Post Oak Savanna The Post Oak Savanna subregions take up most of the area of this ecoregion. Although the savanna receives 35 to 45 inches of precipitation as annual average, many areas look somewhat arid due to a claypan subsoil.
Post oak and
blackjack oak are most common and other oak species include
southern red oak,
bur oak, and
chinkapin oak.
Black hickory is abundant here and there;
cedar elm,
common persimmon,
sugarberry and
eastern red cedar are also conspicuous. As a transitional area between piney woods to the east and prairie to the west, the savanna hosts species from a variety of moisture regimes.
Loblolly pine,
wooly birch and
red maple range in from more humid eastern regions, and there are even
peat bogs. Plants characteristic of arid sites include
honey mesquite,
prickly pear cactus and two species of
yucca.
Northern Post Oak Savanna The landscapes of this ecoregion are generally more level and gently rolling compared to the more dissected and irregular topography of much of Southern Post Oak Savanna. It is underlain by mostly
Eocene and
Paleocene-age formations with some
Cretaceous rocks to the north. Prairie openings contained
little bluestem and other grasses and forbs. The land cover currently has more improved pasture and less post oak woods and forest than the Southern Post Oak Savanna. Some coniferous trees occur, especially on the transitional boundary with the Tertiary Uplands ecoregion.
Loblolly pine has been planted in several areas. Typical wildlife species include
white-tailed deer,
eastern wild turkey,
northern bobwhite,
eastern fox squirrel, and
eastern gray squirrel.
Southern Post Oak Savanna This ecoregion has more woods and forest than the adjacent prairie ecoregions, and consists of mostly hardwoods compared to the pines to the east in the South Central Plains. Historically a post oak
savanna, current land cover is a mix of
post oak woods, improved pasture, and rangeland, with some invasive mesquite to the south. A thick understory of
yaupon and
eastern redcedar occurs in some parts. The ecoregion is underlain by
Miocene,
Oligocene,
Eocene, and
Paleocene sediments. Sand exposures within these Tertiary deposits have a distinctive sandyland flora, and in a few areas unique bogs occur. The endangered
Houston Toad is associated with the deep sandy soils of this ecoregion.
San Antonio Prairie This ecoregion is a narrow, 100-mile long region occurring primarily on the
Eocene Cook Mountain Formation. Upland Alfisol prairies were dominated by
little bluestem and
yellow indiangrass and contained a different mix of grasses and forbs than the dark, clayey, more calcareous soils of the Northern
Texas blackland prairies. Since the 1830s, settlement clustered along the Old San Antonio Road (
Texas State Highway 21 in the south,
Old San Antonio Road in the north) within this narrow belt of prairie land. Currently, land cover is a mosaic of woodland, improved pasture, rangeland, and some cropland.
Northern Prairie Outliers The small, disjunct areas of this ecoregion have a blend of characteristics from the
Texas blackland prairies and the East Central Texas forests. The northern two outliers, north of the
Sulphur River, occur on
Cretaceous sediments, while south of the river,
Paleocene and
Eocene formations predominate. A mosaic of forest and prairie occurred historically in this and adjacent regions.
Burning was important in maintaining grassy openings, and woody invasions have taken place in the absence of fire. The tallgrass prairies included
little bluestem,
big bluestem,
yellow indiangrass, and
tall dropseed. Current land cover is mostly pasture, with some cropland.
Bastrop Lost Pines '') is an endangered species occurring in the East Central Texas forests. This ecoregion is an outlier of relict
loblolly pine-
post oak upland forest occurring on some dissected hills. It is the westernmost tract of southern pine in the United States. The pines mostly occur on gravelly soils that formed in
Pleistocene high gravel,
fluvial terrace deposits associated with the ancestral
Colorado River, and sandy soils that formed in
Eocene sandstones (such as the
Weches Formation). The Lost Pines are about west of the Texas pine belt of the South Central Plains and occur in a drier environment with of average annual precipitation. In this area, the deep, acidic, sandy soils and the additional moisture provided by the Colorado River contribute to the occurrence of pines, which are thought to be a relict population predating the last glacial period. The largest population of the
Houston Toad occurs in this ecoregion.
Floodplains and Low Terraces This ecoregion contains floodplain and low terrace deposits downstream from the
Texas blackland prairies and upstream from the Texas coastal plains. It includes only the wider floodplains of major streams, such as the
Sulphur,
Trinity,
Brazos, and
Colorado rivers. In addition, it covers primarily
Holocene deposits and not
Pleistocene deposits on older, high terraces. The bottomland forests contain
water oak,
post oak,
elms,
green ash,
pecan,
willow oak and
bald cypress to the east, and to the west some
hackberry and
eastern cottonwoods. The northern floodplains tend to have more forested land cover, while in the south the Brazos and Colorado River floodplains are characterized by more cropland and pasture. ==Fauna==