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Northwestern Cross Timbers

Ecoregion on Great Plains, OK

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The rolling hills of the Northwestern Cross Timbers are underlain by Permian-age sandstone, siltstone, and shale. Blackjack oak – post oak savanna occurs on sandy soils, tall grass prairie is native on fine-textured soils, and forests dominated by sugar maple grow in the shelter of larger canyons. Eastern redcedar is native to fire-protected areas; it is now common due to the combined effects of grazing and fire suppression. Ecoregion 29h is drier than other parts of the Cross Timbers (29). Tree density and height are less than in the Northern Cross Timbers (29a), but greater than in the Cross Timbers Transition (27o). Cultivation and overgrazing has largely destroyed the native prairie. Ecoregion 29h includes the canyons of Caddo and Canadian counties and their permanent, spring-fed streams. Most of these streams are highly modified by upstream flood control, channelization, and erosion; they mostly have fine sandy substrates and tend to be entrenched, wide, and shallow. Unmodified reaches occur; they have greater habitat diversity and more sensitive species than modified reaches.

— EPA

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