to conflicting fire management and sup-
pression objectives, most notably in the national parks and