The West Gulf Coastal Plain includes western Louisiana, eastern Texas, southeastern Oklahoma, and southern Arkansas. It is the western edge of the Austroriparian Sub-Province, which covers most of the southeastern United States. At about 95° or 96° west longitude (roughly central Texas), the eastern flora begins to give way -- over a 300 km floristic transition -- to the western one. By 99° west, the transition is virtually complete and the flora is western.
Physiographically the West Gulf Coastal Plain is part of the Coastal Plain Province, which consists of most of the North American geologic coastal plain, which stretches from Massachusetts to Texas to northern Veracruz. The coastal plain is an elevated sea bottom of low relief and rises inland to only about 250 meters. The Mississippi Embayment is a northward extension that occurs to southern Illinois.
The West Gulf Coastal Plain is divided into Tertiary uplands and Quaternary lowlands. Hilly topography in the north gives way to flat terraces near the coast. Many rivers cut through the area northwest to southeast: to the east, the Mississippi and Atchafalaya Basin; westward the Red, Ouachita, Calcasieu, Sabine, Neches, Trinity, San Jacinto, and Brazos rivers.
Precipitation ranges from 30 inches in the west to 60 inches in the east. Humidity is high, winters mild and summers hot.
THE FLORA
The West Gulf Coastal Plain has an eastern, notably southeastern, flora of about 3900 species, and is dominated by pine, oak and hickory forest.
VEGETATION REGIONS
The West Gulf Coastal Plain has, traditionally, been divided into four regions.
Oak-Pine-Hickory Forest predominates in the uplands in southern Arkansas, northern Louisiana, southeastern Oklahoma, and northeastern Texas. Many early accounts suggest that short leaf pine dominated drier areas, often in pure stands, and that loblolly pine was the predominant conifer on lower, more mesic sites. However, most forests in the oak-pine-hickory region were dominated by oaks and monospecific pine areas were rare.
Longleaf Pine Forest is by far the best studied section of the West Gulf Coastal Plain. While this region has become typed as upland, open, longleaf pine savanna, a multiplicity of other communities exist within it.
Post Oak Savanna of the West Gulf Coastal Plain is characterized largely by an absence of pine and by the presence of oak-dominated forest. This area has received desultory attention over the years, but there is an increasingly substantial literature on it, notably on the xeric sandylands and muck bogs. Post oak woodlands, which characterize the area, also occur in other parts of the West Gulf Coastal Plain, notably in drier hill top sites.
Prairie that extends from Texas to Canada and covers most of central North America forms the western, relatively dry, fringe of the West Gulf Coastal Plain. Prairies, however, were not confined to its edge and, ranging in size from one to hundreds of hectares, once occurred in most counties and parishes in the West Gulf Coastal Plain.
These areas are not floristically distinct and really refer to perceived differences based on tree species. For example, the longleaf pine region is defined by the presence of longleaf pine.
REFERENCES
MacRoberts, M.H. and B.R. MacRoberts. 2003. West Gulf Coastal Plain ecoregions. Sida 20:1247-1276.
MacRoberts, M.H. and B.R. MacRoberts. 2004. The post oak savanna ecoregion: a floristic assessment of its uniqueness. Sida 21:399-407.
Diggs, G.M., B.L. Lipscomb, M.D. Reed, and R.J. O’Kennon. 2006. Illustrated flora of east Texas. Sida, Bot. Misc. 26:1-1594.
