Ecoregions of Texas, AS-199 - Texas Commission on Environmental ...

Level IV Ecoregion24a. Chihuahuan Basins and PlayasArea (sq. mi.) 12,625Deep depressions or grabens filled with sediment to form flat to rolling basins. BasinsPhysiographyeither alluvial basins surrounding major river (Pecos, Rio Grande) or internallydrained (Salt Basin). Streams ephemeral.Elevation /1200-4500 /Local Relief (feet)25-500Upper Pecos, Hueco, and Salt Basins: Holocene, Pleistocene, and late Tertiaryalluvium and erosional materials from surrounding mountains, includingSurficial Geology;unconsolidated basin deposits, silt, sand, and gravel. Lower Pecos: Holocene andBedrock GeologyPleistocene limestone residuum and colluvium; Upper and Lower Cretaceouslimestones.Mollisols (Calciustolls), Aridisols (Haplocalcids, Petrocalcids, Calcigypsids),Soil Order (Great Groups)Entisols (Torriorthents, Torripsamments)Basins, fans, and footslopes: Langtry, Zorra, Lozier, Shumla (lower Pecos R. valley),Common Soil SeriesHueco, Wink (Hueco Basin), Delnorte, Nickel, Reakor, Hoban, Upton (Pecos Riverbasin). Salt Basin alkali flats: Holloman, Reeves. Sand sheets and dunes: Bluepoint.Soil Temperature /Thermic, Hyperthermic/Soil Moisture Regimes Aridic, Ustic AridicMean Annual Precipitation (in.) 8-14Mean Annual Frost Free Days 220-250Mean Temperature (F) 25/60;(Jan. min/max; July min/max) 67/97Saline flats and alkaline playa margins: fourwing saltbush, seepweed, pickleweed,and alkali sacaton. Gypsum land: gyp grama, gyp mentzelia, and Torrey ephedra.VegetationDesert shrub land: creosote bush, tarbush, yuccas, sandsage, blackbrush, tasajillo,lechuguilla, and ceniza.Shrubland, remnant grassland, or barren land. Wildlife habitat, limited grazing, someLand Cover and Land Useurban and military land, some irrigated cropland.24b Chihuahuan Desert GrasslandsThe Chihuahuan Desert Grasslands occur in areas <strong>of</strong> fine-textured soils, such assilts and clays, that have a higher water retention capacity than coarse-textured, rockysoil. The grasslands occur in areas <strong>of</strong> somewhat higher annual precipitation (10 to 18inches) than the Chihuahuan Basins and Playas (24a); these include elevated basinsbetween mountain ranges, low mountain benches and plateau tops, and north-facing highmountain slopes where natural erosion or the effects <strong>of</strong> overgrazing have not removedthe soil. Desert grasslands also occur at lower elevations in areas <strong>of</strong> denser clay soil or where the water tableis near the surface, for example, in dry lake basins (playas) and alluvial riparian areas. In his 1857 report toCongress, William Emory made regular observations on the grasslands his expedition encountered duringthe U.S./Mexico Boundary Survey. Since Emory’s time, many <strong>of</strong> these grasslands in West <strong>Texas</strong> have beenreplaced by desert shrubland (Gehlbach <strong>199</strong>3). The evidence suggests that grazing levels in the late 19th andearly 20th centuries were unsustainable, and desert shrubs invaded the fragmented grass cover. Effectivemanagement strategies for grasslands take into account their fragile and erosive nature.The character <strong>of</strong> grasslands and their species assemblages differ between semidesert, mid-elevation, andmountain grasslands. In low elevation semidesert areas (2000 to 4500 feet) with lower rainfall, the arealcoverage <strong>of</strong> grasses may be sparse, 10% or less. Typical species include black, blue, and sideoats grama(Bouteloua spp.), bush muhly (Muhlenbergia porteri), beargrass (Nolina arenicola), and galleta (Pleuraphisjamesii), with scattered creosotebush (Larrea tridentata), acacias (Acacia spp.) and cacti (Opuntia spp.).When the grass cover becomes fragmented, it may be replaced by burrograss (Scleropogon brevifolius).Burrograss can only be eaten by cattle when young, and it spreads more aggressively because it can set seedeven in the driest weather when other grasses are dormant (Cottle 1931). At the same elevation range, in valleybottoms and ancient lake beds with a high water table and clay soil, gramas are much less prevalent. The10<strong>Ecoregions</strong> <strong>of</strong> <strong>Texas</strong>