Ecology
College:
A license to burn?
Thoughts on prescribed prairie fire
by Dr. Thomas
Rosburg
Note:This
article was first published in the Winter 2005 edition of INHF's
quarterly magazine.
"The foremost lesson is that fire can never be 100% positive for all species."
So you just discovered you have a prairie remnant on your property
and-because you've heard that prairies should be burned-you figure
you better toss a match in it. STOP.
Yes, fire is one of the most important ecological factors affecting
many native communities and an important management tool used
for their conservation. But the use of prescribed fire (fire applied
with specific intentions, i.e., a prescription) implies knowledge
of fire ecology. Before you light that match, you must be able
to answer some questions: What is the purpose of this burn? How
can its benefits be maximized and its negatives minimized?
Direct
Effects
Direct effects of fire are those that occur due to the heat and
flames. One of the most important of these is the mortality of
organisms. The relatively treeless landscapes of prairies and
savannas are created and maintained in part by the mortality fire
imposes on woody plants.
Resprouts from the trunk's base or roots often occur in some species,
but these are much reduced in size compared to the tree and do
not compete with prairie plants as effectively as did the tree.
Species unable to resprout, like eastern red cedar, are very effectively
controlled by fire as long as they are small enough for the fire
to consume.
Many other organisms are especially susceptible to direct mortality
from heat and flames. Insects, amphibians and reptiles, bird eggs
in ground nests and small mammals will likely succumb if overtaken
by flames. Likewise, herbaceous plants that are actively growing
will be topkilled and stressed.
If these are non-native species, then the fire-induced mortality
and stress has a positive effect on the prairie's quality. However,
if they are native prairie species, the effect could be negative
(i.e., loss of fire-negative species).
Often prairie or savanna remnants are degraded by exotic or non-prairie
species (like smooth brome, Kentucky bluegrass, honeysuckle, or
elm). Thus, a properly timed fire that inflicts stress and mortality
on them will most likely do more good than harm. However, because
our prairies now occur as fragmented and isolated remnants, concern
for fire-negative species is essential.
Research indicates that nearly 50% of prairie insect fauna are
fire-negative species. Although fire initially reduces fire-negative
populations and insect diversity, recovery typically occurs through
recolonization from unburned habitat and from rare survival within
the burn. Among 63 fire-negative species observed in one study,
the mean recovery time for populations to return to preburn abundance
was 14.5 months. The survival strategy of prairie-adapted fire-negative
insects is rapid population growth and mobility. Just a few individuals
either surviving the fire or colonizing the area later can provide
the nucleus for a rapidly expanding population.
Another group of potential fire-negative species are small mammals.
Species most susceptible to direct fire mortality are those using
surface nests of plant debris (e.g. voles). In a study that followed
seven radio-telemetered meadow voles during a backfire, three
escaped to underground burrows, two emigrated, one escaped by
lying on the bare soil of a gopher mound, and one was killed (perhaps
due to an observer preventing access to a burrow). Backfires (burning
into the wind) provide more opportunity for escape than do headfires
and can increase survival of small mammals and reptiles.
Indirect
Effects
Indirect effects of fire are those that occur after the fire as
a result of the direct effects.
One of the most important indirect effects is the stimulation
of prairie plant growth and reproduction due to changes in the
microclimate following removal of the litter. Increased light
availability during the first few weeks of growth provides additional
energy for photosynthesis and increases soil temperature, which
favors quicker root growth and increased microbial abundance.
Higher microbial activity results in higher mineralization, the
breakdown of organic matter in the soil to nutrients that plants
can absorb.
The removal of litter also allows more rainfall to reach the ground
before evaporating. This provides for greater early season soil
moisture and higher inputs of nitrogen due to natural levels of
nitrate dissolved in rain. A common fallacy is that the ash remaining
after a prairie fire provides a fertilizer effect from the nutrients.
Not so. There is increased availability of nutrients after a prairie
fire, but it's not from the ash. It's from the increased nitrogen
inputs in rainfall and nitrogen fixation (bacteria associated
with legumes) and the increased mineralization from microbes.
Most prairie soils are extremely fertile from centuries of growth
and decay of roots. Thus, the nutrients in the ash are a relatively
minor addition.
The growth of prairie plants is enhanced by fire in other ways.
As the plants begin growth in the spring after a burn, their new
leaves are in a sunny, open environment. This promotes formation
of "sun leaf" characteristics and convective cooling
by winds, both of which optimize the leaves for photosynthesis.
In unburned prairie, where new growth is surrounded by litter,
leaf temperatures can exceed the optimum for photosynthesis (due
to less convective cooling) and leaves develop shade characteristics,
which hampers photosynthesis when the leaf emerges above the litter
into the sun.
Fire also removes plant pathogens from the prairie and reduces
the incidence of disease. In a way, fire protects the prairie
from itself because prairie unburned for many years begins to
suffocate under its own litter. The increased growth, flower and
seed production that results from fire are positive for all prairie
organisms. Plants provide the energy, nutrients and habitat for
insects, birds and mammals to prosper.
However, there are some indirect effects that are negative for
overall prairie diversity, at least temporarily. Small mammals
that forage on leaves or in the litter layer and birds that nest
in the litter are temporarily without habitat until prairie regrowth
has occurred. Fire positive species (native grasses) may become
overly competitive and decrease other species (native forbs).
Some mammal species may emigrate to avoid habitat changes and
the increased predation which can occur. Although increased predation
is negative for the prey, it's also positive for the predator.
And so the foremost lesson is that fire can never be 100% positive
for all species.
Weighing
Outcomes
It's clear that prairie and savanna burn management must consider
the biology of fire-negative species to ensure all prairie biodiversity
is conserved. For fire-negative species, fire is both a provider
and a taker. The direct effects of fire threaten their immediate
existence, yet the indirect effects ensure their long-term survival.
Although susceptible to fire-induced mortality, these species
need the prairie habitat that fire maintains and they benefit
from the increased growth and flower production after a burn.
The challenge nowadays is to continue that precarious relationship
in a highly altered landscape. Consequently, prescribed burns
should mimic the pattern that occurred on the native landscape
in which prairie insects, small mammals and other fire-negative
species evolved.
Although pre-settlement fires were surely large, there were always
unburned areas of prairie somewhere on the landscape. Therefore,
never burn more than a third of a site at one time and let unburned
areas within the burn unit remain unburned rather than relighting
them. Diversify the timing of burns so that when negative effects
occur on prairie species, they are spread among more species rather
than the same few.
Land managers (both public and private) need to realize that every
choice made concerning management has its own negative and positive
consequences, even doing nothing.
Prescribed fire is one of those choices and probably has the most
potential of any to affect the ecology of a remnant, whether it
is a prairie, savanna, woodland or fen.
Fire should be used, perhaps even more aggressively than typically
done, but only after careful ecological thought and planning.
 Click here to see the results
of a prairie burn.
Thomas Rosburg
is an Associate Professor of Biology at Drake University in Des
Moines. He is also on the Iowa Natural Heritage Foundation's advisory
board. Rosburg's research specialty is plant ecology.
For more about prescribed
prairie burns, go to www.inhf.org
For more information, e-mail Cathy
Engstrom, Director of Communications, or call (515) 288-1846.
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