The principal trees of oak savannas are of course the oaks, but shagbark hickory (Carya ovata), black cherry (Prunus serotina), and white or jack pine are occasionally present.

In Midwestern oak savannas the principal oaks are bur oak (Quercus macrocarpa), white oak (Q. alba), black oak (Q. velutina), and northern pin oak (Q. ellipsoidalis). Other species of oaks found in various Midwestern savannas are listed here:

Latin names: bur oak (Quercus macrocarpa), white (Q. alba), black (Q. velutina), post (Q. stellata), blackjack (Q. marilandica), red (Q. rubra), swamp-white (Q. bicolor), chinkapin (Q. muehlenbergii), Hill's (northern pin) (Q. ellipsoidalis). See Silvics of North America for characteristics.

In Wisconsin Curtis categorized the oaks in terms of the soil characteristics where they were most common: northern pin (most xeric), black (better soils and more moisture), red (deep, well-drained soils; most tolerant of all the oaks), chinkapin (restricted range in Wisconsin; southernmost; Grant County), bur (2 races, one without large caps; great range of habitats; acid sands, deep slit loams, rocky calcareous soils, wide moisture tolerance, very shade intolerant, very important in southern Wisconsin, less so in the north, white (slightly less tolerant than bur), swamp white (wet mesic).

West coast and southwestern savannas have completely different species of oaks. California, in particular, has a large number of oak species, many of which grow only there.

The two major oak groups

There are two major oak groups, the red oak and the white oak groups. Oaks of the red oak group have leaves with pointed lobes whereas those of the white oak group have leaves with rounded lobes. Oaks of the red oak group include red, black, and northern pin. Oaks of the white oak group include white, bur, and swamp white oak. Hybridization may occur between species of each of the two oak groups but not across the two groups.

Red oak group: black oak leaf and acorn	White oak group: white oak leaf and acorn	White oak group: bur oak leaf and acorn. The hairy fringe of the cap is distinctive and makes it easy to recognize this acorn. The large indentation in the leaf is also characteristic.

In the white oak group, acorns mature at the end of the first season of formation, whereas those in the red oak group require two seasons to mature. Another important difference is that the water-conducting vessels (xylem) of the white oak group are plugged with intrusions (tyloses), whereas the xylem of the red oak group remain open. Because of these differences in xylem, wood of the red oak group fills with water, causing the wood to sink, whereas wood of the white oak group floats. A consequence is that wood of the red oak group is more susceptible to decay than the wood of the white oaks. An additional difference is that members of the white oak group are resistant to oak wilt where those of the red oak group are sensitive (see below).

White oak

The white oak is an outstanding tree and is widespread across eastern United States. It is of great economic interest in the lumber industry and is also widely used as staves for the manufacture of barrels. It is found from southern Maine to northern Florida, and west to the Iowa/Kansas border. It grows on a wide range of soils with a wide range of moistures except on the driest sites, or on poorly drained bottom land. It can be a major or exclusive component of oak savannas throughout its range.

Bur oak

The bur oak grows farther to the west than any of the other Midwestern oaks, but does not grow in the south or along the east coast. Historically, the bur oak was an important bottom-land species, thriving where other oak species are not able to grow. Most of these bottom land oaks have been eliminated, although bur oaks remain in floodplain forests along the Missouri River in North Dakota and along watercourses in the Great Plains.

The bur oak remains widespread throughout its range on dry uplands and sandy plains where it is often associated with calcareous soils. In the Great Plains it may be a pioneer tree invading prairie grasslands, and it is frequently planted there in shelterbelts. It is frequently found as a fringe between the prairie and upland forest in the prairie/forest border (eastern Iowa and northern Illinois). In protected locations along the Missouri River in North Dakota, very old bur oaks (>400 years) can be found.

The bur oak is a slow growing tree and is very intolerant of shade. However, it is very fire resistant and hence a principal tree of oak savannas. In Curtis’s detailed study of tree composition in Wisconsin savannas, the bur oak was the dominant tree (see table below).

