The Great Plains is the grasslands of the central United States, but precise delineation of this region has evaded agreement due to the transition between Great Plains grasslands and forests of the eastern United States. After comparing Great Plains delineations in readily available GIS (geographic information system) layers, Rocky Mountain Research Station scientist Brice Hanberry established a northeastern boundary using evidence from historical tree surveys during the 1800s.
Hanberry defined the Great Plains using historical reconstructions along the eastern extent of predominantly tallgrass prairie to resolve a problem that has eluded general agreement.Tallgrass prairie is continuous with tallgrass savanna and thus, precise division is not possible, even with perfect records and additionally, boundaries varied over centuries. Nonetheless, historical evidence from the 1800s of whether or not an area was regularly treed in surveys (i.e., every 0.8 km) or classed more extensively as prairie or savanna in reconstructions helped differentiate tallgrass prairie from tallgrass savanna, resolving the difficulty that caused disagreement in other definitions.
Unlike designations in other maps, Missouri was not tallgrass prairie at landscape scales, whereas most of Iowa, some of Illinois, and even a small part of Indiana and Wisconsin were prairie, with limited tree cover. These boundaries generally aligned with independent sources of reconstructed vegetation, but ecotonal areas remain where unclear separation between grassland and forest potentially could be reclassified.
In this definition, the Great Plains had an extent of 2.29 million km2 in 15 states including Indiana and Wisconsin, and 2.19 million km2 after removal of open forests in Oklahoma and Texas. Clarifying the historical eastern boundary provides a solution to the problem of competing versions of the Great Plains region.