The term "oak savanna" refers to a plant community with scattered "open-grown oaks". surrounded by grasses and forbs. Other terms sometimes used are "oak opening" and "oak barren." In contrast to an oak forest, which has a closed canopy (approaching 100%), the savanna canopy ranges from 10% to 60%. Although oak savannas are found in other parts of the United States, in southern Wisconsin and throughout the upper Midwest they were once a dominant vegetation type. Originally, about 30% of the vegetation in southern Wisconsin was oak savanna.

It has been estimated that only about 0.01% of the original oak savanna of Wisconsin still remains. The remaining fragments of oak savanna thus constitute a rare and highly endangered community.

Fortunately, Pleasant Valley Conservancy has around 30-50 acres of oak savannas, and restoration work has created one of the best examples of the oak savanna type in southern Wisconsin.

How do we recognize an oak savanna? The key characteristic of the oak savanna is the open-grown oak, a tree that has developed in the open, away from other trees. Savanna oaks usually have large lower branches, an indication that they developed without competition from nearby trees. The photo below, of a bur oak, is an example.

The presence in the habitat of open-grown oaks is a good indication of a savanna. Even if the habitat was later heavily invaded by other trees and exotic brush, the open-grown oaks may still be present.

Large areas of highly degraded but potentially good oak savannas still exist in the Midwest, especially on the south-facing slopes and ridge tops. Examination of old air photos, such as the 1937 ones taken by the U.S. Soil Conservation Service, show many highly degraded areas that still have open-grown trees. The existence of these open-grown oaks at Pleasant Valley Conservancy was a principal reason why restoration work was undertaken.

Aesthetics of the Oak Savanna Landscape

Surveys of attitudes and perceptions have shown that the oak savanna landscape rates highly in the public mind. Open forests with large, relatively old trees are considered very attractive. Other pleasing factors of the oak savanna include the high plant diversity, the presence of wild flowers, the large openings surrounded by trees, and the extensive vistas and overlooks. These are characteristic of parks, and the park-like setting of oak savannas is appealing, just as it was to early travelers through the Midwest.

Classic open-grown bur oak in a restored savanna at Pleasant Valley Conservancy. This unit has been burned annually for the past six years.

The Importance of Fire Probably the most common reason why the oak savanna community has disappeared is because of lack of fire. This is a fire-controlled vegetation. The oaks themselves are fire-resistant, whereas weedy trees such as walnut, elm, maple, and ash are not. In presettlement times, fire was used by Native Americans to keep the woods open and to control woody shrubs. As late as the 1950s, farmers were using fire for similar purposes and to bring about early "green-up" of their pastures. However, protection from fire became an established practice of the U.S. Forest Service, as well as the Wisconsin Conservation Commission (the forerunner of the Department of Natural Resources). Although fire is a real danger in the coniferous forests of northern Wisconsin, not so for southern oaks. We know now that it was wrong to prevent fires in the southern oaks. Oaks do not suffer from the disastrous crown fires that rage through the coniferous forest. Except under unusual circumstances, fire in an oak woods is confined to the ground, to the leaf litter. Oak leaves are unusually susceptible to fire. When they burn, weedy woody vegetation such as prickly ash, buckthorn, and honeysuckle is killed, thus keeping the woods open. An open woods encourages the growth of grasses and flowering plants, which are part of the glory of an oak savanna.

Ideas have changed completely about fire in the woods. Without fire, restoration of an oak savanna is difficult, expensive, and generally unsuccessful. Research in Wisconsin, Illinois, Minnesota, Missouri, and Iowa has shown clearly that fire is not the enemy of the oak forest, but its friend.

Restoration of an Oak Savanna A principal activity in oak savanna restoration is the "daylighting" of oaks, removing trees that are crowding out the oaks. The bur oak in the above photo is a good example. This oak, whose age is estimated at about 200 years, was virtually invisible when restoration work began, being completed crowded with elm, cherry, and buckthorn. Once daylighted, the lower branches could thrive again, and the tree has been saved for posterity. Note also the other open-grown oaks farther up the hill. The open-grown oak is a handsome specimen which has often survived from the period before European settlement.

Oak Savanna Locations at Pleasant Valley Conservancy The savanna habitats at Pleasant Valley are in the areas where fire would have been most common. These include the south-facing slope and the part of the ridge top that is nearest to the south slope. Because the south-facing slope receives intense sunlight, it is much drier (more xeric) than the north-facing slope. Historically, the whole south slope was prairie, and fire would have spread quickly up into the savannas.

Digital photos taken with a fish-eye lens have been used to measure the openness of the canopy. The principal savanna area is on the upper south slope and the ridge top, where many large open-grown bur oaks are present. In this savanna zone, the canopy ranges from about 25 to 70% cover. The oak woods, where there are no open-grown oaks, has canopy cover around 90%.

At Pleasant Valley Conservancy, we have oak savannas of two types, bur oak and white oak. The bur oak savanna is predominantly on the top of the ridge, on the dolomite cap. The white oak savanna is mainly on the Jordan sandstone just below the dolomite. As a result of restoration, the full magnificence of the oak savanna can be appreciated. A hike along the upper ridge trail takes one through the heart of our oak savanna.