Ecoregions of Texas, AS-199 - Texas Commission on Environmental ...

29c Western Cross TimbersThe Western Cross Timbers ecoregion covers the wooded areas west <strong>of</strong> the GrandPrairie (29d) on sandstone and shale beds <strong>of</strong> Pennsylvanian, Permian, and LowerCretaceous age. The Eastern and Western Cross Timbers (<strong>Ecoregions</strong> 29b and 29c) in<strong>Texas</strong> are the southern portion <strong>of</strong> a larger area (Ecoregion 29) that extends into Oklahomaand Kansas. The entire region is a mosaic <strong>of</strong> oak woodland and prairie that forms thetransition between the eastern deciduous forest and the Great Plains. The oak woodlandis concentrated on sandstone substrates while prairie grasses dominate on surrounding limestone formationsor interior limestone inclusions. Trees in the Western Cross Timbers are drought-stressed; they experienceerratic precipitation and seasonal extremes in temperature. Trees growing under such conditions may beseveral hundred years old and no taller than 20 to 30 feet (Stahle et al., 2003).The landscape has cuesta topography that is expressed as lines <strong>of</strong> sandstone ridges with a gentle dip slopeon one side and a steeper scarp on the other (Spearing <strong>199</strong>1). The soils are mostly fine sandy loams with claysubsoils that retain water. Some researchers contend that these woodland areas would be savanna-like if theyexperienced fire, although one early account described the Cross Timbers as “an immense natural hedge” orbelt <strong>of</strong> thick impenetrable forest. It is likely that with more frequent fire there were more prairie openingsbetween the belts <strong>of</strong> forest, and that closed canopy woodland and park-like savannas were both present.As in the Eastern Cross Timbers (29b), the dominant trees are post oak and blackjack oak with anunderstory <strong>of</strong> shrubs and grasses. The prairie openings historically contained taller grasses such as big bluestem(Andropogon gerardii), yellow Indiangrass (Sorghastrum nutans), and switchgrass (Panicum virgatum)growing on deeper soil, and shorter grasses such as sideoats grama (Bouteloua curtipendula), buffalograss(Buchloe dactyloides), and silver bluestem (Bothriochloa laguroides ssp. torreyana) growing on shallowsoil. The grassy understory is better developed on the red, gravelly soils <strong>of</strong> Pennsylvanian age (Francaviglia2000). The riparian vegetation in the eastern portion <strong>of</strong> Ecoregion 29c resembles that <strong>of</strong> the Grand Prairie(29d) and Eastern Cross Timbers (29b), but, farther to the west, eastern riparian species are gradually replacedby typically western species such as little walnut (Juglans microcarpa), netleaf hackberry (Celtis reticulata),and mesquite (Prosopis glandulosa) (Bezanson 2000). The variety <strong>of</strong> ecosystems in the Cross Timbers, i.e.,prairie, open woodland, and thicket, provide diverse habitats for wildlife. In addition, the abundant acorns area staple food source for wild turkeys (Meleagris gallopavo), prairie chickens (Tympanuchus spp.), raccoons(Procyon lotor), squirrels (Sciurus spp.), and deer (Odocoileus spp.). The area has a long history <strong>of</strong> coal, oil,and natural gas production from the Pennsylvanian sandstone, limestone, and shale beds. Deeper soils in theeastern part <strong>of</strong> this ecoregion support a dairy industry, pastureland, and cultivation <strong>of</strong> forage sorghum, silage,corn, and peanuts.The height <strong>of</strong> oak trees in the Cross Timbers is affected bythe depth <strong>of</strong> soil. Here, stunted oaks grow at the edge <strong>of</strong> asandstone outcrop covered with lichen. Photo: R.E. Rosiere,Tarleton State UniversityThe yearly acorn crop from Cross Timbers oaks is an importantfood source for the wild turkey (Meleagris gallopavo).Photo: Albert Lavalle, National Wild Turkey Foundation<strong>Ecoregions</strong> <strong>of</strong> <strong>Texas</strong> 39