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Tertiary Uplands (35a)
The rolling Tertiary Uplands, gently to moderately sloping, cover a large area in eastern Texas, southern Arkansas, and northern Louisiana. The rolling Tertiary Uplands are dominated by commercial pine plantations that have replaced the native oak– hickory–pine forest.

Ecoregion 35a is underlain by poorly-consolidated Tertiary sand, silt, and gravel; it lacks the Cretaceous, often calcareous rocks of the Creataceous Dissected Uplands (35d) and the extensive Quaternary alluvium of Ecoregions 35b, 35g, and 73 (Mississippi Alluvial Plain). In eastern Texas and Louisiana, Tertiary deposits are mostly Eocene clays, silts, and sands, with some Paleocene sediments to the west. Soils areloamy, well-drained, thermic Ultisols or moderately drained Alfisols, typically with sandy and loamy surface textures. In many areas of Ecoregion 35a, commercial pine plantations have replaced the native shortleaf pine–hardwood forest. Prominent native trees include shortleaf pine, loblolly pine, southern red oak, post oak, black oak, white oak, hickories, and sweetgum. American beautyberry, sumac, greenbriar, and hawthorn are common understory species. The sandier areas often have more bluejack oak, post oak, and stunted pines. Scattered, stunted, sandhill woodlands also occur. Grasses in the area include mid and tall grasses such as yellow Indian grass, pinehill bluestem, narrowleaf woodoats, and panicums.

The landscape is dissected by numerous small streams. Waters tend to be stained by organics, thus lowering water clarity and increasing total organic carbon and biochemical oxygen demand levels. Most streams have a sandy substrate and a forest canopy. Many do not flow during the summer or early fall. However, in sandhills, spring-fed, perennial streams occur; here, total dissolved solids, total suspended solids, alkalinity, and hardness values are lower than elsewhere in the South Central Plains. Water quality in forested basins is better than in pastureland. Oil production has lowered stream quality in the south.

Timber production is the dominant land use, along with livestock grazing, poultry production, and oil and gas activities. Major settlements in the region include much of Little Rock; El Dorado, Arkansas; Texarkana; Ruston, Louisiana; and Longview, Tyler, and Nacogdoches in Texas. Protected areas include Caddo Lake National Wildlife Refuge, Poison Springs State Forest, much of Sabine National Forest, and Sabine Wildlife Management Area.

Floodplains and Low Terraces (35b)
The Floodplains and Low Terraces of Ecoregion 35 comprise the western margin of the southern bottomland hardwood communities that extend along the Gulf and Atlantic coastal plain s from Texas to Virginia. Region 35b is nearly level, veneered by Holocene alluvium, and contains natural levees, swales, oxbow lakes, and meander scars. Longitudinal channel gradients are low and are less than in the Ouachita Mountains (ecoregion 36). Soils include Alfisols, Inceptisols, Vertisols, and Entisols and are generally somewhat poorly drained to very poorly drained, clayey and loamy.

Large parts of Ecoregion 35b are frequently flooded. Forested wetlands are characteristic, but pastureland also occurs. Cropland is far less common than in the Red River Bottomlands (35g). Active, meandering alluvial river channels are dynamic systems, with erosion and deposition reworking the topography of levees, ridges, and swales. Overbank flooding, subsurface groundwater, and local precipitation recharge water levels in backswamps, pools, sloughs, oxbows, and depressions of this floodplain region.

Potential natural vegetation is southern floodplain forest as in the Mississippi Alluvial Plain (ecoregion 73); it is unlike the oak–hickory–pine forest of the higher, better drained, and lithologically distinct Tertiary Uplands (35a) and Cretaceous Dissected Uplands (35d). Water oak, willow oak, sweetgum, blackgum, American elm, red maple, Southern red oak, swamp chestnut oak, and loblolly pine are typical. Understory flora include holly, grape genus, poison ivy, crossvine, greenbriar, and a variety of ferns and mosses. Bald cypress and water tupelo occur in semi-permanently flooded areas. Spanish moss hangs in these trees, and floating aquatic plants often occur. On wet flats, backswamps, and swamp margins that are seasonally flooded, overcup oak, water hickory, water elm, sweetgum, green ash, and red maple occur. River banks may contain black willow, sycamore, and eastern cottonwood. Wetness and flooding present severe limitations for agriculture. A few of the higher terraces may have some pasture, but most of the region has deciduous forest land cover. Silviculture activities range from selective tree removal to clearcutting to, in some areas, replacement with pine monoculture. Reservoirs have inundated large areas of this habitat and have altered downstream hydrology. The bottomland forests provide important wildlife habitat with a high diversity of species.

