East-Central Texas Post Oak Savanna

This ecological system is found in east-central Texas in a broad, northeast/southwest-trending band located west of the Upper West Gulf Coastal Plain, northwest of the Coastal Prairie, and east and south of the Blackland Prairie ecoregions. It exhibits some floristic and physiognomic variation across this northeast-southwest gradient, losing some eastern species and picking up some species with more western affinities. It is distinguished from the nearby prairie by the higher density of trees and diversity of woody species. The system differs from the floristically similar Crosstimbers Oak Forest and Woodland (CES205.682) in that it generally occurs on Tertiary (primarily Eocene) geologic formations on the east-central Texas Plains, while the related Crosstimbers ecological system occupies Cretaceous and older formations of the interior plains. Floristically, Post Oak Savanna (at least north of the Colorado River) contains species of more eastern affinities such as Callicarpa americana, Sassafras albidum, Cornus florida, Vaccinium arboreum, Ulmus alata, and particularly Ilex vomitoria, the latter species being absent from Crosstimbers Oak Forest and Woodland (CES205.682). Post Oak Savanna generally occurs on sandy or loamy soils, often underlain by a claypan subsoil. Rainfall ranges from about 120 cm in the northeastern part of the range to about 70 cm in the southwest, where it becomes increasingly erratic. Therefore moisture is often limiting during part of the growing season. The system was historically characterized as having significant areas of graminoid cover with species composition resembling that of nearby prairie systems, punctuated by short, stunted woodlands and forests dominated by Quercus stellata and Quercus marilandica. Drought, grazing, and fire are the primary natural processes that affect this system. Much of this system has been impacted by conversion to improved pasture or crop production. Overgrazing and fire suppression have led to increased woody cover on most extant occurrences and the invasion of some areas by problematic brush species such as Juniperus virginiana var. virginiana and Prosopis glandulosa in the southern part of the system's range. These factors have also led to decreases in native grass cover allowing for annual grasses and forbs to invade.

This system represents a transition from the woodlands and forests of East Texas to the prairies to the west, specifically the Blackland Prairie. Savannas and woodlands are typically dominated by Quercus stellata, Quercus marilandica, and Carya texana. Large areas of woodland, particularly in the south and east, are dominated or codominated by Quercus fusiformis or Quercus virginiana (east of the Brazos River). Other species, such as Quercus incana (on more xeric sites), Ulmus alata, Ulmus crassifolia, Quercus nigra, Juniperus virginiana, and Prosopis glandulosa, can also be present in the overstory. To the east, Quercus falcata, Quercus nigra, Liquidambar styraciflua, Pinus echinata, Pinus taeda, and Carya tomentosa (= Carya alba) may be conspicuous in the overstory. Shrubs may attain significant cover in the understory, with species including Ilex vomitoria (often dominant), Callicarpa americana, Sideroxylon lanuginosum, Crataegus spp., Ilex decidua, Toxicodendron radicans, Smilax bona-nox, Juniperus virginiana, and Symphoricarpos orbiculatus. To the south, this system grades into vegetation more characteristic of southern Texas, with Quercus fusiformis and Prosopis glandulosa becoming the primary overstory components, and shrubs of southern Texas such as Acacia rigidula, Forestiera angustifolia, Condalia hookeri, Colubrina texensis, Eysenhardtia texana, Opuntia engelmannii var. lindheimeri, and Diospyros texana becoming increasingly conspicuous understory components. To the east, Vaccinium arboreum, Morella cerifera, Diospyros virginiana, and Cornus florida may be common components of the understory. On some sites, Ilex vomitoria can form a nearly continuous, sometimes impenetrable, dense shrub layer. Mid- and tallgrass species, including Schizachyrium scoparium, Sorghastrum nutans, and Panicum virgatum, are frequent in the understory where light penetration supports herbaceous cover, and also form prairie patches within the savanna, particularly on tighter soils. Other grasses present include Andropogon gerardii, Bothriochloa laguroides ssp. torreyana, Paspalum plicatulum (to the south), Nassella leucotricha, Dichanthelium spp., Aristida spp., and Sporobolus cryptandrus. Non-native grass species such as Bothriochloa ischaemum var. songarica, Paspalum notatum, and Cynodon dactylon may dominate some sites. Forbs are often conspicuous and may include species such as Croton capitatus, Gaillardia pulchella, Monarda punctata, Rudbeckia hirta, Phlox drummondii, Commelina erecta, Acalypha radians, Verbesina virginica, Aphanostephus skirrhobasis, Froelichia gracilis, Cnidoscolus texanus, and many others (Elliott 2011).

This system is typically located on irregular plains in the East Central Texas Plains (Level III Ecoregion 33) of EPA (Griffith et al. 2004), composed of sedimentary formations of Tertiary age, including Eocene sands such the Queen City, Sparta, and Carrizo sands, as well as the Wilcox and Claiborne groups. The system also occupies other Tertiary formations such as the Goliad and Willis formations, as well as portions of the Quaternary Willis Formation. This system occupies gently rolling to hilly topography. It is moderately dissected by drainages. It usually occurs on sandy to sandy loam soils, often with a marked clay subsurface horizon. Soils of this system are generally Alfisols, are typically acidic to neutral, and range from shallow to moderately deep. Typical Ecological Sites include Claypan Savannah, Claypan Prairie, Sandy Loam, Sandy, and Deep Sand (Elliott 2011). Rainfall ranges from about 120 cm in the northeastern part of the range to about 70 cm in the southwest, where it becomes increasingly erratic.

Drought, grazing, and fire are the primary natural processes that affect this system. This system is intricately tied with some occurrences of West Gulf Coastal Plain Herbaceous Seep and Bog (CES203.194). The sandy soils and underlying geologic strata that support this system serve as recharge areas for groundwater that supports seeps and bogs along hillsides and at the heads of drainages supporting West Gulf Coastal Plain Herbaceous Seep and Bog.

Though exact physiognomic condition of this ecological system during presettlement times is unknown, reconstruction of this history suggests that density of woody vegetation is higher today than historically (Campbell 1925, Tharp 1926, McBride 1933, Parmalee 1955, Midwood et al. 1998, Singhurst et al. 2004, Stambaugh et al. 2011b). Factors influencing the primary processes affecting this system, in particular, overgrazing and altered fire regimes, are likely responsible for this change in physiognomy, including invasion of some areas by problematic brush species such as Juniperus virginiana var. virginiana (to the north) and Prosopis glandulosa (to the south). These factors have also led to decreases in native grass cover allowing for annual grasses and forbs to invade. In addition, much of this system has been impacted by conversion to exotic pasture grasses Cynodon dactylon and Paspalum notatum. Other invasive species issues include Ligustrum sinense, Melia azedarach, Triadica sebifera, Ailanthus altissima, feral hogs, and red imported fire ants (TPWD 2012a). Early land uses, including grazing, then farming, and today urban and rural development, infrastructure development, and lignite coal mining, have resulted in the clearing of vast areas (Parmalee 1955, Bartlett 1995, Loucks 1999). Other threats include fragmentation and erosion (Bartlett 1995, Loucks 1999). Impacts of the altered composition and structure of vegetation regrowth since original land clearing are not well-studied and the vast majority of what remains is under private ownership. Less than 1% of the ecological system is under conservation management (Bezanson 2000).

This ecological system is found in east-central Texas in a broad, northeast/southwest-trending band located west of the Upper West Gulf Coastal Plain, northwest of the Coastal Prairie, and east and south of the Blackland Prairie ecoregions. An arm extends along the Red River in north Texas.