A license to burn? Thoughts on prescribed prairie fire

This article was first published in the Winter 2005 edition of INHF's quarterly magazine.

by Thomas Rosburg

"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.

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.