Oak Savannas

Purpose of this web site

This web site has been created to provide land managers, ecologists, foresters, land owners, and interested laypeople with the tools needed to restore oak savannas in Midwestern North America.

The oak savanna was once one of the most common vegetation types in the Midwest but is today highly endangered. Intact oak savannas are now one of the rarest plant communities on earth. However, many degraded oak savannas still remain and can be restored. The detailed information in this web site shows the way.

Savanna Oak Foundation, Inc.

This web site has been created and is maintained by the Savanna Oak Foundation, Inc., a nonprofit foundation dedicated to the oak savanna community. The Foundation was established by Tom and Kathie Brock of Madison, Wisconsin, who manage Pleasant Valley Conservancy, a Wisconsin State Natural Area, which has extensive restored oak savannas.

Another web site maintained by the Savanna Oak Foundation is Pleasant Valley Conservancy, which describes in detail the restoration work on oak savannas and other fire-dependent ecosystems being carried out at this outstanding State Natural Area in southern Wisconsin.



Two views of the same bur oak savanna. Right, spring burn. Left, same view the following September.

 

What is an oak savanna?

A savanna is generally defined as a plant community where trees are a component but where their density is "...so low that it allows grasses and other herbaceous vegetation to become the actual dominants of the community." (Curtis, The Vegetation of Wisconsin). Savannas are found throughout the world, but the dominant trees differ. In North America, a major type of savanna has oaks as the principal trees. Three major areas of oak savanna in North America are found: 1) California and Oregon on the west coast; 2) Arizona, New Mexico, and Mexico in the southwest; and 3) the Midwestern United States. This web site deals exclusively with Midwestern oak savannas, of which many restorable sites exist. Oak savannas in the Midwest are most commonly found in a climatic zone intermediate between woodland and prairie, which is often called the prairie/forest border.

Savannas are often defined in terms of the openness of the tree canopy. Thus, the upper limit between savanna and forest is generally considered to be a tree canopy with 50% coverage. Therefore, if more than one-half of the ground area is in the sun at noon in midsummer, the vegetation is classed as a savanna. It the canopy has greater than 50% tree canopy coverage, the vegetation is called a woodland or forest. The lower canopy coverage, between savanna and prairie, is generally considered to be 10% tree coverage, although these upper and lower limits are only approximate.

Another term sometimes used as equivalent to oak savanna is “oak opening”, which refers to the open-grown characteristic of the trees. Early travelers in the Midwest remarked at the “park-like” character of the vegetation. Some alternate terms occasionally used to describe a savanna-like setting are barren, brush prairie, glade, and open woodland.

The oak savanna landscape
A landscape is a mosaic of vegetation types, with sizes varying with the topography and other characteristics of the land. The term “oak savanna landscape” refers to a natural area or a complex of natural areas with a wide diversity of species. The dominant trees of the oak savanna are several major species of oaks. Within and among this oak tree canopy are numerous smaller trees, shrubs, grasses, and forbs. Thus, although the oaks are the most obvious plants, we must keep in mind the high diversity that is present. This diversity is important in the changes that take place in the landscape with time. Gradual climate change, year-to-year variation in weather, and external disturbance (logging, disease, wind, etc.) may lead to the elimination of individual trees or even wide swaths of the forest, but in a diverse natural area, other individuals usually move in and fill the gaps.

How does a savanna differ from a woodland?
Two major types of oak ecosystems in the Midwest are savanna and woodland. The essential character of an oak savanna is the presence of open-grown oaks. When a single tree grows isolated from all other trees, it does not face competition and its lower limbs are able to spread out and become large and substantial. A savanna oak generally develops in an open area where competition is reduced, which in the Midwest are usually prairies. Thus, oak savannas and prairies are closely linked ecologically as well as topographically.

The openness of the oak savanna is usually maintained by fire, and of the major tree species in the Midwest the oaks are uniquely fire resistant. Over time, these scattered oaks develop into large trees and each open-grown tree receives maximum sunlight and there is little competition between individuals. Oak savannas generally develop in drier areas, on south- or southwest-facing slopes or other areas where many other tree species are unable to compete.

