Central forest-grasslands transition


The Central Forest/Grassland Transition Zone (hereafter the CTZ) extends from northern Illinois, across much of Missouri, and into eastern Kansas, Oklahoma, and Texas. The CTZ is one of the larger savanna-type ecoregions, covering more than 380,000 km2, and along with the Upper Midwest Forest/Savanna Transition Zone, separates the Eastern Deciduous Forests from the tallgrass and mixed grass prairies. The CTZ can be distinguished from the Central U.S. Hardwood Forests and other forested ecoregions to the east by its mixture of savanna, prairie, and woodlands. It can be delineated from the tallgrass prairie and Central and Southern Mixed Grasslands to the west by the higher tree and shrub densities. Annual precipitation ranges from 600-1040 millimeters (mm), with wetter areas supporting a more closed tree canopy.

caption WWF

The uniform soil type (mollisols) unites this wide-ranging ecoregion. The major disturbance regimes were clearly fire and drought. The intensity, frequency, and areal extent of fires, combined with drying periods, probably kept the boundaries of this ecoregion in a state of flux. Unfortunately, virtually no intact habitat remains in the CTZ because this ecoregion is one of the most converted of U.S. ecoregions. Almost all of this unit is intensively farmed for corn and soybeans.

Biological Distinctiveness

caption Bullsnake (Pituophis melanoleucus sayi), United States. (Photograph by USFWS)

The CTZ is one of the richer ecoregions in North America due to its size and location as the ecotone between the Great Plains and the Eastern Deciduous Forest. It is also much richer than the other two transition ecoregions, the Upper Midwest Forest/Savannah Transition Zone, and the Canadian Aspen Forest and Parklands. The CTZ ranks among the top ten ecoregions for reptiles, birds, butterflies, and tree species. It shares a strong affinity with the adjacent grassland ecoregions in that many of the tallgrass prairie species can be found in the understory layer. The CTZ also shares much of the fauna of the adjacent grassland ecoregions; these species persist in the ecotones and openings within the ecoregion. Oaks and hickories are the dominant tree species throughout the unit but often occur at low to moderate densities. Typical oaks are blackjack oak (Quercus marilandica) and post oak (Quercus stellata) in the southern part of this ecoregion. Bison (Bison bison) were abundant in this ecoregion in the presettlement period.

Conservation Status

Habitat Loss and Degradation

Less than one percent of the remaining habitat is considered to be intact. Conversion to intensive production of corn and soybeans is almost complete.

Remaining Blocks of Intact Habitat

caption Chain O'Lakes State Park, Illinois, United States. (Photograph by Jason Lindsey) The extremely high level of habitat loss translates into few remaining blocks of intact habitat. All of the remaining units are small. The most important example is the Emiquon floodplain forests in western Illinois, an important wetland and migratory stopover.

Other sites with high potential include:

  • Goose Lake Prairies and the Midewin National Grassland - northeastern Illinois
  • Palos Savanna, 52.5 km2 of moderately fragmented savanna - northeastern Illinois
  • Kankakee Sands, a savanna-wet prairie that is the site of a TNC restoration program, a 32 km2 area with macro-site expansion to 80 km2 - on the Illinois-Indiana border
  • Osage Plains prairie fragments, an important site for the prairie chicken (Tympanuchus cupido), the richest and largest fragments of tallgrass prairie in this ecoregion -Missouri
  • Cross Timbers area, an oak savanna with tallgrass prairie understory * Arbuckle Uplift native grassland - southeastern Oklahoma
  • Indiana Dunes Lake Shore grassland savanna - northern Indiana
  • Emiquon floodplain forest - western Illinois

Degree of Fragmentation

caption Chiwaukee Prairie, Ilinios, United States. (Photograph by Robert Shaw/Wild Perceptions)

Degree of fragmentation is extremely high in this ecoregion among remaining pieces of natural vegetation.

Degree of Protection

There are many small protected areas and larger ones along the Mississippi and Illinois rivers. Prairie State park is protected, as are some scattered remnants of the Osage Plains. Willow Slough and Goose Lake Prairie receive partial protection.

Types and Severity of Threats

Because almost all of this ecoregion has already been converted to corn and soybean production, there is little chance for further extensive conversion to occur. The Osage Plains faces some threats of degradation.

Suite of Priority Activities to Enhance Biodiversity Conservation

Restoration and immediate protection of remaining fragments must be the key activities in an ecoregion like the CTZ that has been so dramatically altered from its original state.

Important activities include:

  • Taking advantage of high potential for acquiring floodplain forests as a result of recent floods
  • Identifying clusters of land to acquire to connect and restore aggregates of large substantial areas

Conservation Partners

  • Illinois Department of Natural Resources
  • Indiana Department of Natural Resources
  • Midwest Science Center
  • Missouri Department of Conservation
  • MoRAP
  • The Nature Conservancy
  • The Nature Conservancy - Midwest Regional Office
  • The U.S. Fish and Wildlife Service
  • The U.S. Forest Service Midewin Tallgrass Prairie

Relationship to other classification schemes

The CTZ combines Omernik ecoregions 54 (Central corn belt plains), 40 (Central irregular plains), and 29 (Central Oklahoma/Texas plains). It corresponds to Küchler unit 73, a mosaic of two other units (66-bluestem prairie, and 91-oak hickory forest). The CTZ corresponds roughly to an area that spans about eight sections in Bailey.

Additional information on this ecoregion


Disclaimer: This article is taken wholly from, or contains information that was originally published by, the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.






Fund, W. (2014). Central forest-grasslands transition. Retrieved from


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