Settlements of the Floodplains and Low Terraces include parts of Beaumont and Lake Charles; typically these narrow and wet regions are less developed. Protected areas include Pond Creek National Wildlife Refuge, Little River National Wildlife Refuge, Felsenthal National Wildlife Refuge, Upper Ouachita National Wildlife Refuge, Sabine Island Wildlife Management Area, and Trinity River National Wildlife Refuge.

Pleistocene Fluvial Terraces (35c)
The Pleistocene Fluvial Terraces are nearly level, poorly-drained, periodically wet, underlain by Pleistocene unconsolidated terrace deposits, and often covered by pine–hardwood flatwoods. The broad flats and gently sloping stream terraces are lower and less dissected than the Tertiary Uplands (35a), but higher than the floodplains of 35b and 35g. Soils are typically well to poorly drained Alfisols and Ultisols with loamy and sandy surfaces. Loblolly pine and oaks are common and are adapted to the prevailing hydroxeric regime; pastureland and hayland are less extensive. A vertical sequence of terraces occurs. The lowest terrace is nearly flat, clayey, and has extensive hardwood wetlands. Higher terraces become progressively older and more dissected; they are dominated by pine flatwoods, pine savanna, prairie, or a mixed forest of pine and oak; flatwood wetlands are less extensive than on the lowest terrace. The mid-level terrace is veneered with windblown silt deposits (loess). Streams tend to be mildly acidic and stained by organic matter. They have more suspended solids, greater turbidity, and higher hardness values than the Tertiary Uplands (35a).

The southernmost piece of 35c, around Marksville, Louisiana, is a Pleistocene terrace with some similarities in soils and historical natural vegetation to that of the Lafayette Loess Plains (34j). The prairie has been converted to agriculture. Terraces are less extensive in Texas and Oklahoma, occurring mostly along the Red River, with some smaller terraces along the Sulphur River. In Texas, current land cover is mostly pine–hardwood forest, with post oak, Shumard oak, and eastern redcedar woods to the west. In Arkansas, loblolly pine is more common on the terraces than shortleaf pine, perhaps influenced by the seasonal wet-dry regime.

Settlements include White Hall, Arkansas; much of Pine Bluff, Arkansas; Sparkman, Arkansas; East Camden, Arkansas; Bastrop, Louisiana; Hermitage, Arkansas; Minden, Louisiana, and Ashdown, Arkansas. Protected areas include Cane Creek State Park, Seven Devils Wildlife Management Area (WMA), Crossett Experimental Forest WMA, and Lafayette WMA (Arkansas).

Cretaceous Dissected Uplands (35d)
The nearly level to hilly Cretaceous Dissected Uplands ecoregion has a greater drainage density than other parts of the South Central Plains.

Native vegetation is largely oak–hickory–pine forest. Today, woods and pastureland are common. Water quality in forested watersheds tends to be good, and is better than in pastureland. Streams generally have lower total dissolved solids values and much lower total organic carbon values than the Tertiary Uplands (35a) and Pleistocene Fluvial Terraces (35c), although turbidity, total suspended solids, and hardness values are slightly higher. Longitudinal stream gradients and Ouachita Mountain influences are greater than in ecoregions 35a or 35c.

Water quality in forested watersheds tends to be good, and is better than downstream of pastureland. Surface waters usually have lower total organic carbon values and slightly higher turbidity, total suspended solids, and hardness values than the Pleistocene Fluvial Terraces (35c). Streams lying in the Muddy Boggy Creek and Blue River watersheds often originate in, and share many aquatic species with, the Arbuckle Uplift (29g); alkalinity, hardness, and conductivity values are influenced by local springs, and increase westward. Streams in the eastern part of the Cretaceous Dissected Uplands drain the more rugged, lithologically distinct Ouachita Mountains (ecoregion 36); aquatic flora and fauna assemblages are influenced by the Ouachitas and are distinct from western streams.

