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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 [NA0415], 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 mm, with wetter areas supporting a more closed tree canopy. 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.
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 [NA0415], and the Canadian Aspen Forest and Parklands [NA0802]. 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. 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 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:
Degree of Fragmentation 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.
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:
 Relationship to other classification schemes The CTZ combines Omernik (1995) ecoregions 54 (Central corn belt plains), 40 (Central irregular plains), and 29 (Central Oklahoma/Texas plains). It corresponds to Küchler (1985) 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 (1994). Prepared by: S. Robinson, Terry Cook, Steve Chaplin, and Eric Dinerstein This text was originally published in the book Terrestrial ecoregions of North America: a conservation assessment from Island Press. This assessment offers an in-depth analysis of the biodiversity and conservation status of North America's ecoregions. All text by World Wildlife Fund © 2001 For more general information on this ecoregion, go to the WildWorld version of this description.
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