PROTECTING WISCONSIN'S BIODIVERSITY

 
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Oak Opening

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Photo by Eric Epstein

 

Counties with Mapped NHI Occurrences

Oak opening is an open savanna community with a heavy prairie component. Oak openings tend to be dominated by members of the white oak group, especially bur oak (Quercus macrocarpa), sometimes white oak (Q. alba), and locally in southwestern Wisconsin, chinquapin oak (Q. muhlenbergii). Black oak (Q. velutina) and shagbark hickory (Carya ovata) may also be present. Canopy closure can vary widely. The best quality sites have a canopy with 5-40% closure but can be as high as 60% closure, often with large canopy gaps in between trees. "Canopy closure" is the proportion of shrub and ground layer that is covered in shade or shade flecks at noon on a sunny day. Canopy trees tend to have short, thick trunks with wide-spreading crowns, unlike the tall straight trunks and narrower crowns of forest-grown trees.

Oak openings historically experienced near-annual surface fires. As a result, the subcanopy is essentially absent in good quality sites. Tall shrubs are also sparse, though may include American hazelnut (Corylus americana) and shrubby oak saplings, also known as oak grubs. The ground layer of oak openings is typically dominated by a matrix of native grasses and sedges, especially little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), needlegrass (Hesperostipa spartea), and Leiberg’s panic grass (Dichanthelium leibergii). Where oak openings occur on wet-mesic sites, sedges (e.g., Carex pellita) or bluejoint grass (Calamagrostis canadensis) may dominate.

Typical forbs are numerous and have significant overlap with prairies, barrens, bedrock glades, and oak woodlands. Some of the best indicators include leadplant (Amorpha canescens), white camas (Anticlea elegans), Canada milkvetch (Astragalus canadensis), kitten-tails (Besseya bullii), bastard toadflax (Comandra umbellata), prairie coreopsis (Coreopsis palmata), veiny pea (Lathyrus venosus), whorled loosestrife (Lysimachia quadrifolia), prairie phlox (Phlox pilosa), Seneca snakeroot (Polygala senega), eastern shooting-star (Primula meadia), and early buttercup (Ranunculus fascicularis). Additional oak opening indicator species can be found in the Coarse-level Monitoring Protocol for Oak Openings (Carter et al, 2023).

Today, oak opening is one of the rarest plant communities in the Upper Midwest with very few high-quality sites in existence. Sites that were not converted to agriculture or settlements were quickly degraded by grazing resulting in the loss of prairie and savanna flora. When grazing declined fire suppression became the norm, causing a rapid growth of trees and shrubs. Oak openings historically occurred in a variety of landscape settings. The best remaining sites are found in the Driftless Region on steep west- to south-facing slopes where they are associated with dry prairies on bluffs. In glaciated southern Wisconsin, they occur on gravelly kames, eskers, and along end moraines, especially in the southern Kettle Moraine region. A variant of oak openings can also occur on sandy alluvial terraces and islands associated with large river systems, such as the Wisconsin, Black, and Chippewa Rivers, where they are sometimes also referred to as floodplain savannas. Historically, oak openings also occurred on mesic sites, but these have essentially been extirpated by fire suppression, intense grazing, and conversion to other land uses.

 
 
 
Key Environmental Factors

Oak openings can be differentiated from oak woodlands by a more open canopy (especially apparent in 1930s aerial photos). In contrast, the best oak woodlands tend to have a higher degree of canopy closure (rarely less than 41%, and often up to 65% or more). Oak openings tend to have open-grown limb architecture, and often a higher proportion of bur oak. In contrast, oak woodlands tend to have mature trees intermediate between open grown and forest grown architecture and have a higher abundance of white oak. Oak woodlands also lack the high representation of prairie flora that is often present in high-quality oak openings.

Oak openings can be differentiated from oak barrens by their dominance of bur oak and sometimes white oak. In contrast, oak barrens are usually dominated by black or Hill’s oak, sometimes with pines co-dominant. Oak openings tend to have a matrix of prairie grasses in more open, sunnier areas and occur on a wide range of soil types from sandy loam to clay loam soils. In contrast, oak barrens occur on sand. Oak barrens may also have prairie grasses but often dominated by Pennsylvania sedge (Carex pensylvanica), poverty grass (Danthonia spicata), and savanna running sedge (Carex siccata). In addition, oak barrens are more likely to have locally abundant sand-loving species such as three awn grasses (Aristida spp.), fall witch grass (Digitaria cognata), love grasses (Eragrostis capillaris and E. spectabilis), goat’s-rue (Tephrosia virginiana), flax-leaved aster (Ionactis linariifolia), dotted horsemint (Monarda punctata), hairy hawkweed (Hieracium longipilum), and sweet-fern (Comptonia peregrina). Oak openings occur rarely on sand. When they do, it is usually associated with wet-mesic alluvial terraces of large sandy-bottomed rivers. In these situations, tree species composition (more bur oak and less black or Hill’s oak) and the absence of oak barrens species, as described above, are points for differentiating oak opening.

Oak openings can be differentiated from prairies by having at least 5% closure of canopy trees. This equates to at least one large, mature tree per acre (Curtis, 1959). While Curtis acknowledged this to be a somewhat arbitrary dividing line, it is still a useful division. Given nearly a century of fire suppression, however, many prairies may now have scattered oak trees. Oaks also tend to invade frequently burned prairies. Thus, savannas often grade into adjacent community types with no clear line between them. While this may cause consternation when trying to strictly classify sites, it is also evidence that these community types are closely related and shifted back and forth over time and space. Historical aerial photos from the late 1930s (accessible via Wisconsin Historic Aerial Imagery Finder) and original public land survey notes from 1832-1866 (accessible via Wisconsin Public Land Survey Records) are useful in determining the circa 1800s historical community for a given site.

Wildlife Action Plan

Species of Greatest Conservation Need (SGCN) are associated with habitats (or natural communities) and places on the landscape. Understanding relationships among SGCN, natural communities and ecological landscapes help us make decisions about issues affecting SGCN and their habitat and how to respond. Download the Wildlife Action Plan association score spreadsheet to explore rare species, natural communities and ecological landscape associations

Conservation actions respond to issues or threats, which adversely affect species of greatest conservation need (SGCN) or their habitats. Besides actions such as restoring wetlands or planting resilient tree species in northern communities, research, surveys and monitoring are also among conservation actions described in the Wisconsin Wildlife Action Plan because lack of information can threaten our ability to successfully preserve and care for natural resources.

For more information, contact

Ryan O'Connor
Conservation Biologist
tel:+1-608-354-2383