The Western Gulf coastal grasslands () are a subtropical grassland ecoregion of the southern United States and northeastern Mexico. It is known in Louisiana as the "Cajun Prairie", Texas as "Coastal Prairie," and as the Tamaulipan pastizal () in Mexico.
Setting The ecoregion covers an area of , extending along the shore of the Gulf of Mexico from southeastern Louisiana (west of the Mississippi Delta) through Texas and into the Mexican state of Tamaulipas as far as the Laguna Madre. Specific areas include a number of barrier islands, and the resacas or natural levees of the Laguna Madre. The coast is vulnerable to tropical storms that can seriously damage habitats.
This ecosystem, in part, has edaphic origins: the soils of this ecoregion feature poor internal drainage, often under alfisols and vertisol. In combination with frequent fires, tree seedlings have a harder time settling, allowing grasses and other herbaceous species to dominate. This process is understood in the context of "subtropical hyperseasonal grasslands", similar to the Florida dry prairie.
However, there are interspersed areas of higher sand and silt content that break up the otherwise heavy clay environment: these areas typically correspond to floodplain/riparian zones, and are more permissive of tree-growth, featuring galleries or even closed-canopy bottomland expanses.
Climate From Southwest Louisiana west to the Upper Texas coast, the climate is wet humid subtropical, featuring significal annual precipitation. The climate becomes more arid farther south along the Texas coast into northeastern Mexico, though precipitation totals still remain high enough for the humid subtropical classification.
Flora
The natural habitat of the area is a mix of tallgrass prairie similar to those found in inland Texas, with Indiangrass (Sorghastrum nutans), big bluestem (Andropogon gerardi), little bluestem (Schizachyrium scoparium), and switchgrass (Panicum virgatum) the primary tallgrass species that are typical of the coastal prairie, with several other shorter grasses and many herbaceous and woody species.
Separating the grassier habitats are bottomland forests and woodlands, which are present within the floodplains of the region's many waterways. In the wetter climate covering the zone from Southwest Louisiana west through the upper Texas coast, these bottomlands contain many species typical in forests elsewhere across the Southern United States, such as the southern live oak, bald cypress, magnolia, loblolly pine, post oak, and southern hackberry. Farther south, from the lower one third of the Texas coast and through the Tamaulipan stretch, the drier climate supports shrubby areas of honey mesquite (Prosopis glandulosa), huisache (Vachellia farnesiana var. farnesiana), lime prickly-ash (Zanthoxylum fagara), and Texas persimmon (Diospyros texana). Ferns such as the Christmas Fern (Polystichum acrostichoides), Cinnamon Fern (Osmundastrum cinnamomeum) and the Bracken are all found here.
Fauna
This coast is rich in wildlife, and 700 species of birds, animals and reptiles have been counted here, although many are now threatened or endangered. This coast is a critical habitat for the Attwater's prairie chickens (Tympanuchus cupido attwateri), over one million of which inhabited the prairie in Texas and Louisiana in the 19th century, but extreme reduction of their habitat put them on the U.S. endangered species list in 1967. Another endangered bird of the coast is the whooping crane (Grus americana). Many species of wading birds, shorebirds, and other waterbirds are abundant. Birds in the Mexican part of the region include Morelet’s seedeater (Sporophila morelleti), red-billed pigeon (Columba flavirostris), brown jay (Cyanocorax morio), Neotropic cormorant, white-winged dove (Leptotila verrequxi) and Audubon's oriole (Icterus graduacauda).
Mammals of the area include bobcats, collared peccary, white-tailed deer, eastern cottontails, with ocelot (Leopardus pardalis), Gulf Coast jaguarundi (Puma yagouaroundi cacomitli), southern yellow bat (Lasiurus ega), and Mexican spiny pocket mouse (Liomys irroratus) more abundant in Mexico. Rancho Nuevo beach in Tamaulipas and along the Texas coast are the only nesting sites in the world for the Kemp's ridley sea turtle (Lepidochelys kempii) while other herpetofauna of the southern part of the ecoregion include Río Grande chirping frog (Eleutherodactylus cystignathoides) and Mexican white-lipped frog (Leptodactylus fragilis).
Threats and preservation Less than 1% of the ecoregion remains in pristine condition, almost entirely in Texas, while most of the coast has been converted to farmland, including rice paddies, grazing land, or urban areas including Houston, Texas. Estuaries and other coastal wetlands are better preserved than the prairie and indeed the protected areas of the coast are mainly sanctuaries for waterbirds.
Human use Largely because of the region's flatness and grassland potential, a higher percentage of the land is in cropland than in bordering ecological regions. Rice, grain sorghum, cotton, and soybeans are the principal crops. Urban and industrial land uses have expanded greatly in recent decades, and oil and gas production is common.Griffith, G.E., Bryce, S.A., Omernik, J.M., Comstock, J.A., Rogers, A.C., Harrison, B., Hatch, S.L., and Bezanson, D., 2004, Ecoregions of Texas (color poster with map, descriptive text, and photographs): Reston, Virginia, U.S. Geological Survey (map scale 1:2,500,000). ; . Accessed 2024-03-25.
