Fire - Agroecology: The Ecology of Sustainable Food Systems - page 135

On the other hand, some vegetation types are in a sense
kept healthy by periodic fire, because the fire removes old
and dying individuals, returns stored nutrients to the soil,
and stimulates renovation by new or younger individuals.
Many larger animals can avoid fire by moving away
from it, but even when they are killed by fire, their popu-
lations in the burned area can recover through recoloniza-
tion from nearby unburned areas. Some animals actually
seek out recently burned areas because of the concentra-
tion of new growth and forage for feed, or because the
ash can aid in removal of parasites such as ticks and fleas.
Following a fire there is an immediate reduction in
the populations of nearly all soil-dwelling organisms,
including fungus, nitrifying bacteria, spiders, millipedes,
and earthworms. Many die as a result of the high tem-
peratures, but some organisms are impacted by the
changes in pH that follow the fire or by the flush of certain
nutrients into the soil that comes from burned organic
matter. After a fire, however, there is fairly rapid recolo-
nization, especially by bacteria that are stimulated by the
increase in pH.
On the whole, fire can have both negative and positive
impacts on the environment, but regardless, it must be
remembered that the intensity, duration, and frequency of
fires in natural ecosystems are incredibly variable. From
one year to the next, conditions that favor fire are going
to vary tremendously. And when a fire does occur, it
effects will not be uniform. Some areas will be burned
very thoroughly, whereas a short distance away the same
type of ecosystem may be spared the impacts of fire
completely.

to fire. Other fire-dependent plants will not flower until
after a fire, or will become senescent unless exposed to
periodic fires.

FIRE IN AGROECOSYSTEMS

Fire has a long history of use in agriculture. But from an
agroecological perspective, there can be good fires and
bad fires, overuse or underuse of fire, and careful or care-
less use of fire. The challenge is the appropriate applica-
tion of the knowledge of the ecological impacts of fire.

S HIFTING C ULTIVATION

The agroecosystem with the longest history of fire use
is shifting cultivation, or slash and burn agriculture.
Shifting cultivation with the use of fire continues today
to be the most important form of subsistence agriculture
in many parts of the world. Although thought to be
practised primarily in the tropics, fire-based shifting
cultivation was used in early agriculture even in Europe,
where wheat and barley were grown on a 10 to 25 years
fallow cycle (Russell, 1968). Although it might seem
quite simple to clear, burn, and plant, good shifting
cultivators have learned through experience that the timing
of all activities, especially the fire, make the difference
between a sustainable system and a degrading system.
Shifting cultivation works when the system is allowed
enough time for natural successional processes to
restore the soil fertility lost through disturbance and
crop harvest (Figure 10.4).
Immediately following a fire, nutrient mobility in the
system is quite high, often resulting in high leaching
losses. This accentuates the need for a fallow period in
order to recover the lost fertility. Crops in slash and burn
systems need to quickly pick up the nutrients added to the
soil from ash, or else leaching will remove them or invad-
ing noncrop plant species will begin to capture them.
Depending on soil types, climatic regimes, and cropping
practices, the rate of nutrient loss varies considerably. But
studies have shown that the loss can be rapid and high,
especially for nutrients such as calcium, potassium, and
magnesium (Ewel et al., 1981; Jordan, 1985; Nye and
Greenland, 1960). Repeated fires in short succession, as
well as soil cultivation, can accelerate nutrient loss even
more (Sanchez, 1976).
Shifting cultivation systems are generally thought to
be able to sustain relatively low human population levels.
In well-managed shifting cultivation systems, most of
the soil carbon and nitrogen remains following a fire, the
root mat stays intact and alive, the soil surface is pro-
tected by some form of biomass cover, and even soil
mycorrhizae survive. As a result, nutrient loss and soil
erosion are minimized, and the system is sustainable.

PLANT ADAPTATIONS TO FIRE

In any location where fire has a long evolutionary history,
most plants and at least a few of the animals have devel-
oped adaptations to fire. It is interesting that the adapta-
tions that provide resistance to fire in plants are in many
cases also traits that enable the plants to deal with excess
light or drought stress.
Plants can be adapted to fire in three different ways.
Fire resistance: Plants with fire resistance have traits
that help prevent the living parts from being burned in a
fire. These traits include such characteristics as thick bark,
fire-resistant foliage, or a litter mat that will support fre-
quent but less damaging fires.
Fire tolerance: Fire-tolerant plants have traits that
allow the plant to survive being burned in a fire. A com-
mon fire-tolerant trait is the ability to resprout from the
crown following a fire.
Fire dependence: Fire-dependent plants actually
require fire for reproduction or long-term survival. Some
fire-dependent plants have seeds that need fire before they
will germinate, or cones that will not open unless exposed