Oak Savannas in Wisconsin
by
Richard A. Henderson
Wisconsin Department of Natural Resources
Eric J. Epstein
Wisconsin Department of Natural Resources
| Figure. Gross range of presettlement oak savanna in central North America (adapted from Nuzzo 1986 and Smeins and Diamond 1986). The shaded area represents the general region in which oak savannas occurred, although this region was not uniformly savanna. Significant blocks of nonsavanna vegetation, such as prairie or forest, were also present within it. Nor was oak savanna totally restricted to this region. Small, disjunct outliers existed as far east as Ohio and as far west as the Dakotas. | |
Oak savannas have probably been in North America for some 20-25 million years (Barry and Spicer 1987), expanding and contracting with climatic changes and gaining and losing species (on a geologic time scale) through evolution and extinction. For the past several thousand years, such savannas have existed as a relatively stable band of varying width and continuity, from northern Minnesota to central Texas (Figure). |
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| At the time of European settlement (ca. 1830), oak savanna covered many millions of hectares. It varied somewhat in species composition from north to south and east to west, but structure and functions were probably similar throughout. In the upper Midwest (Minnesota, Wisconsin, Michigan, Iowa, Illinois, Indiana, and Missouri) there were an estimated 12 million ha (29.6 million acres) of oak savanna (Nuzzo 1986). Wisconsin's portion was 2.9 million ha (7.3 million acres; Curtis 1959). As the Midwest's rich soils were used for agriculture and fire was suppressed, this ecosystem all but disappeared from the landscape throughout its range. Today, oak savanna is a globally endangered ecosystem. | ||
Status |
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Oak savanna. Courtesy R. Henderson, Wisconsin Department of Natural Resources |
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| In the early to mid-19th century, the oak savanna ecosystem was thoroughly fragmented and nearly totally destroyed throughout its range. Most of its acreage suffered from clearing and plowing, overgrazing, or invasion by dense shrub and tree growth caused by lack of fire, lack of grazing, or both. Consequently, oak savanna now shares equal billing with tall-grass prairie as the most threatened plant communities in the Midwest and among the most threatened in the world. Only a little more than 200 ha (500 acres) of intact examples of oak savanna vegetation are listed in the Wisconsin State Natural Heritage Inventory, or less than 0.0001 (0.01%) of the original 2.9 million ha (7.3 million acres)--a fate repeated throughout this community's entire range (Johnson 1986; Smeins and Diamond 1986). A tallying of known oak savanna sites in the upper Midwest (Missouri northward) in 1985 (Nuzzo 1986) listed only 133 sites totaling 2,600 ha (6,420 acres), or only 0.0002 (0.02%) of the estimated presettlement extent of the community. Most of what remains are dry and wet savanna types. Richer, more productive soil savanna is now nearly nonexistent. |
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| Fortunately, most of the biota that was associated with savanna, especially the vertebrates, have either adapted to the changed landscape or have managed to survive in suboptimal habitat (e.g., the fringes of other less devastated communities, such as oak forests). This situation is precarious for many species, however, and their long-term future is doubtful. Vertebrates have been successful because major elements of the savanna structure are still well represented in various edge habitats, including wooded pastures, lawns, and woodlots. The fact that the plant species may be different in surrogate savannas has not affected savanna vertebrate species for the most part. | ||
| Oak savanna vegetation has not fared as well. Many species that were probably savanna specialists are now uncommon and are found only in the fringes and openings of oak woods, brushy areas, and lightly grazed pastures. A few examples are giant false-foxglove (Aureolaria grandiflora), yellow pimpernel (Taenidia integerrima), pale Indian-plantain (Cacalia atriplicifolia), New Jersey tea (Ceanothus americanus), sessile-leaved eupatorium (Eupatorium sessilifolium), and white death-camas (Zigadenus elegans). Two likely savanna specialists, purple milkweed (Asclepias purpurascens) and wild hyacinth (Camassia scilloides), are now listed as endangered in Wisconsin. Three others--kitten-tails (Besseya bullii), cream gentian (Gentiana alba), and Virginia lespedeza (Lespedeza virginica)--are listed as threatened. | ||
| Most bird species found in Wisconsin savannas are still doing well today (e.g., American robin [Turdus migratorius], indigo bunting [Passerina cyanea], and brown thrasher [Toxostoma rufum]). Only one oak savanna/woodland bird, the passenger pigeon (Ecopistes migratorius), has become extinct, and another, the wild turkey (Meleagris gallopavo), was extirpated but is now restored; however, both of these were lost because of unregulated hunting and not because of habitat loss. | ||
| Recently, however, a number of savanna birds have not thrived or have begun to decline throughout their range, including the northern flicker (Colaptes auratus), red-headed woodpecker (Melanerpes erythrocephalus), vesper sparrow (Pooecetes gramineus), northern bobwhite (Colinus virginianus), warbling vireo (Vireo gilvus), and field sparrow (Spizella pusilla). The orchard oriole (Icterus spurius) and yellow-breasted chat (Icteria virens) are on Wisconsin's list of special concern. The loggerhead shrike (Lanius ludovicianus) and barn owl (Tyto alba) are on Wisconsin's endangered species list, and Bell's vireo (Vireo bellii) is now on Wisconsin's threatened species list (D.W. Sample and M.J. Mossman, Wisconsin Department of Natural Resources, personal communication, 1994). Although loss of habitat has not been the cause of decline in all these species, it certainly is affecting many of them. The abandonment and loss of savanna/woodlot pastures in the past few decades may be playing a role in some of the recent declines of savanna bird species. | ||
