Cofrin Center for Biodiversity

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Plants

In the Midwest, bur oak (Quercus macrocarpa) is the tree species most characteristic of oak savannas. 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.

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). Eiten (1986) has defined a savanna as follows:

"A savanna is any area where scattered trees and/or shrubs and other large persistent plants occur over a continuous and permanent groundlayer visually dominated by herbs, usually graminoids."

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. According to Bacone et al. (1994), more than 800 species of vascular plants have been recorded from savanna remnants. Of these, 28 species are listed as threatened or rare. However, 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. More complete lists of typical savanna plant species can be found in Bray (1955), Curtis (1959) Madany (1981), Packard (1993), and Hujik (1994).

Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects