Native Plant Community and Habitat Spotlights: Oak Savanna

Native Plant Communities

Southern Mesic Prairie

ground view of yellow and purple flowers

Mesic prairies exist on level to gently rolling sites that are more protected from drought stress than other prairie communities due to higher soil moisture. This tallgrass dominated community once covered large areas of Minnesota but is now considered “imperiled” based on its rarity and threats facing the remaining examples. Common species include big bluestem, Indian grass, and prairie dropseed, which are mixed with a suite of other grasses and forbs such as smooth blue aster, wood lily, and tall meadow-rue. Trees are typically absent from this community except where fire suppression has allowed invasion by woody species. The only common woody species in this community include low growing semi-shrubs like leadplant and prairie rose and sparse patches of wolfberry, gray dogwood, American hazelnut, and wild plum.

The fertile soils and gentle relief of mesic prairies are ideal for row crop agriculture, which has resulted in the conversion of almost all of this plant community. The remaining examples are threatened by modern fire suppression. Recurrent fire is essential for the existence of mesic prairies, as environmental conditions are otherwise suitable for tree growth and rapid conversion to forest in the absence of fire. To add habitat diversity to your property, you should consider creating or enhancing open spaces as prairies. Avoid planting trees on these sites and implement management activities that mimic the frequent fires that once shaped this ecosystem.

Southern Mesic Savanna

single large oak tree without leaves

This unique native plant community, which is the namesake of the entire subsection, is considered “critically imperiled” based on its rarity and threats facing the remaining examples. These savannas formed where streams, lakes, and steep hillsides protected them from the intense prairie fires. Under these conditions, fires recurred frequently enough to prevent trees and shrubs from dominating, but allowed fire-tolerant trees to become established. These trees occur as dispersed individuals with an open-grown form or as scattered small clumps with total tree cover typically 25 to 50 percent and rarely greater than 70 percent. Bur oak is the most common tree species in these savannas but northern pin oak may also be present. Tall grasses such as big bluestem, Indian grass, and prairie dropseed dominate the ground-layer but a diversity of other grasses and forbs, such as heath aster and goldenrod, are also present.

This plant community has faced many threats such as conversion to row crop agriculture and fire suppression. The remaining plants are under threat by a number of invasive species such as Kentucky bluegrass, smooth brome, buckthorn, honeysuckle, and multiflora rose. Monitoring the understory and edges of your savanna for these invasive species—and taking fast action to control them through selective/spot herbicide treatment, mechanical removal, and prescribed fire—can help make this biologically unique ecosystem as healthy and resilient as possible.

Southern Mesic Oak-Basswood Forest

veiw of an oak-basswood forest during the summer

This mesic hardwood community typically contains a nearly closed canopy of basswood, northern red oak, and sugar maple on rolling hills and north-facing valley slopes. The density and abundance of seedlings and saplings in the shrub and sub-canopy layers is dependent on the available light filtering though the canopy and usually contains ironwood, sugar maple, basswood, and bitternut hickory. The forest floor of this community often erupts with a diversity of ephemeral flowers in the spring that take advantage of available light before the forest canopy leafs out and they often fade thereafter. Some common ground-layer species include zigzag goldenrod, large-flowered bellwort, and Virginia waterleaf.

Catastrophic disturbances were historically rare in this community, while small disturbances that created a mix of canopy layers were far more common. Tailoring your firewood harvesting strategy by removing single trees or small clusters will create small gaps in your woods. This will allow some of the sugar maple and basswood saplings to develop and create a mix of tree ages which gives your forest more vertical diversity. Other species such as northern red oak and bitternut hickory need more sunlight to reach maturity so creating some medium to large gaps—up to an acre-in your woods will encourage these species. Whether you choose to create small or large gaps, you will help create diverse age groups among your trees similar to historic disturbances, these gaps will create better wildlife habitat and help your woods be more resistant to environmental stress.

Southern Mesic Maple-Basswood Forest

view of a maple-basswood forest

Present on sites that have been historically protected from fire, this deciduous forest is most often found on middle and lower slopes on north and northeast facing bluffs. The canopy is strongly dominated by sugar maple, with lesser amounts of basswood and northern red oak present. Sugar maple, bitternut hickory, basswood, prickly gooseberry, and chokecherry are common in the shrub-layer. Important ground-layer species include Virginia waterleaf, bloodroot, yellow violet, large-flowered bellwort, wild leak, blue cohosh, and early meadow-rue.

A forest with mixed age classes, heights, and tree species will be more resilient to Minnesota’s changing climate. To diversify your forest, create openings by conducting scattered removals of individual trees ready for harvest or removing small patches of trees. This will encourage smaller sugar maple and basswood trees in the understory to grow into the gaps created by the harvest while discouraging sun-loving trees like red and bur oak. Repeat harvesting individual or small patches of trees every 10-20 years to maintain a diverse maple-basswood dominated forest. If you want to increase the amount of oaks, clearcut your woods one to two years after pre-planting oaks to a density up to 600 seedlings per acre. Remove competing trees and shrubs until oaks dominate the canopy. Care must be taken to protect remaining canopy and understory trees from damage during the harvest.

Habitat Spotlight:

Oak Savannas

view of oak trees forest

Oak savannas have scattered trees above a layer of prairie grasses and forbs. Bur oak is the most common savanna tree in Minnesota and typically has a spreading form that results from growing in an open environment. The savanna is a transitional ecosystem between the tallgrass prairie and woodland environments, so it is an important habitat for both woodland and prairie wildlife. This habitat is particularly valuable to a number of bird species that depend on open woodlands for feeding and nesting such as red-headed woodpeckers, eastern bluebirds, loggerhead shrikes, and eastern whip-poor-wills. The open environment was maintained largely by fire. In the absence of fire, woodland species gradually fill the savanna, closing the canopy and shading the prairie grasses and forbs. Prior to European settlement, oak savanna was one of the most common ecosystems in Minnesota, covering roughly 10 percent of the state. Fire suppression, agriculture, and urban growth have reduced the extent of oak savanna coverage to le s than one-tenth of one percent of its original acreage, making it one of the rarest plant communities in the region. Given the significance of this habitat type, the acreage that has been lost, and its importance to a diversity of wildlife, you should consider actions to restore or preserve any oak savannas on your property.

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