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Fire Effects in the Northern Great Plains

Information provided by Dr. Mark Petersen of the Fort Keogh Livestock and Range Research Laboratory in Miles City.

Fire and grazing are natural and important processes in maintaining grasslands in the Great Plains and elsewhere.  Active fire suppression has in fact been recognized as a key disruptive force in rangeland ecosystem integrity throughout the world.  Fire’s ecological effects are numerous and complex.  Fire can manipulate nutrient dynamics, soils, vegetation, and animals.

Primary factors affecting community response are timing, frequency, and intensity of fire relative to the biology of organisms examined.  Fire effects on total plant productivity in the northern Great Plains are neutral to positive.  Native perennial grass productivity generally increases following fire.  Neutral responses in total productivity occur when increases in native perennial grasses are offset by reductions in annual grasses and forbs, which are predominantly non-native and non-preferred species.  Fire causes an immediate reduction of standing dead material and litter.  The combustion of standing dead material is a loss of forage in the near term.  The reduction in litter can alter light and moisture relations at the soil surface, promoting increased productivity and discouraging establishment of non-native species.  Reduction of standing dead material and litter as well as improved forage quality of new growth also attract grazers to burned sites.

Response to fire can be species-specific, allowing targeted control of native weeds, such as purple threeawn, pricklypear cactus and juniper.  Invasive non-native weeds, such as annual bromes are also susceptible to selective control with fire and fire can kill seeds of some noxious invasive species.

Fire has even been shown to selectively control pest grasshopper species.   As an evolutionary process, fire cannot be substituted with any other management option.

The dominant perennial grasses in the northern Great Plains are resistant to fire.  Western wheatgrass, threadleaf sedge, needle-and-thread, and blue grama were exposed to fires with a wide range of fuel loads (up to 8020 lb/ac) and hot, dry weather to determine the probability of fire-induced mortality.  No western wheatgrass or threadleaf sedge died.  To reach a 0.5 probability of mortality for needle-and-thread and blue grama, surface temperatures  exceeding lethal limits had to last 10.5 and 7.5 minutes, which required more than 7100 lb/ac of fuel.  Most combustion in grassland fuels is completed in 30 seconds to 2.5 minutes and fuel loads in the northern Great Plains are typically 600-2700 lb/ac.

Species such as Japanese brome can be reduced by fire through direct mortality of exposed plants and seeds, and indirect reduction through alterations in the microenvironment that reduce successful germination and establishment.  Established perennial weeds with protected buds, such as leafy spurge, are not harmed by fire, but seeds near the soil surface, in the litter, or in the canopy can be quite vulnerable to fire-induced mortality.

As with plant species, animals are directly and indirectly affected by fire.  For example, ticks and grasshoppers that are in the plant litter or canopy  experience direct mortality from fire.

Additionally, we have shown some of the primary pest grasshopper species can be selectively controlled with fire.  Migratory grasshoppers show little egg mortality because they lay eggs deeply in the soil, whereas white-whiskered grasshoppers lay their eggs near the soil surface and commonly show 86% mortality.

Fire Ecology References available upon request. Email [email protected].