The native range of bur oak (Quercus macrocarpa) across the United States. In the western part of its range, it is more commonly found in moister habitats, such as gullies and ravines. Not shown is its distribution in southern Manitoba and southern Ontario. Redrawn from Silvics of North America.

Black and Hill's Oak Savannas

Many savannas in sandy areas contain black oaks (Quercus velutina) as their principle oak tree species. The black oak is a common, medium-sized tree of eastern and midwestern United States. Although it grows best on moist, rich, well-drained soils, it is often found on poor, dry, sandy flats or hillsides. It is relatively intolerant to shade and seedlings generally die within a few years when growing under full overstories. This species is more sensitive to fire than bur or white oak and its bark is often damaged by fire, creating entry points for decay fungi. It is much more sensitive to oak wilt than bur or white oak (see below).

A related species is Hill's (or northern pin) oak (Q. ellipsoidalis), which also is found on sandy savannas. This species is common in many of the savannas in Minnesota

Because they are able to establish itself in sandy habitats, they are often found in tallgrass savannas, where big bluestem and Indian grass are principal herbaceous species.

The Karner blue butterfly, an endangered species, is often associated with black oak savannas, because of the presence of wild lupine (Lupinus perennis). Karner blues require lupine as a food source, and this plant species grows preferentially in sandy areas. Since this is the habitat where black oaks grow, the two species are often found together. Conservation of Karner blues thus often focuses on the establishment of the black oak savanna.

Oak Wilt

The disease oak wilt, caused by the fungus Ceratocystis fagacearum, is potentially a major factor in the survival of oaks in nature. It has probably been part of North American oak forests for over 100 years and is present wherever oaks are present. Although trees of both the red and white oak groups can become infected, those of the red oak group are much more susceptible. When a tree of the red oak group is infected, it may lose all of its leaves quickly, and dies. Trees of the white oak group generally lose only leaves on a branch or two and may survive the infection.

Oak wilt can move from tree to tree by means of sap-feeding beetles. The fungus produces a sweet-smelling odor that attracts the beetles. The beetles then fly to healthy trees and initiate infection. Another important means of spread is by means of root grafts. The roots of trees especially of the same species form underground grafts which permit the spread of the fungus. Root grafts are more common on members of the red oak group.

Root grafts are more likely to occur in a dense forest than in an open one and oak wilt is less likely to develop in an oak savanna than in an oak woods. One factor in opening up a woods and creating a savanna could be oak wilt. In fact, Kline at the University of Wisconsin-Madison Arboretum introduced oak wilt intentionally into thousands of oaks in the 1980s in order to restore an area to oak savanna. The motivation here was that since oak savannas are so rare, oak wilt could aid in producing one. Opening up the canopy increased the fuel for prescribed burns, which were then able to move through the area and kill undesirable woody species.

Disturbance of an oak forest creates openings that aid in producing a savanna. Oak wilt is one type of disturbance.

Hill's oak savanna at the Helen Allison Savanna Scientific and Natural Area, a Nature Conservancy preserve which is adjacent to the University of Minnesota Cedar Creek Natural History Area. This savanna is part of an extensive glacier-deposited sand plain in which the original vegetation was oak barrens and oak openings.

Black oak savanna at Fort McCoy Oak Barrens State Natural Area, Wisconsin. This area is very sandy and is part of huge sand deposits of glacial Lake Wisconsin. This is a typical example of a "barrens" type of savanna, common on impoverished sandy soils. This State Natural Area contains one of the least disturbed barrens in Wisconsin. In addition to black oak, Hill's, bur, and white oaks are also present. Fort McCoy is owned by the U.S. Department of Defense. Although large areas are "off limits" except by special permission, the area where this photo was taken is open to the public.

Burns, Russell M. and Honkala, Barbara H. 1990. Silvics of North America. Volume 2, Hardwoods. U.S. Forest Service, Department of Agriculture, Agriculture Handbook 654.

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

Kline, Virginia M. 1983. Use of oak wilt to control oak invasion of prairie, Proceedings of the Eighth North American Prairie Conference pp. 162-164