Oak Savannas and Woodlands
Savannas are generally defined as ecosystems made up of grasslands with a moderate tree canopy. A wooded area in which half or more of the sunlight reaches the ground can be considered a savanna. If the canopy shades more than half the ground cover, this falls into the category of “woodland.”
The savannas that grace the Midwest landscape are oak savannas. The oak savanna is an ecosystem that functions as a transition biome between the tall grass prairies of western Minnesota, the hardwood forests to the east, and the coniferous forests to the northeast.
The oak savanna was once a common ecosystem in the Midwest, but today it is highly endangered. Intact oak savannas are now a rarity. However, many degraded oak savannas still remain along rivers and streams, but also in upland areas. The municipality of St. Cloud includes many oak savannas along the Mississippi and Sauk Rivers. These savannas, although often overrun by invasives like Buckthorn and crowded by fast-growing trees, can be restored.
The beauty of the oak savanna is its proportion of tree to open space. Neither dense forest nor open prairie, the oak savanna strikes a lovely balance between the two, creating an attractive habitat for humans. Historical documents show that European settlers often camped en route and settled in such places, dawn no doubt to the shade of the oaks, not to mention the sense of permanence and protection they create.
The openness of the oak savanna is usually the result of fires. Before human settlement, fires were caused by lighting. When humans arrived to North America some 12,000 years ago, much of the Midwest was covered with ice. As the glaciers receded, the land gradually became inhabited by native peoples who altered the landscape through their land management strategies, one of which was the use of fire.
It is now believed that native peoples practiced burning. This was done for a variety of reasons, but an important consequence was that the oak savanna landscape flourished. The fires would kill off much of the young woody vegetation of other trees such as maples, walnut, and basswood, while enhancing the grass and wildflower ground cover.
Oak savannas are found most commonly in hill country, from Minnesota to Texas, because these areas burn better. Fires generally move uphill more rapidly and with higher flame heights and more intense heat.
In level sites, savannas may be more common in sandy than loamy soil, again because they burn hotter. In addition to trees, scrub-brush is often a major component of a degraded oak savanna. Shrubs are fire sensitive and top-killed by fire but not always eradicated.
Over time, scattered oaks develop into large trees and each open-grown tree receives maximum sunlight. Oak savannas generally develop in drier areas, on south or southwest-facing slopes or other areas where other tree species are unable to compete.
The open nature of the oak savanna results in the establishment of numerous kinds of prairie plants, both grasses and forbs. If the tree canopy is very sparse, the vegetation will be more prairie-like than woodland-like. On the other hand, when the tree canopy approaches 50%, prairie plants will not grow as well but many woodland plants will thrive. Because of the scattered nature of the oaks, some parts of an individual savanna will be very open and other parts more closed.
The Bur Oak
Of the major tree species in the Midwest, Bur Oaks are uniquely fire-resistant. Their deeply fissured bark allows them to absorb and radiate heat from fires. Moreover, the exposed limbs of a smaller Bur Oak may be killed by a fire, but it will often regrow what was lost in the following year.
In Minnesota, there are two primary families of oaks: the red oak and white oak. Both are valuable trees for many reasons, one of which is that they both produce acorns (unlike, say, maples), which are a primary source of food for many woodland creatures. Both white oaks and Bur Oaks are long-lived trees, with the white oaks living up to, and even exceeding, 200 years.
Oak Savanna Vegetation
A major activity in any savanna restoration is the recovery of the understory vegetation, principally flowers and grasses. Depending upon the history of a site, its understory may be rich in species, impoverished, or without native plants.
There are many residential developments in Central Minnesota in Oak Savannas. Often the back yards consist of lawn that meets an oak wood, but usually these savannas and woodlands have been degraded. The tell-tale sign of degradation is a thick understory of brush, making it too thick to traverse for human and wildlife alike. Often this underbrush consists of invasive, non-native plants such as Buckthorn and Honeysuckle.
Restoring a Woodland/Oak Savanna
The first step in restoring a woodland lot is clearing the undesirable underbrush. Sometimes this is done selectively so as to preserve the native species that are there, such as Hazelnut or Chokecherry. Other times the woodland is so degraded that a thorough clearing of the underbrush is necessary. Often, there is the need to thin the stand of trees so as to allow in more light.
Our goal when we plan a backyard woodland or savanna project is to design it so that it becomes part of the residential landscape. Too often woods are "wasted spaces," a mere backdrop for the landowner. The effect of a good design is that a backyard no longer ends where the turf grass meets the woods; rather, the woodland becomes an extension of the residential landscape. Clearing the understory of invasive plants such as Buckthorn and Honeysuckle, and removing undesirable trees such as Siberian Elm, as well as by designing paths and perhaps a sitting area, one can incorporate a great asset such as woodland into your landscape.
Usually we continue to eliminate non-native plants for a growing season, working them out of the soil so that they will not germinate at a later time. Seeding in woodland plants can take place in the late fall, before the ground freezes, but after the temperatures drop enough that the seed will not germinate that fall.
We also grow plugs so that we can add specific shade-tolerant natives at a later time.
As you can see, a woodland or oak savanna restoration can be staged over several years. We work with homeowners to help them meet their budgets and continuously improve their land until it is where they want it to be.