Settlements include (in Arkansas) Arkadelphia; Hope; Lockesburg; (in Oklahoma) Hugo; Bennington; and Caney. Protected areas include Crater of Diamonds State Park, some portions of Ouachita National Forest, and Boggy Depot Park.

Southern Tertiary Uplands (35e)
The Southern Tertiary Uplands ecoregion of Texas and Louisiana generally covers the remainder of longleaf pine range north of the Flatwoods (35f) on Tertiary sediments. The region of Tertiary geology is more hilly and dissected than the Flatwoods (35f) to the south, and soils are generally better drained over the more permeable sediments. Soils are Ultisols and Alfisols with silt loam to loamy sand textures. The Pliocene-age to Eocene-age geology contains a variety of siltstones, sandstones, and calcareous and acidic clays.

Historical vegetation was dominated by longleaf pine–bluestem woodlands, but a variety of forest types were present, including shortleaf pine–hardwood forests, mixed hardwood–loblolly pine forests, and hardwood-dominated forests along streams. On more mesic sites, some American beech or magnolia–beech–loblolly pine forests occurred. Some small, scattered prairies with many rare plants are associated with areas of calcareous clay soils. Some sandstone outcrops of the Catahoula Formation have distinctive barrens or glades in Texas and Louisiana that contain several rare species. Forested seeps in sand hills support acid bog species including southern sweetbay, hollies, wax-myrtles, fetterbush, insectivorous plants, orchids, and wild azalea; this vegetation becomes more extensive in the Flatwoods (35f). Currently, the ecoregion in Texas and Louisiana has more pine forest than the oak–pine and pasture land cover more typical to the north in the Tertiary Uplands (35a). Large parts of the region are public National Forest land, such as Kisatchie National Forest. The Southern Tertiary Uplands ecoregion also covers the majority of Louisiana’s longleaf pine range west of the Mississippi River.

Settlements include (in Louisiana) Chatham; Clarks; Jena; Pineville; Leesville; (in Texas) Jasper; Lufkin; and Conroe. Protected areas include Jimmie Davis State Park, Big Cypress State Park, Esler Field Wildlife Management Area (WMA), Kisatchie National Forest (four large tracts), Alexander State Forest, Fort Johnson North WMA, Clear Creek WMA, the southern portion of Sabine National Forest, Angelina National Forest and Bannister WMA, Davy Crockett National Forest, and Sam Houston National Forest.

Flatwoods (35f)
The Flatwoods ecoregion occurs on mostly flat to gently sloping Pleistocene sediments. The northern boundary is generally along the Hockley Scarp or the division between Pleistocene and older Pliocene/Miocene formations to the north. Soils are poorly to moderately well drained Alfisols and Ultisols with mostly silt loam textures. They are generally more poorly drained than many areas of the Southern Tertiary Uplands (35e) to the north, and tend to be less clayey than in the Southern Subhumid Gulf Coastal Prairies (34b) and Northern Humid Gulf Coastal Praries (34a) to the south. This ecoregion once was dominated by longleaf pine flatwoods and savannas, but also supported other mixed pine–hardwood forest types. The longleaf pine community was characterized by longleaf pine over a great diversity of herbaceous species, with the structure and composition varying from the more open forests of the wet savannas to the more closed forests of the dry flatwoods. Savanna wetlands on the Montgomery Formation and prairie areas on the Beaumont Formation were most likely larger in the Flatwoods than in the Southern Tertiary Uplands (35e) and Tertiary Uplands (35a) to the north. The pine wetland savannas had scattered longleaf pine along with with shrubs of sweetbay, wax myrtle, titi, and holly, and a diverse herbaceous layer of grasses, sedges, and, in some places, insectivorous plants and orchids. These wetland savannas are one of the rarest habitat types in Texas with only a few small fragments receiving any protected status. Other similar sites have been greatly modified or have become overgrown and less diverse.

Flatwood landscapes are characterized by pimple mounds, small hillocks that are abundant across the flats. On steep slopes, along streams and other areas where fire was less frequent, forests contained loblolly pine, sweetgum, white oak, southern red oak, willow oak, blackgum, and hollies. Although greatly reduced from its original extent, longleaf pine continues to characterize a significant portion of the area. Beech–magnolia was not extensive, occurring in narrow areas along some streams and mesic slopes.

