The Tamaulipan mezquital (Spanish: Mezquital Tamaulipeco), also known as the Brush Country, is a deserts and xeric shrublands ecoregion in the Southern United States and northeastern Mexico. It covers an area of 141,500 km2 (54,600 sq mi), encompassing a portion of the Gulf Coastal Plain in southern Texas, northern Tamaulipas, northeastern Coahuila, and part of Nuevo León.
Distribution The Sierra Madre Oriental range to the west separates the Tamaulipan mezquital from the drier Chihuahuan Desert. The Tamaulipan matorral is a transitional ecoregion between the mezquital and the Sierra Madre Oriental pine-oak forests to the west and the Veracruz moist forests to the south. The Western Gulf coastal grasslands, known as the Tamaulipan pastizal south of the border, fringe the Gulf of Mexico. The Edwards Plateau savannas lie to the north, and the East Central Texas forests and Texas blackland prairies to the northeast.
Geography
The Bordas Escarpment transects this ecoregion, running from near the southern limit northwest to the vicinity of Rio Grande City, Texas, arching back northeast to the vicinity of Choke Canyon Reservoir in Texas. The region east of the Bordas Escarpment is flat with deep, sandy soils, of which vast areas both north and south of the border have been converted to agricultural use. West of the Bordas Escarpment the topography changes to gently rolling hills with a thin layer of high calcium soil over a thick layer of caliche. Some large portions of the west are poorly suited for cultivation but good for ranching and excellent wildlife habitat. A series of Cenozoic strata of limestone, sandstone, siltstone, and claystone occur throughout most of the region. These strata are slightly tilted, gently slanting seaward, with increasingly recent exposures to the east, including some Quaternary bodies at the coast transitioning into the sea, and a few older Cretaceous exposures and outcrops appearing in some far western interior areas. Elevations generally range from near sea level to 300 meters, transitioning at ca. 300–500 meters into the Sierra Madres, Edwards Plateau, and Chihuahua Desert in western areas. A series of small mountain ranges occur in the region, isolated on the coastal plain between the Sierra Madre Oriental and the Gulf of Mexico. These ranges are not part of the strata of the Gulf Coast Plain or the complexly folded and faulted Jurassic and Cretaceous formations of the Sierra Madre Oriental. The Sierra de San Carlos, (and Sierra de Tamaulipas just beyond the southern limit of this ecoregion) are plutons, igneous rock intrusions of nepheline syenite to gabbroic composition, emplaced during the Mid-Tertiary with a few minor sierras of alkaline basalt flows occurring locally (e.g. in the vicinity of Llera, Tamaulipas). The Coastal Sand Plain, sometimes referred to as the "Wild Horse Desert", is a region of Quaternary sand deposits extending about 60 miles inland from the Laguna Madre. It is bordered by Baffin Bay to the north and the Lower Rio Grande Valley to the south. Quaternary to Tertiary marine sedimentary strata, which may be exposed in western areas, underlie the sand sheet. The sand sheet is six feet deep in many places with maximum dune elevations reaching 30 feet. Although the area is generally flat, the unstable sands often form a dune and swale topography where vegetated dunes are interspersed with active, unstable, wind-blow dunes. The active dunes attain areas as large as 250 acres, predominantly in the eastern third. Grasslands cover much of the sand plain where seacoast bluestem (Schizachyrium littorale) is dominant in all but the wettest areas. These grasslands are banded with live oak (Quercus virginiana) groves, forming dense forest in spots, and smaller honey mesquite (Prosopis glandulosa) mottes in discontinuous belts which are sometimes connected with one another and collectively cover at least a quarter of the sand sheet. Numerous ephemeral, internally drained ponds form where subsidence and blow-outs occur. Hydrology: The region is generally semi-arid however, various wetlands are present. The Laguna Madre is one of only five hypersaline coastal lagoons (or negative estuaries) in the world and a significant ecosystem in itself, laying between the Gulf of Mexico and the Tamaulipan mezquital ecoregion. As one moves inland from the hypersaline lagoon, a relatively narrow band of freshwater marshlands are encountered running north to south, albeit occurring fragmented and irregularly. These marshes are dynamic, fluctuating with rainfall and tropical storm activity, and alternating with wet and dry periods. When and where present, these marshlands can be locally, surprisingly extensive with deep marshes and both permanent and ephemeral freshwater ponds that support populations of salamanders (Siren intermedia), turtles (Trachemys), and millions of migrating waterfowl, such as redheads (Aythya americana) in the winter.
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