EPA Level IV ecoregions
The Western Gulf Coastal Plain is a Level III ecoregion in the US Environmental Protection Agency's scheme of US ecoregions. It is roughly coextensive with the Western Gulf coastal grasslands, with the important exception that the EPA-defined area terminates at the national boundary, while the natural ecoregion extends into northeastern Mexico. In the EPA's definition, the principal distinguishing characteristics of the Western Gulf Coastal Plain are its relatively flat topography and mainly grassland potential natural vegetation. Inland from this region the plains are older, more irregular, and have mostly forest or savanna-type vegetation potentials. The EPA identifies ten Level IV ecoregions: Northern Humid Gulf Coastal Prairies; Southern Subhumid Gulf Coastal Prairies; Floodplains and Low Terraces; Coastal Sand Plain; Lower Rio Grande Valley; Lower Rio Grande Alluvial Floodplain; Texas–Louisiana Coastal Marshes; Mid-Coast Barrier Islands and Coastal Marshes; Laguna Madre Barrier Islands and Coastal Marshes; and Lafayette Loess Plains.
Northern Humid Gulf Coastal Prairies (34a)
Quaternary-age deltaic sands, silts, and clays underlie much of the Northern Humid Gulf Coastal Prairies on this gently sloping coastal plain. The original vegetation was mostly grasslands with a few clusters of oaks, known as oak mottes or maritime woodlands. Little bluestem, Indiangrass, brownseed paspalum, gulf muhly, and switchgrass were the dominant grassland species, with some similarities to the grasslands of the Texas Blackland Prairies (ecoregion 32). Almost all of the coastal prairies have been converted to cropland, rangeland, pasture, or urban land uses. The exotic Chinese tallow tree and Chinese privet have invaded large areas in this region. Some loblolly pine occurs in the northern part of the region in the transition to the South Central Plains (ecoregion 35). Soils are mostly fine-textured: clay, clay loam, or sandy clay loam. Within the region, there are some differences from the higher Lissie Formation to the lower Beaumont Formation, both of Pleistocene age. The Lissie Formation has lighter colored soils, mostly Alfisols with sandy clay loam surface texture, while darker, clayey soils associated with Vertisols are more typical of the Beaumont Formation. Annual precipitation varies from in the southwest portion to in the northeast, with a summer maximum.
Southern Subhumid Gulf Coastal Prairies (34b) The Southern Subhumid Gulf Coastal Prairies ecoregion is drier than Ecoregion 34a to the north, not only receiving less annual precipitation, but also typically experiencing summer drought. Annual precipitation ranges from in the southwest to in the northeast, with May and September peaks. Soils are hyperthermic compared to thermic in most of Ecoregion 34a. Little bluestem, yellow Indiangrass, and tall dropseed were once dominant grasses. Eragrostoid grasses, including the genera Bouteloua, Buchloe, Eragrostis, Hilaria, and Setaria increase in importance in Ecoregion 34b compared to 34a. Invasive species such as honey mesquite and huisache are a concern. Almost all of the coastal prairies have been converted to other land uses: cropland, pasture, or urban and industrial.
Floodplains and Low Terraces (34c)
Covering primarily the Holocene floodplain and low terrace deposits, the Floodplains and Low Terraces ecoregion, especially to the southwest, has a different bottomland forest than the floodplains of the South Central Plains (ecoregion 35). Bottomland forests of pecan, water oak, southern live oak, and elm, are typical, with some bald cypress on larger streams. The Brazos and Colorado River floodplains are a broad expanse of alluvial sediments, while floodplains to the south are more narrow. Soils include Vertisols, Mollisols, and Entisols. Large portions of floodplain forest have been removed and land cover is now a mix of forest, cropland, and pasture.
Coastal Sand Plain (34d) The Coastal Sand Plain ecoregion provides a distinct break in both vegetation and surficial materials from the fine-textured soil grasslands of the Southern Subhumid Gulf Coastal Prairies (34b) to the north. This sand sheet landscape consists of active and (mostly) stabilized sand dune deposits with lesser amounts of silt sheet deposits (silt and fine sand) to the north. This depositional plain is characterized by a closed internal drainage system with only occasional discontinuous drainage remnants due to sand movement. Closed depressions pond water in response to seasonal and tropical storm precipitation. Soils developed on these parent sediments are Entisols and Alfisols with thick sand surfaces. The dominant grasses on the coastal sand ridges and islands extend inland covering parts of the sand plain. Vegetation is mostly mid and tall grasses such as seacoast bluestem (Schizachyrium littorale), switchgrass, gulfdune paspalum (Paspalum monostachyum), fringeleaf paspalum, sandbur, purple threeawn, pricklypear, and catclaw with an overstory of southern live oak and honey mesquite trees. The potholes have a variety of bulrushes and sedges. Most of the Coastal Sand Plain has been moderately to heavily grazed, and large areas have been converted to non-native range or pasture grasses. The region has little cropland compared to the Southern Subhumid Gulf Coastal Prairies (34b).
Lower Rio Grande Valley (34e)
The Lower Rio Grande Valley ecoregion once supported dense, diverse grassland and shrub communities and low woodlands. However, mesquite, granjeno (spiny hackberry), and a variety of brush and shrub species invaded the landscape. Now, it is almost all in cropland, pasture, and urban land cover. The region is underlain by a mix of Quaternary clays and sands with some Miocene-age sediments of the Goliad Formation at the western edge. Mollisols are extensive, and the soils are deep, mostly clay loams and sandy clay loams. The freeze-free growing season is often over 320 days compared to 250–260 days along the northern Texas coastal area of the Northern Humid Gulf Coastal Prairies (34a). Along with the Lower Rio Grande Alluvial Floodplain (34f), the Lower Rio Grande Valley contains important nesting grounds for the white-winged dove, a favored hunting species in southern Texas.
Lower Rio Grande Alluvial Floodplain (34f)