Spring, 2009

Restoring the Oak Savanna

The Belwin Conservancy’s 1,338–acre preserve plays host to a huge range of plants and animals. The wetlands, woodlands, fens and forests on our land are all part of a unique patchwork of ecosystems that cover this landscape with a bewildering array of diversity. Part of what makes our work so rewarding and challenging is the complexity that we work with every day.

This part of Minnesota is a transitional zone. Prairies dominate the landscape to the south and west while woodlands rule in the north and east. At the Belwin Conservancy we have large pockets of both; typical for this area. It is often the case in nature that transitional areas are some of the most interesting. They tend towards greater diversity and complexity. Between the prairie and the forest is where we find the oak savanna.

Oak savanna is the most endangered ecosystem in the Midwest. Today less than 0.01% of its original extent remains. We are deeply fortunate to have some large areas of oak savanna at the Belwin Conservancy although much of what remains is overgrown and in poor health. It is our duty to restore this endangered landscape where we can. To that end, for the last several years we have been working on one of the largest oak savanna restorations in Minnesota.

Lake Edith Panorama

The Oak Savanna

Savannas are special places, and it is not an exaggeration to say that all people are drawn to them in some way. When you think of sitting outside on a warm summer day, chances are that you are picturing yourself in a savanna whether you know it or not. These sun-dappled meadows overhung by majestic shade trees are so ingrained in us that we create them wherever we go. After all, our yards and parks with their turf lawns and shade trees are just man-made savannas designed long ago to echo the natural article.

There are many kinds of savanna in the world, but in this region it was the oak savanna that was once found in great abundance. Oak savannas are dominated by loosely scattered bur or white oaks in a diverse mix of grasses and forbs that thrive in the mosaic of sunlight found in the understory. Because the trees only cover 10-50% of the ground beneath them, enough sunlight penetrates the canopy to sustain life on the ground as well as the treetops.

Numerous animal species depend on the diversity of habitat and sheer productivity endemic to an oak savanna. The trees are used for nesting, perching, and feeding by a variety of wildlife. The natural holes, hollows and dead snags provide space for birds and animals to make dens. The acorns produced by the oaks also provide a major source of nutrition for a huge variety of animals including deer, turkeys, ruffed grouse, and numerous small mammals.

Native Americans also depended on the oak savanna, relying on acorns as an important source of nutrition. Because they are so easily stored, acorns could be stockpiled as a hedge against leaner times. To help the trees maintain a healthy acorn crop, Native Americans conducted the first controlled burns in this region to spur their growth.

Lower Classroom Savanna Prescribed Burn

As it is on the open prairie, fire is critical to the health of the oak savanna. Regular burns keep the understory open, assist certain species in germination and restock the soil with nutrients. Oak savannas exist in a fragile balance and it doesn’t take much to tip them one way or another. In this area for example, regular fires were eliminated long ago, allowing these once majestic places to become overgrown.

Restoration

The strategy to restore oak savanna depends on the structure and density of the trees in the restoration. To create new savanna, open areas are first seeded to prairie and then scattered oak saplings are planted. Although they are fire tolerant as adults, these saplings need to be protected during prescribed burns until they develop the thick bark characteristic of the species. As the trees mature, more shade-adapted grasses and flowers are planted among them.

If the restoration begins with an overgrown savanna as it did in our case, the non-native trees and shrubs including buckthorn, and native species crowding the oaks need to be removed. This is called “daylighting” the oaks and the goal is to give the existing trees enough space to regain their open-grown shape, and to allow enough sunlight to penetrate the canopy and encourage oak saplings in the understory. Usually, native grasses and flowers are planted; however, depending on the land use history, there may be pockets of existing savanna vegetation or seed that will thrive once sunlight reaches the ground.

Spreading Prairie Seed

We have been working for years to restore an oak savanna at our Lake Edith Site. This spot once hosted a vast savanna covering the steep slopes surrounding the lake. Although it was heavily grazed, the oaks are clearly visible in aerial photos from the late 1940s. Since then, the slopes have quickly filled in and the oaks were drowned in a rising green sea of buckthorn.

Walking through these areas now, you can pick out the slow-growing bur oaks amongst the other trees of the forest. These old oaks still remember when they grew freely in the sun; their open-grown shape betrays their history. In the thick forest of today, they look out of place and sickly.

We are using every method at our disposal to restore this particular savanna: we are planting trees in the open fields of our Lake Edith Site and we are daylighting the overgrown oak trees along the slopes. This is very slow work, especially when done by hand as it was in 2008. That winter we were only able to restore 15 acres.

This year, due to a grant we received from the Minnesota Department of Natural Resources, the pace of this restoration has greatly increased. The grant funded the removal and staging of woody material generated during the process of restoration. This woody debris is in turn used by District Energy St. Paul as biofuel in their combined heat and energy plant.

Recently Restored Oak Savanna

In the course of performing this type of restoration, dealing with brush and woody debris has always been one of the principal limiting factors. By harvesting this debris on an economic scale, what was once a liability for the Belwin Conservancy becomes the fuel to help us perform this kind of restoration on a much larger scale.

In 2009 we have restored over 70 acres of oak savanna. Much of the structure is now in place, but we have much more work to do getting native vegetation established in the understory. With vigilance, we will make sure that these areas remain open and in time we hope to see the animals and birds that rely on oak savanna make a comeback.