This region is warmer, wetter, flatter, less dissected, and lower in elevation than the Tertiary Uplands (35a) and Southern Tertiary Uplands (35e) to the north, with a greater presettlement fire frequency. Streams are low gradient and sluggish. In the eastern part of the region near the boundary with the Red River Bottomlands (35g), more loess occurs, the landscape becomes more dissected, and it supports a mixed hardwood–pine forest. Almost all of the Big Thicket National Preserve is within this region. The area has a long history of modification, particularly by the lumber, railroad, and oil and gas industries that contributed to boom and bust cycles of development and occupance.

Settlements include (in Louisiana) Moss Bluff; DeRidder; (in Texas) Lumberton; Atascocita; Humble; and The Woodlands. Protected areas include Chicot State Park, Marsh Bayou Wildlife Management Area (WMA), West Bay WMA, Big Thicket National Preserve, and Kirby State Forest.

Red River Bottomlands (35g)
The Red River Bottomlands contain the floodplain and low terraces of the Red River within the South Central Plains (ecoregion 35). The region occurs in Texas, Oklahoma, Arkansas, and Louisiana, and includes the highly meandering main channel of the Red River, oxbow lakes, meander scars, ridges, and backswamps.

The Holocene alluvium associated with Red River deposition developed well to somewhat poorly drained Vertisols and Entisols with clayey and loamy, reddish-brown, neutral to calcareous surfaces. The lithology contrasts with the Pleistocene terrace deposits of Pleistocene Fluvial Terraces (35c), and the Tertiary sediments of the Tertiary Uplands (35a). Natural vegetation of the bottomland hardwood forests included trees such as water oak, sweetgum, willow oak, southern red oak, Nutall oak, honey locust, water locust, eastern red cedar, blackgum, blackjack oak, overcup oak, river birch, red maple, green ash, and American elm. There are also some plant distribution differences between the floodplains of ecoregions 35g and 35b. Western species, such as bur oak and Durand oak, were native to the Red River Bottomlands but were typically absent from the Mississippi Alluvial Plain (ecoregion 73).

Currently in the Red River Bottomlands, most of its natural woodland has been cleared for cropland and improved pasture, although some woodland still occurs in very poorly drained and frequently flooded areas. The broad, nearly level bottomlands are often dominated by agriculture, with more cropland than other floodplains of the South Central Plains (ecoregion 35). In Oklahoma, The main crops are soybeans, grain sorghum, alfalfa, corn, and wheat; In Texas, soybeans, sorghum, wheat, and cotton; and in Louisiana, cotton, soybeans, corn, wheat, and rice, with some sugarcane in the southern portion. The Red River carries high silt loads and is almost continuously turbid compared to other rivers of the South Central Plains, although silt load has lessened in Louisiana with the construction of the river's lock and dam system. The Red River historically carried high silt loads and was almost continuously turbid compared to other rivers of Ecoregion 35. With the construction of the lock and dam system throughout its extent in Louisiana, silt load has lessened. The Louisiana portion of the Red River Bottomlands shares similarities in natural vegetation and other biota, hydrology, and land uses with parts of the Mississippi Alluvial Plain (ecoregion 73).

Settlements, all in Louisiana, include Shreveport; the eastern part of Natchitoches; Alexandria; Cheneyville; and Bunkie. Protected areas include Spring Bank Wildlife Management Area (WMA), Soda Lake WMA, John Franks WMA, Red River National Wildlife Refuge (NWR), Loggy Bayou WMA, Bayou Pierre WMA, Elbow Slough WMA, Grand Cote NWR, Lake Ophelia NWR, and the Acadiana Conservation Corridor.

Blackland Prairie (35h)
The level to rolling Blackland Prairie characteristically has dark soils derived from underlying Cretaceous marl, chalk, and limestone. Prairie was common or dominant during and shortly after the Hypsithermal Period in the middle of the Holocene Epoch. By the late 18th century, the Blackland Prairie was a mosaic of woodland, savanna, and prairies. Today, the fertile soils of the Blackland Prairie are mostly used for pastureland, woodland, and hayland. Only a few prairie remnants still occur and are mostly limited to the thin, droughty soils of cuesta scarps.

Settlements include Gurdon, Prescott, McNab, Ozan, Saratoga, and Rocky Comfort. Protected areas include Terre Noire Natural Area Wildlife Management Area (WMA), Ozan WMA, Hope Upland WMA, Rick Evans Grandview Prairie WMA, and Raymond Gary State Park.