Cross Timbers - As An Ecoregion

As An Ecoregion

The Cross Timbers are defined by the United States Environmental Protection Agency as Ecoregion 29, a Level III ecoregion. Some organizations and maps refer to the Cross Timbers ecoregion as the Central Oklahoma/Texas Plains. The Cross Timbers are contained within the WWF Central forest-grasslands transition ecoregion.

The woodland and savanna portions of the Cross Timbers are mainly post oak and blackjack oak on coarse, sandy soils; fire suppression in recent years has increased forest density and allowed Eastern redcedar to invade as well. The short, stout oaks that grow in the Cross Timbers were no good for timber and those that were not cleared for farmland constitute one of the least disturbed forest types in the eastern United States (MADISON), with some 890,000 acres (3,600 km2) of old-growth forest scattered throughout the region. These old-growth forests contain millions of post oak from 200 to 400 years old and redcedar over 500 years old. The prairie portions are chiefly tallgrass on finer, dry soils; overall, the Cross Timbers are not as arable as the surrounding ecoregions. Today, land use is a mixture of rangeland, pastures, and farmland. The area has also been an important site of oil extraction for over eighty years.

Geologically speaking, the Cross Timbers are underlain by Pennsylvanian and Cretaceous-era sandstone and limestone that has been moderately dissected, giving the region a gently to moderately rolling topography, including some cuestas. Although local relief is relatively low, it is generally greater than that in the surrounding ecoregions, although this is not the case with the Flint Hills in Kansas.

Ecologically, the EPA includes the Cross Timbers as part of the vast Great Plains, which comprise Level I Ecoregion 9.0, stretching from central Alberta in Canada to northern Mexico. More specifically, the Cross Timbers fall into Level II Ecoregion 9.4, the smaller South Central Semi-Arid Plains. In southern Oklahoma, the Cross Timbers are literally located on the very edge of the Great Plains, as they border directly onto parts of Level I Ecoregion 8.0, the Eastern Temperate Forests; elsewhere, the Cross Timbers are separated slightly from the Eastern Temperate Forests. In turn, the Cross Timbers are themselves subdivided into nine Level IV Ecoregions: