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.

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.