Fire - Agroecology: The Ecology of Sustainable Food Systems - page 134

accompanying release of CO 2 , a loss of nitrogen and sulfur
in volatilized forms, and the deposition of minerals.
After fire there is usually a reduction in soil moisture-
holding capacity, although with the removal of vegetative
cover, actual moisture availability in the soil can increase
because of reduced demand. Soil aggregate size is
reduced, bulk density goes up, and permeability and water
infiltration rates are reduced. Often there is also an
increase in rainfall runoff and nutrient leaching, and the
possibility of greater soil erosion until the soil is covered
once again with vegetation. It is not uncommon just after
a fire for the immediate surface of the soil to actually be
water repellent, but this condition is usually overcome
after some exposure to moisture.
Generally speaking, most of the abiotic effects listed
above are of a rather short-term nature. Regeneration of
the vegetation, coupled with replacement of soil organic
matter, leaching rainfall, and plant modification of the
burned conditions, rapidly begins the process of recovery.
In the case of severe fire intensity following excessive fire
suppression and abnormal fuel buildup, or in the case of
a fire burning thick organic layers of peat or muck that
reaccumulate at a very slow rate, abiotic conditions can

be altered for longer periods of time. Unnaturally frequent
fires, usually human induced, can also lead to more lasting
change.

B IOTIC F ACTORS

Obviously, any living plants or animals caught in the path
of a fire are in peril. Plants that are not adapted to fire
are easily killed, especially if the bark type does not
protect the living cambium. If the fire is hot enough and
other conditions are right, living plant matter can be
killed, dried out, and ignited very rapidly, reducing all
above-ground material to ash. Then, if the plants do not
sprout from below-ground structures, recovery will only
begin with the germination of seeds. Seeds of some spe-
cies of plants are killed by fire, whereas others are either
stimulated by the breaking of specific dormancy factors
or by the creation of soil conditions that favor germination
and establishment (Figure 10.3).
Repeated fire can retard the vegetation recovery pro-
cess to the point that another vegetation type, more toler-
ant of fire, can establish dominance. The conversion of
shrubland to grassland is a good example of this process.

FIGURE 10.3 Fire response by pines. Young lodgepole pines reestablish following devastating crown fires that killed the parent
trees in Yellowstone, Wyoming.