In contrast, in an oak woodland the trees grow in a crowded situation, with each tree shading its neighbors. The lower limbs of these trees receive inadequate sunlight and eventually die. Woodlands develop in moister areas, on north- or northeast-facing slopes or in lowlands where tree growth is favored. In a woodland, the faster growing trees overtop slower growing ones and become dominant. The slower growing, shaded, trees will eventually die. The characteristic structure of a healthy woodland consists of tall straight trees without significant lower branches and these trees have top canopies that spread out, making it possible for them to capture maximum sunlight. The trees of a woodland can also be oaks, although maple, basswood, beech, and other species may dominate, depending on the environmental conditions.

Because of the open nature of the savanna, there will be many areas of scattered light and shade, where herbaceous plants can thrive even during the summer. An oak woodland, in contrast, is so shady during summer that significant herbaceous plants are only found before the tree canopy has leafed out (these plants are therefore called “spring ephemerals”).

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.

In addition to the prairie-like and woodland-like herbaceous plants, there is a third category, the savanna specialists, that grow best at intermediate light intensities. Thus, the diversity of plants in an oak savanna is higher than either a prairie or woodland, because it has species representing all three categories of plants: prairie plants, savanna plants, and woodland plants.

An open-grown tree
The first step in recognizing an oak savanna is to look for open-grown trees. The best example of an open-grown tree (whether oak or other species) is a “street tree.” Cities plant trees widely spaced along streets so that they do not crowd each other. Such trees are generally planted as saplings and are cared for until they reach maturity. A large street tree is a glorious object, and because it takes many years to mature it is irreplaceable. Yard trees, planted by homeowners, are also open-grown trees, and provide shade and comfort to the home.

A typical open-grown oak that has developed in an urban settling. Because there was no competition, the branches remain alive and intact close to the ground.

Open-grown oaks are also found in the wild, as part of oak savannas. In a well-managed savanna open-grown oaks are readily recognized. However, most savannas that exist today are highly degraded and although open-grown oaks may be present, they are generally crowded from overgrowth of weedy trees or woody shrubs. Although large lower limbs may be present, they are often dead. In some case, the only evidence that a tree was once open-grown is the presence of large knots where branches once stood. A major task of oak savanna restoration is the liberation of open-grown oaks from competition, a process called "daylighting the oaks."

Note that the open character of the forest alone does not signify savanna. If an oak forest has been logged, it may be open, but the remaining trees were not open-grown. Such trees can be distinguished from open-grown ones by the lack of lower branches, or the by the absence of the large knots that show where lower branches were once present. In degraded savannas, the large lower limbs that have died (due to shading) may remain on the tree for many years and can still be seen. Also, lower branches that are in a moribund state due to shading in a degraded savanna may actually revive once the tree is opened up.

The savanna oaks in this natural area have also grown in the open, although with much less purity of line than the street tree because of less favorable environmental conditions.  

 




Fire is a critical element in maintaining a savanna, and the savanna (like the prairie) is considered a “fire-dependent ecosystem.” Due to periodic fire, the tree canopy of an oak savanna remains open, with wide spaces between the branches. Among hardwood trees, oaks are uniquely resistant to fire.

The two principal fuels of an oak savanna fire are grasses and oak leaves. Among hardwood trees, oaks are unique in producing leaves that are well adapted to fire. Oak leaves contain flammable chemicals. In addition, oak leaves remain in curled positions on the forest floor, so that fire moves readily from one leaf to another.


The tree canopy of an oak savanna


The character of the tree canopy serves to define an oak savanna. A canopy cover from 10% to 50% is often used as a criterion. The tree canopy can be estimated visually, or measured quantitatively using vertical photography with a fish-eye lens.

However, the best indication of savanna character is the presence of historic open-grown oaks. This is especially useful when decided whether a degraded savanna is suitable for restoration. The presence of open-grown oaks serves as an indicator that the site had savanna characteristics in its past. Using this criterion, many degraded but potentially restorable oak savannas can be identified that would otherwise be overlooked. (See Leach and Givnish for further information)

The more open the canopy, the more “prairie-like” is the savanna. The available sunlight affects two major environmental factors of the understory: the total solar radiation, and the available moisture (the latter indirectly, since a shadier environment will not dry out as rapidly as a sunnier one). A habitat with 10% canopy is essentially a prairie, with 50% is a nice savanna, whereas a habitat with 80% canopy it is essentially a woodland.

However, even with 10% canopy, groundlayer plants growing close to a large tree will be more shaded than those farther from the tree. The oak savanna habitat is highly heterogeneous.


Note the lush, almost prairie-like, vegetation in parts of this white oak savanna. Across this savanna, there are wide variations in sunlight.  

