Texas Cross Timbers Oak Forest

This system is primarily found within central Texas and Oklahoma, ranging north to southeastern Kansas and east into eastern Oklahoma. It is distinct from the surrounding prairie by the higher density of tree species. The area consists of irregular plains with primarily sandy to loamy Ustalf soils that range from shallow to moderately deep. Rainfall can be moderate, but somewhat erratic, therefore moisture is often limiting during part of the growing season. Short, stunted Quercus stellata and Quercus marilandica characterize and dominate this system. Other species, such as Carya texana, Carya cordiformis, Quercus prinoides, Ulmus crassifolia, and other Quercus spp., can also be present within their respective ranges. The understory often contains species typical of the surrounding prairies, in particular Schizachyrium scoparium. Shrubs such as Rhus spp. may also be present. Drought, grazing, and fire are the primary natural processes that affect this system. Overgrazing and conversion to agriculture, along with fire suppression, have led to the invasion of some areas by problematic brush species such as Juniperus virginiana and Juniperus ashei and Prosopis glandulosa farther south in Texas and Oklahoma. It has also led to decreases in native grass cover allowing for annual grasses and forbs to invade.

This system is generally described as a savanna or woodland, distinguished by its dominance by short, stunted Quercus stellata and/or Quercus marilandica. It occurs in southwest/northeast-trending bands separated by the Grand Prairie. Other species in the canopy may include Ulmus crassifolia, Quercus fusiformis, Celtis laevigata, and Juniperus virginiana. The understory may have been historically dominated by Schizachyrium scoparium, but current understory composition may be largely determined by land-use history and grazing pressure. Carya texana, Carya cordiformis, and Quercus prinoides are lacking from Texas examples and are mainly present in stands of this system in the northern Crosstimbers of Oklahoma (L. Elliott pers. comm. 2011). In the east, where precipitation is greater, tallgrass species such as Andropogon gerardii and Sorghastrum nutans may be important components of the understory or occupy prairie patches. In the drier west, shortgrass species such as Bouteloua dactyloides (= Buchloe dactyloides) become more conspicuous. Other graminoid species that may be present include Schizachyrium scoparium, Paspalum setaceum, Sporobolus compositus, Bouteloua curtipendula, Bouteloua hirsuta, Bouteloua rigidiseta, Bothriochloa laguroides ssp. torreyana, Nassella leucotricha, and Aristida spp. Non-native species such as Cynodon dactylon and Bothriochloa ischaemum var. songarica frequently dominate the herbaceous layer. With the disruption of a natural fire cycle, branching of overstory species may be continuous to near ground level, reducing light penetration and leading to reduced herbaceous cover. The shrub layer may contain species such as Smilax bona-nox, Rhus glabra, Rhus trilobata, Crataegus spp., and Symphoricarpos orbiculatus. Sites dominated by Prosopis glandulosa, sometimes with Ziziphus obtusifolia as a common shrub component, are particularly common to the west. Sites dominated by junipers (including Juniperus virginiana, Juniperus ashei, and Juniperus pinchotii, depending on the site) are also frequently encountered. Prairie openings and inclusions tend to occur on tighter soils. Shrubs such as Rhus spp. may also be present. Other species may include Celtis laevigata, Cercis canadensis, Cotinus obovatus, Fraxinus albicans (= Fraxinus texensis), Gleditsia triacanthos, Juniperus ashei, Juniperus virginiana var. virginiana, Quercus fusiformis, Quercus buckleyi, Quercus velutina, Ulmus alata, and Ulmus americana (Elliott 2011).

This system is located on irregular plains composed of sandy to loamy Ustalf soils. These soils range from shallow to moderately deep. Rainfall can be moderate, but sporadic, leading to periods of limiting moisture. This system also includes smaller patch woodlands dominated by Quercus stellata occurring over Mollisols and scattered throughout the limestone uplands of the eastern Edwards Plateau and Lampasas Cutplain of Texas, locally referred to as "Redlands" (B. Carr pers. comm. 2005). The eastern occurrences of this system are associated with sandy members of the Cretaceous Woodbine Formation, while western occurrences occupy soils derived from the sands of the Cretaceous Trinity Group (such as Paluxy, Antler, and Twin Mountain-Travis Peak sands). Further west, in the fringe of the western Crosstimbers, the system occurs on more rugged, rocky and gravelly sites derived from Pennsylvanian formations. The landforms are gently rolling, moderately dissected uplands, and irregular plains becoming more rugged in the western fringe of the distribution of this system. Soils are sands or sandy loams, some with a claypan. Ecological Sites typical of the eastern expressions include Sandy Loam, Tight Sandy Loam, Claypan Prairie, Sandstone Hill, and Sandy. Those more typical of the western expressions include Sandy Loam, Loamy Sand, Tight Sandy Loam, Sandy, and Clay Loam (Elliott 2011).

Drought, grazing, and fire primarily influence this system. Overgrazing and conversion to agriculture have allowed for the invasion of eastern red-cedar (Juniperus virginiana), Ashe's juniper (Juniperus ashei), and honey mesquite (Prosopis glandulosa) in some areas. Decreases in native grass cover associated with overgrazing can also lead to an increase in invasive annual grasses and forbs.

This system is primarily found within central Texas and Oklahoma, with the northern extent reaching into southeastern Kansas in the Cross Timbers (EPA level III ecoregion 29). It also includes the "Lower Canadian Hills" and "Osage Cuestas" in eastern Oklahoma and the Edwards Plateau Woodland, Semiarid Edwards Plateau and Broken Red Plains of Texas (37e, 40b, 30a, 30d, 27i of EPA, respectively).