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A disappearing ecosystem: Minnesota’s oak savannas

Oak savannas — open grassland studded by tall, spreading oak trees — once covered 10 percent of Minnesota.

Oak savannas — open grassland studded by tall, spreading oak trees — once covered 10 percent of Minnesota, mostly in the southeast quarter of the state. They are an attractive ecosystem for animals such as deer, turkeys, and red-headed woodpeckers. Before European immigration, indigenous people valued the savannas for the good hunting they provided, fostering and maintaining them through the use of regular fire. In 2017, only about one percent of the savannas that existed 200 years ago remain.

The oak savanna is a transitional ecosystem, between prairie and woods. In Minnesota it consists mainly of scattered bur oak trees amid grasslands. Historically, the state’s savanna lands existed mostly in a band from south of what became the Twin Cities to the Iowa border. This was the northern extension of savannas that covered most of present-day southern Wisconsin and southern Iowa, northern Missouri and Illinois, and large fractions of Oklahoma and Texas.

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Oak savannas can exist and persist only where there is regular fire. Without fire, the transition zones tend to fill with brush, shrubs, and fire-intolerant trees; the savannas turn into woods. Before large-scale Euro-American immigration, prairie fires took place regularly in Minnesota, but natural fires, caused by lightning, did not happen widely enough or often enough to maintain Minnesota’s vast savanna land. That oak savanna was a human creation.

Before the arrival of settler-colonists, American Indians made frequent use of fire all across North America, for hunting and to create and maintain open land. Compared to oak savanna, forest was not good hunting country, nor did it produce much food. The great food-producing animals of the Midwest — bison, deer, and elk — fed well in the grasslands and savannas, where they were also the easiest to hunt. Indigenous people understood this, and set fires annually to preserve and extend open country. Fire also fertilized the grasslands, to the benefit of the animals and their human hunters.

Minnesota’s oak savanna country occupied the territory between the Big Woods of the southeast and the open prairie of the southwest. The then-dominant tree species of southeastern Minnesota — elm, maple, basswood — did not tolerate fire well; regular fire keeps them in check. The mature bur oak, by contrast, tolerates fire well because of its thick, cork-like bark. It does best where the canopy is open and undergrowth is kept low.

Regular fires eliminate competitive tree species and control undergrowth (browsing animals, such as bison, also help). Prairie, together with bur oaks (and, secondarily, red oaks) and regular fire form oak savanna. The oak trees create bountiful crops of acorns, a favorite food of turkeys, deer, and other animals, so the savanna made ideal hunting grounds. The indigenous people of southern Minnesota undoubtedly observed this and understood the relationship between their fire practices and the desirable savanna landscape.

European farming practices and urban growth destroyed most of the savanna in the nineteenth century. Oak trees got in the way of the plow, and the crop farmer had no use for fire. There was no place for fire in towns and cities, either. And so the savanna melted away.

The destruction of savannas drastically reduced habitat for the species best-adapted to them. The red-headed woodpecker, which prefers savannas over all other habitats, was hit particularly hard, its numbers vastly reduced. Audubon Minnesota reported in 2014 that the woodpecker’s population had dropped from 94,000 in 2004 to 20,000 in 2012, mostly due to continuing loss of oak savanna.

In the late twentieth century the value of the savanna began to be recognized once again, and scattered efforts at preservation or restoration of oak savannas have been undertaken by the state, some city park departments, the University of Minnesota, and private groups like the Nature Conservancy. Conservation and renewal projects were undertaken at Agassiz Dunes in Polk County, the Helen Allison Savanna and Cedar Creek Ecosystem Science Reserve in Anoka County, and Ottawa Bluffs in LeSueur County. Still, the survival of oak savanna habitats remains in doubt.

For more information on this topic, check out the original entry on MNopedia.