Some herbaceous plants of the oak savanna represent a unique vegetation type

The plant species diversity in a quality oak savanna is very high, higher than in either a prairie or woodland. There are three categories of herbaceous plant species in oak savannas:

1. Many sun-loving prairie plants find suitable conditions in parts of the savanna.
2. Shade-tolerant woodland plants are also able to find suitable conditions in other parts of the savanna.
3. Finally, there are unique savanna species (savanna specialists) that may be found in either prairie or woodland but find ideal conditions in the partial shade of a savanna.

As you walk through a savanna, keep looking up to study the structure of the canopy. Some areas will be shaded, whereas others will be open. Notice how the light is distributed on the forest floor. And then keeping looking at the understory vegetation to seek out savanna specialists.

The Midwestern oak savanna today

At the time of European settlement, oak savannas in the Midwest amounted to about 50,000,000 acres (20 million hectares) in a mostly continuous band stretching along the eastern edge of the Great Plains from Texas into southern Canada. There were also small areas of oak savanna east into Indiana, Michigan, and Ohio. Link to Map of Midwestern Savannas

Because of its openness, savannas have strong human appeal, and early settlers frequently commented on their attractiveness. Unfortunately, oak savannas were among the earliest areas in the Midwest to be settled. In the main prairie states, where large open prairies were very suitable for agriculture, the settler could avoid the intense summer heat by building among a savanna grove at the edge of a prairie, where large oaks provided cooling shade. Although the trees may not have been cut down, their roots were damaged by nearby digging, rolling over, and walking upon, as well as cutting down branches that offended. The herbaceous layer of the savanna would be quickly eliminated by trampling or mowing to keep down tall “weeds”. Within a few years after a savanna area was settled, its specific character would be unalterably ruined. In addition, many high quality savannas in the Midwest were heavily grazed, which did not destroy the large oaks but did destroy the herbaceous vegetation.

Much of this vast savanna area was destroyed during the settlement period in the latter half of the 19th century. At most, Midwestern savannas today amount to only about 30,000 acres, most of which is degraded. Most remaining individual savannas are small, usually less than 100 acres. The rarity of oak savannas has led to them being listed as “globally imperiled”.

There are, however, much larger amounts of restorable oak savannas, primarily in hill country where agriculture is less dominant. The Driftless Area of the Midwest, which constitutes a large mostly hilly area in southwestern Wisconsin with small outliers in Illinois, Iowa, and Minnesota, probably has the largest amount of restorable oak savanna in the Midwest. A number of Wisconsin State Natural Areas are being managed for oak savanna. Also, savannas are being managed and restored in Minnesota, Iowa, northern Illinois and Indiana, southwestern Michigan, and northwestern Ohio. Farther west, a large number of substantial savannas are under management in Missouri.

In Illinois, oak savanna restoration has become an important activity in a suburban band around the large Chicago metropolitan area. Also, extensive restoration work is under way by the Nature Conservancy and others in the Oak Openings Region of northwestern Ohio. Indiana Dunes National Lakeshore is undertaking oak savanna restoration at that location.

The importance of oak savanna restoration has been recognized by the Natural Resources Conservation Service (U.S. Department of Agriculture) through its Wildlife Habitat Incentives Program, and special “points” are given for evaluating WHIP proposals for private landowners that involve significant savanna habitat. The state of Wisconsin has promoted oak savanna restoration by private landowners through its Landowner Incentive Program. The oak savanna has also been singled out for concern by state agencies of most of the Midwestern states that have significant restorable savannas.

Treatened and Endangered Species of Oak Savannas

Several plant species and some birds that are threatened or endangered are found preferentially in oak savanna habitats. Among the plants are giant yellow hyssop, purple milkweed, cream gentian, upland boneset, wild hyacinth, and yellow pimpernel. The red-headed woodpecker, a now relatively uncommon bird, also prefers the savanna habitat.

For more details on the savanna plants.
For more details on savanna animals.

Purple milkweed (Asclepias purpurascens)
now rare but often found in restored oak savannas.

Red-headed woodpecker (Melanerpes erythrocephalus) prefers open savanna areas.



Leach, Mark K. and Givnish, Thomas J. 1988. Identifying highly restorable savanna remnants. Transactions Wisconsin Academy Sciences, Arts, and Letters 86: 119-128.

Curtis, John T. 1959. The Vegetation of Wisconsin. University of Wisconsin Press, Madison.

 
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