Fire - Agroecology: The Ecology of Sustainable Food Systems - page 131

10 Fire

Fire is a major form of environmental change or distur-
bance. It removes dominant plant species, displaces ani-
mals, returns nutrients to the soil, and burns accumulated
litter on the forest floor. Nearly all the vegetation of the
earth has been influenced in some way by fire. Periodic
fires of varying frequencies and intensities are thought to
occur in most ecosystems, especially in regions with pro-
nounced dry seasons.
The most common fires are natural in origin, but
anthropogenic (human-induced) fires have a consider-
able history as well. There are reports in the literature
of charcoal deposits in tropical rain forest areas dating
back as far as 6000 B.P., many of which appear to be
associated with human activity. Before the development
of early agricultural tools, fire may have been the most
important “tool” early humans had for vegetation
management.
Some natural vegetation types that have evolved in
areas where fire is relatively frequent are actually
dependent on fire for their long-term stability; these
include certain prairie, savanna, shrub, and forest types.
Chaparral is probably the best-known fire-dependent
vegetation, often being described as a “fire climax”
community (Figure 10.1).
In early ecological research, fire was not studied
much, because it was seen only as a destructive force, and
because it was hard to observe its actual effects. More
recently, however, detailed studies of fire in ecosystems
such as California chaparral have helped make fire an
important topic of ecological investigation. Today, fire is
seen as an integral part of many ecosystems, as witnessed
by the rising use of controlled or prescribed burns in the
management of parks and nature reserves. Fire plays very
important roles in agroecosystems as well: it is an impor-
tant part of the practice of shifting cultivation, and is used
to manage crop residue, kill weeds, and clear slash
following logging.

It is still important today, with over 70% of the wildfires
in the western U.S. being caused by lightning strikes. In
very recent geologic time, humans have become another
important “source of ignition.” Humans have used fire
since the Paleolithic, as long as 500,000 years ago. Fire
was probably used first for the hunting or herding of
animals, and then evolved into a vegetation management
tool. Burning may have been used to provide better feed
for animals, or even to promote the presence of certain
plants that served as food or materials sources. Eventually,
fire became a tool to prepare the ground for planting, with
evidence thus far showing that early slash and burn agri-
culture began about 10,000 years ago.
From an ecological perspective there are primarily
three types of fires (Figure 10.2):
Surface Fire: This is the most common type of fire.
Fire temperatures are not too hot, with flames burning the
trash, grass, or litter that has accumulated on the surface
of the soil. Such a fire can move along under a forest
canopy and not burn the trees. Changes that occur in soil
conditions during a surface fire are usually short lived,
although the understory vegetation can be greatly altered.
Surface fires can be used to either control or promote the
growth of weedy or invasive vegetation, depending on the
circumstances.
Crown Fire: This type of fire can be very damaging
for some types of vegetation, whereas it may be an integral
part of rejuvenating other types. During crown fires, the
canopy of the vegetation is consumed, and usually the
mature plant species are killed. Crown fires are usually
very fast moving and often combine with a surface fire to
burn everything above the soil surface.
Ground or Subsoil Fire: This type of fire is not
very frequent, but when it does occur, it can be very
destructive. It is characteristic of soils that are high in
organic matter, especially peat or muck soils. Organic
matter in the soil can be burned down to the mineral
soil layer. These are usually slow fires, with more
smoke than flame, that dry the soil as they burn. Roots
and seeds in the soil are killed, and animal habitats are
severely altered.
Any individual fire can combine aspects of all three
fire types. In general, the intensity of a fire is very closely
related to the frequency of fires in the area.

FIRE IN NATURAL ECOSYSTEMS

A fire can occur in an ecosystem when three conditions
are met: an accumulation of sufficient fuel or organic
matter, dry weather, and a source of ignition. For millions
of years, lightning was the primary source of ignition.

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