| Most amphibian and reptile species that were closely associated with the historic oak savanna in Wisconsin are doing at least moderately well today, although two reptiles associated with savanna habitat are on the Wisconsin list of endangered species and are suffering from habitat loss: the western slender glass lizard (Ophisaurus attenuatus) and the eastern massasauga rattlesnake (Sistrurus catenatus). The eastern massasauga is also under consideration for federal listing. | ||
| Our knowledge of oak savanna invertebrates is limited; we know little about what species were characteristic or restricted to oak savanna, let alone their current status. Some reliable status information does exist for savanna Lepidoptera (moths and butterflies), however; of this group, the Karner blue butterfly (Lycaeides melissa samuelis) is listed as federally endangered while phlox flower moths (Schinia indiana) and tawny crescent butterflies (Phyciodes batesii) are under consideration for federal listing. The frosted elfin butterfly (Incisalia irus) is listed as threatened in Wisconsin, and four savanna skippers (Erynnis persius, Hesperia leonardus, H. metea, and Atrytonopsis hianna) and the buck moth (Hemileuca maia) are considered rare in the state. Other globally rare insects thought to have been part of the oak savanna ecosystem include the federally listed American burying beetle (Nicrophorus americanus) and the red-tailed leafhopper (Aflexia rubranura), which is under consideration for federal listing. | ||
Threats |
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| Threats to the future survival of oak savanna throughout its range can be summarized into four categories. The first, loss of recovery opportunities, can be attributed to accelerating succession to tree and shrub species that produce dense shade; a lack of recruitment and eventual death of long-lived plants surviving now only in suboptimal habitat; changes in pasturing practices through either increasing or decreasing grazing pressure; and an increasing rate of rural home building in key savanna areas. The second threat is lack of understanding about the community by the public, professional resource managers, and scientists. Resistance to the use of prescribed fire, especially in wooded areas, and lack of understanding about the importance of fire in maintaining biodiversity are the third threat; invasion by aggressive non-natives (i.e., honeysuckle, buckthorn, and reed canary grass) is the fourth. | ||
Recovery Potential |
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| In the absence of active management, the future of oak savanna looks bleak in Wisconsin and throughout its entire range. The increasing abandonment of lightly to moderately grazed wooded pastures and the accelerating succession of oak woodlots toward heavy shade-producing trees and shrubs are likely to lead to the further decline and possible loss of much of the remaining savanna flora and fauna, including eventual declines of the oaks themselves. | ||
| If oak savanna habitats are actively managed, however, their recovery potential in Wisconsin and throughout their range is substantial (Holtz 1985; Bronny 1989; R.A. Henderson, Wisconsin Department of Natural Resources, unpublished data). Many degraded sites in the dry and wet ends of the spectrum can be recovered with relative ease. Mesic, richer soil savannas will take more time and work, but recovery is still feasible. The native species that formerly inhabited oak savannas can be reintroduced with a reasonable amount of effort (Packard 1988), but the options available are quickly decreasing. | ||
| In Wisconsin alone, hundreds if not thousands of hectares of overgrown but still retrievable oak savanna exist on public lands and thousands more on private lands. Although Wisconsin may be above average in potential for savanna recovery, similar situations exist in other states. Much of this land, especially low productivity sites, could be restored within a few decades simply by thinning trees, brushing, and burning. Well-drained, rich soil sites, however, will require more work and time to restore. Some plant reintroductions may be necessary, but much can be accomplished with fire alone. Light grazing may also have potential as a savanna management tool. | ||
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Wisconsin Department of Natural Resources Bureau of Research 1350 Femrite Dr. Monona, WI 53716 |
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| References | |
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Barry, A.T., and R.A. Spicer. 1987. The evolution and palaeobiology of land plants. Croom Helm, London. 309 pp. Bronny, C. 1989. One-two punch: grazing history and the recovery potential of oak savanna. Restoration and Management Notes 7(2):73-76. Curtis, J.T. 1959. The vegetation of Wisconsin. University of Wisconsin Press, Madison. 657 pp. Holtz, S.L. 1985. Cutting and burning a degraded oak barrens: management techniques that stimulate natural disturbance. M.S. thesis, University of Wisconsin, Madison. 80 pp. |
Johnson, F.L. 1986. Oak-hickory savannahs and transition zones: preservation status and management problems. Pages 345-347 in D.L. Kulhavy and R.N. Conner, eds. Wilderness and natural areas in the eastern United States: a management challenge. Stephen F. Austin State University, Nacogdoches, TX. Nuzzo, V.A. 1986. Extent and status of Midwest oak savanna: presettlement and 1985. Natural Areas Journal 6(2):6-36. Packard, S. 1988. Just a few oddball species: restoration and the rediscovery of the tallgrass savanna. Restoration and Management Notes 6(1):13-20. Smeins, F.E., and D.D. Diamond. 1986. Grasslands and savannahs of east central Texas: ecology, preservation, and management. Pages 381-394 in D.L. Kulhavy and R.N. Conner, eds. Wilderness and natural areas in the eastern United States: a management challenge. Stephen F. Austin State University, Nacogdoches, TX. |