The Effects of Fire on the Prairie/Forest Border in Southern Minnesota
By Erin Withers

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Imagine yourself as a pioneer in the United States in 1850.  You are driven west by the promise of available land where you hope to settle and raise crops. Land, fast becoming a precious commodity in the East, is still the life support of the pioneer and equals wealth and promise. As you forage over rolling hills made by glaciers in the not so distant past, you look for a forested area to clear and cultivate. Your zealousness and hopes of manifest destiny draw you and your family further west where you notice that trees are becoming more scattered until you find yourself surrounded by an endless sea of prairie grass. Still unaware of the richness of the prairie soil and astonished by the different environments, you decide to travel north in search of a forested area on which to lay claim.

This story most likely recounts the experience of many travelers heading into the Minnesota territories in search of land in 1850. The ecotone that many settlers startled upon is called the prairie/forest border. This area occurs as "trees invade the prairie in scattered groves to form woodland and parkland communities" (Wendt, K.M. and Coffin, B.A. 1988.) Although this border is rough and has continuously changed since 1850, the reasons for its existence are still unclear. A wide variety of environmental factors and disturbances have played a role in shaping the prairie/forest border. Factors such as climate and soil type heavily influence the vegetation of the region along with such disturbances as drought, windstorms, and insect plagues (Wendt, K.M. and Coffin, B.A. 1988). But, the most influential force in shaping the prairie/forest border in central and southern Minnesota was fire.

The environment of southern Minnesota had changed a great deal even before the settlers set foot on it. According to pollen analysis, the area known as the Big Woods or Maple-Basswood forest was prairie during the approximate period of 7100 to 5100 radiocarbon years B.P. The data then shows a timely invasion of woodland starting with oak woodland in the eastern Big Woods around 5000 years ago. Pollen evidence suggests that the characteristic trees of the Big Woods -- elm, basswood, sugar maple, and ironwood -- may have reached certain areas of southern Minnesota only several decades before 1850. This late and rapid spread of the Big Woods resulted in the heavily forested area that many settlers saw in 1850 (Grimm, E.C. 1983).

Thus, the land the settlers stepped onto in 1850 in southern Minnesota looked quite different from today. In the1850’s, during the time of the public land survey, surveyors reported that tallgrass prairie still covered one third of the state and was mainly in the west. Oak woodland and brushland along with Big Woods covered much of southern Minnesota to the East (Wendt, K.M. and Coffin, B.A. 1988). One of the major forces in stopping the woodlands from taking over all of southern Minnesota prairie was fire. Topography, climate and vegetation, and the people who lived on the land controlled these fires.

The topography in the region of the prairie-forest border greatly influenced how fire affected the area. Since fire needs a continuous stream of fuel, firebreak areas such as rivers, lakes, and rough topography favored wooded areas (Wendt, K.M. and Coffin, B.A. 1988). As Grimm explains, “ Rough topography and lakes not only generally reduce fire frequency but also produce irregularity in the fire-frequency pattern, such that numerous small sites that are very well protected from fire provide habitats for trees while prairie dominates the landscape” (1983) The topography of Minnesota and its soil was a product of the glaciers that had swept over much of the state several thousand years before and contains both flat and hilly areas. Soil and topography determine the type of vegetation that is able to grow and can thus influence the success of fires in an area. Also, flatter areas burned better since fire travels more slowly up hills. Thus, the pattern of vegetation seen in Minnesota in 1850 is a direct result of the topographical outlay of the land which helped control fire.

Climate and vegetation would also influence fires and thus the prairie/forest border. Some evidence suggests that the spread of the Big Woods coincides with the Little Ice Age. The Little Ice Age would have brought a cooler and perhaps wetter climate that would not favor the frequency of prairie fires needed to hold back the Big Woods (Grimm, E.C. 1983). The vegetation of the two biomes also influenced what effect fire had on them. Many plant species on the prairies were well equipped to survive fire because they were perennial in nature and had deep, extensive root systems. When a fire had destroyed the aboveground vegetation, new plants were able to shoot up from just below the ground surface. Fire also allowed nutrient cycling, helping the prairie grasses to bounce back after their destruction. The high temperatures of the fires wiped out the few trees that could survive the prairie climate of low rainfall and high summer temperatures. The fires that burned across the prairies eventually slowed and died in the shade, moist soils, and lack of fuel in the forests (Tester, J.R. 1995). Thus, there was a continual battle between prairie and forest fought by fire.

Perhaps one of the biggest controllers of fire was man. Prairie fires were most likely started by lightning or by the American Indians for hunting purposes before Euro-American settlement (Wendt, K.M. and Coffin, B.A. 1988). Nicollet describes seeing the effects of these fires when he writes, “It is good testimony in favor of my opinion that all the prairies watered by the Mississippi and the Missouri are the work of the Indians who destroy by fire the rich vegetation to assure themselves of animal food. Let the vast and shorn prairies that we cross remain untouched and the forests, with time, will reappear” (Nicollet, J.N.1976). Some speculate that the spread of the Big Woods in the decades before 1850 may have been due to the decreasing numbers of American Indians. With the western movement of settlers, foreign diseases and fights over land may have killed or pushed away many American Indian groups who played a pivotal role in burning the prairie and thus defining the prairie/forest border (Grimm, E.C.) Early Euro-American settlers also had a large hand in controlling fires. Early settlers believed that forested soil was more suitable for crop growing so they instigated the removal of many forested areas. Lumber was also needed in good supply to the settlers making their way west which also contributed to deforestation. Without trees, these areas would be more likely to burn and thus suppress the spread of the forest. However, the continued settlement of the prairie and surrounding areas brought ditches and highways that served as firebreaks. Also, the settlers did not want their buildings and livestock to be consumed in fire so they would actively suppress fires whenever possible. This action of the pioneers allowed an easy invasion of the prairie by woody plants (Tester, J.R. 1995). Thus, man actively played a role in controlling fire, which changed the dynamics of the prairie/forest border.

The physical setting of central and southern Minnesota in 1850 would have been unlike any other time period in history. The prairie was slowly being taken over by the Big Woods but the prairie/forest border ecotone in between these two biomes would prove to be dynamic. The constant changing of this border was most prominently shaped by the effects of fire. The prairie fires of the time determined the spread of the Big Woods. The topography, the vegetation, and the people who inhabited the land controlled these fires. Ultimately, man and his cultivation of the earth would forever change the land, but leading up to 1850, fire was the main instigator of change in the biomes of Minnesota.

Works Cited

Grimm, E.C. 1983. Chronology and dynamics of vegetation change in the prairie-woodland region of southern Minnesota, U.S.A. New Phytologist 93: 311-350. Accessed via JSTOR 3/3/02.