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":