One of the three ecotonal units separating the vast Great Plains grasslands from the forests of the eastern U.S. is the Upper Midwest Forest-Savanna Transition Zone (hereafter UMTZ). This is the smallest of the three transition units along the eastern and northern edge of the Great Plains. Much of the UMTZ is contained in central and southern Wisconsin and southwestern Minnesota.
The UMTZ is distinguished from the adjacent Northern and Central tall grasslands by the predominance of trees in a mosaic of forests, savannas, and woodlands, and from the Central Forest/Grassland Transition Zone to the south by differences in dominance of major tree species. The UMTZ is essentially an oak, maple, basswood woodland, forest, and savanna ecosystem. The boundaries of this ecoregion were heavily influenced by fire and drought. During periods of extensive burning this ecotone may have covered a much larger area to the east than currently estimated. Today, the oak savanna component of the UMTZ is one of the world's most endangered ecosystems. Much of the original ecoregion has been converted.
The UMTZ contains the last concentration of tallgrass prairie savanna (black soils) in the U.S.. It contains important sites used by geese during migrations and it is home to populations of river otters extending out into the prairies. Riparian areas also support an interesting floodplain forest community.
Despite severe threats to its integrity, much of the fauna of the original savanna has persisted, largely because many species are generalists that can survive in altered habitats. The edges of forests often mimic savannas in terms of tree density, and vertebrates persist in these areas. The original oak savanna vegetation, an important community within this ecoregion, has not adapted as well. At least eleven species of herbaceous plants are now confined to much smaller areas and are considered to be threatened. Additional pressures may come from an overabundance of deer in this ecoregion. Several species of invertebrates are also considered threatened.
Today, less than five percent of the ecoregion is considered intact. The savannas found on the deep rich soils of the Midwest were completely fragmented and nearly entirely destroyed throughout the ecoregion by the early to mid-19th century. Most of the ecoregion was affected by clearing, plowing, or overgrazing, and what remained in semi-natural habitat suffered invasion by woody shrubs as a consequence of fire suppression. Recent estimates of remaining savanna fragments testify to the critical status of this ecoregion: only 2 square kilometers (km2) of intact examples of oak savanna vegetation remain in Wisconsin, or less than 0.01 percent of the original 29,000 km2 present.
Remaining Blocks of Intact Habitat
A 1985 survey of oak savanna across the ecoregion listed a mere 133 sites totaling 26 km2, or 0.02 percent of the estimated pre-settlement extent. The remaining examples are largely on either the wettest or driest sites. The intermediate sites on the most productive soils have been converted.
There are no National Forests in this ecoregion. Remaining blocks of habitat include:
- Baraboo Hills, Devils State Park - southern Wisconsin
- Savanna River Depot, (sandy grasslands and flood plain forests) - northwestern Illinois
- Upper Mississippi Wildlife Refuge (flood plain forests) - eastern Wisconsin, western Minnesota
- Mississippi Bluffs forests (relatively intact) - Minnesota
- Richard J. Dorer State Forest - Minnesota
- Whitewater Wildlife Management Area - Minnesota
- Neceda National Wildlife Refuge (protected and managed wetlands),
- Horicon Marsh - southeastern Wisconsin (stopover site for migratory geese)
- Kettle Moraine State Forest - southeastern Wisconsin
Degree of Fragmentation
As stated above, remaining habitat is highly fragmented. Pioneering field research conducted in isolated forest remnants of this ecoregion contributed greatly to our understanding of the impact of habitat fragmentation on breeding success of songbirds. It was here that the severe impact of brown-headed cowbirds (Molothrus ater) on songbirds was well documented. The absence of large predators has given rise to high populations of smaller predators that prey on songbirds and their eggs and nestlings.
Degree of Protection
The State Department of Natural Resources maintains several hundred very small protected sites, many less than 5 km2. These pockets of vegetation are all isolated.
Types and Severity of Threats
Four main threats to the survival of this ecosystem have been identified: 1) loss of recovery opportunities as second home and residential development spread into more natural areas, 2) lack of general awareness of the globally threatened status of oak-savanna vegetation, 3) fire suppression and misunderstanding about the importance of burning in maintaining the integrity of the ecosystem, and 4) invasion by exotic plants such as honeysuckle (Lonicera sp.) and reed canary grass (Phalaris arundinacea). Grazing of wooded sites by cattle and deer continue to be a problem.
In some areas the forests containing maturing oaks and walnuts are big enough for a second cut. The need to protect these areas from logging is paramount.
If carefully managed, portions of the UMTZ have good potential for recovery. It is estimated that within a few decades thousands of hectares of overgrown oak savannas on public and private lands could be recovered. Restoration techniques involve thinning, removing brush, and burning.
Suite of Priority Activities to Enhance Biodiversity Conservation
- Identify and protect clusters of intact habitat (i.e., source pools) from further fragmentation
- Implement savanna restoration, which has a high probability of success in many areas
- Improve private timber management to prevent conversion and further degradation
- Illinois Department of Natural Resources
- Minnesota Department of Natural Resources
- The Nature Conservancy
- The Nature Conservancy, Iowa Field Office
- Wisconsin Department of Natural Resources
Relationship to Other Classification Schemes
The UMTZ is a combination of Omernik ecoregion 51 (North Central hardwood forest), 52 (Driftless area), and 53 (Southeastern Wisconsin Till Plains). It corresponds closely to Küchler vegetation type 72 (Oak savanna), 90 (Maple-Basswood forest-in part) and 97 (Northern Hardwoods-in part). It also corresponds generally with Bailey sections 222M in part (Minnesota & Northeast Iowa Morainal, Oak Savanna Section), 222L (North Central U.S. Driftless and Escarpment Section), and 222K (Southwestern Great Lakes Morainal Section).
Additional Information on this Ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
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