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Cofrin Center for Biodiversity

Oak Savanna and Oak Openings

The first oak openings were planted around Prairie pond in the Keith White Prairie in 1978. In 2000 and 2001 two large areas of old field and farmland along Scotwood Drive were transformed into oak savanna. The areas were seeded with a prairie mix. Bur oak (Quercus macrocarpa) and black cherry (Prunus serotina) saplings were planted to augment existing Hills oak (Q. ellipsoidalis) at the site.

Student sowing seeds on the newly tilled Oak Savanna site.

Savannas are grassland areas with scattered trees and shrubs that are transitional between open grasslands and woodlands.  The characteristic habitat consists of scattered trees in a matrix of grasses and herbs, creating a "park-like" environment.  In North America the dominant trees in many savannas are oaks, leading to the more specific term "oak savanna." Curtis (1959) set a limit of 50% canopy cover; if more than one-half of the ground is shaded by trees at noon during mid-summer, then stand is considered a forest rather than a savanna. Many authors recognize a gradient of canopy cover ranging from savanna (approximately 5-30% canopy cover) to woodland (30-80% canopy cover) to forest (> 60% canopy cover).

At the time of European settlement in the 1800's oak savannas extended over large portions of Minnesota, Iowa, Missouri, Illinois, Wisconsin, Michigan, Indiana, and Ohio, covering approximately 11-13 million hectares (Nuzzo 1986).  Today, only about 0.02% of the original area of oak savannas remains, and many of the remnants are degraded, threatened, or atypical of presettlement savannas. 

Fire has played a significant role in generating and maintaining savannas in the Northern Lake States.  Bur Oak (Quercus macrocarpa), an important savanna tree in this region. This important plant is capable of forming sprouts from roots or stumps, enabling individuals to persist even after fire (Bray 1955). Other oaks, including black oak (Quercus velutina), Hill's oak (Q. ellipsoidalis), and white oak (Q. alba) are capable of sprouting and often occur in Midwestern savannas. The trees are widely spaced, creating an orchard-like landscape with occasional clumps of trees. The distinction between oak savanna and oak woodland is somewhat arbitrary. Frequent burning leads to fires of low intensity, which rarely kill canopy trees but maintain an open understory.  Higher intensity fires stimulate sprouting by species like Bur Oak, leading to thickets of young woody plants among the grasses.  Settlement in the 1800's was accompanied by widespread replacement of oak savannas by farmland.  Cessation of fires caused a further conversion of remnant savannas to oak forests. In places where fires were frequent, small oaks were inhibited from reaching tree size, so shrub communities predominated.

Besides oaks, other tree species of oak savannas in the western Great Lakes region include black cherry (Prunus serotina), shagbark hickory (Carya ovata), and large-toothed aspen (Populus grandidentata). The understory vegetation is a mixture of herbs, sedges, shrubs, and grasses from both mesic prairies and open forests. No plant species occurs exclusively in oak savannas, and Curtis (1959) identifies only 6 species (Heliopsis helianthoides, Besseya bullii, Orobanche uniflora, Phlox glabberima, Ranunculus fascicularis, and Zygadenus elegans) that are more widespread in oak savannas than in any other Wisconsin plant community. Prominent grasses include big bluestem (Andropogon gerardi), little bluestem (A. scoparius), Panicum spp., Indian grass (Sorghastrum nutans), and porcupine grass (Stipa spartea). Common forbs include flowering spurge (Euphorbia corollata), Comandra richardsiana, wild bergamot (Monarda fistulosa), and hog peanut (Amphicarpa bracteata). Rosaceous shrubs (Rosa sp., Rubus sp.) also are conspicuous in many Midwestern savannas. Many other understory plants occur in oak savannas, the composition varying with degree of openness and soil moisture.

References:

Bray, J.R. 1960. The composition of savanna vegetation in Wisconsin. Ecology 41:721-732.

Curtis, J.R. 1959. The vegetation of Wisconsin. University of Wisconsin Press. Madison, WI. 657 pp.

Nuzzo, V.A. 1996. Extent and status of Midwest oak savanna: presettlement and 1985. Natural Areas Journal 6(2):6-36