Chapter 39
Ecological Subregions of the United States



Chihuahuan Semi-Desert

Two Sections have been delineated in this Province:

These Sections are located in the southwestern conterminous States, including parts of Arizona, New Mexico, and Texas. The area of these Sections is about 85,200 mi2 (220,700 km2).

Section 321A--Basin and Range

Geomorphology. This area, which is in the Basin and Range physiographic province, is located in southeast Arizona and southwest and central New Mexico. Relatively recent episodes of continental rifting, volcanism, erosion, and sedimentation have dominated this Section. Oligocene faulting created the Rio Grande rift in New Mexico and west Texas and initiated volcanism. Subsequent Miocene composite volcanoes emitted silicic lava and ash. Along with Pliocene and Pliestocene mass wasting and cyclic erosion events, and associated with glacial cycles farther north, this combination of processes gradually filled the basins with deep sediments from adjacent mountain ranges. Current erosion cycles dissect these deposits and continue to modify the rift valley through transport and deposition processes. Various landforms comprise about equal areas: (1) plains with low mountains consisting of 50 to 80 percent of gently sloping area and local relief of 1,000 to 3,000 ft; (2) plains with high hills where relief is 1,000 to 3,000 ft; (3) open high hills with relief of 500 to 1,000 ft; and (4) tablelands with moderate relief averaging 100 to 300 ft. Elevation ranges from 2,600 to 5,500 ft (800 to 1676 m).

Lithology and Stratigraphy. Geologic strata consist of an undifferentiated mixture of Quaternary marine deposits, Miocene volcanic rocks, lower Tertiary volcanic rocks, and Lower Cretaceous marine deposits; Permian marine deposits of Ochoan and Guadalupian series; Paleocene continental deposits; Upper Cretaceous marine deposits; Precambrian plutonic and intrusive granitic rocks; Quarternary volcanic rocks; Permian continental deposits of Wolcampian age, and Miocene felsic volcanic rocks; upper Paleozoic marine deposits; Precambrian sedimentary rocks of Pahrump and Unkar groups; Precambrian Mazatal quartzite, Yavapai series, pinal schist, and metavolcanic formations.

Soil Taxa. Types are mostly Torriorthents with Calciorthids, Haplargids, and some Alfisols (10 percent) and Mollisols (10 percent) with a thermic temperature regime, an aridic moisture regime, and mixed or carbonatic mineralogy.

Potential Natural Vegetation. K\"uchler mapped vegtation as trans-Pecos shrub savanna ({\it Flourensia-Larrea}); grama-tobosa desert grasslands; oak-juniper woodland; and mesquite-tarbush desert scrub.

Fauna. Reserved.

Climate. Precipitation ranges from 8 to 13 in (200 to 320 mm); it occurs mostly during July and August. Temperature ranges from 55 to 70 oF (13 to 20 oC) and winters are mild. The growing season lasts 200 to 240 days.

Surface Water Characteristics. There is a low density of intermittent streams and very few associated rivers, most of which originate in distant mountainous areas. Flow rates are low to moderate, except during periods of heavy rain, when large amounts of surface runoff can occur. Dendritic drainage pattern has developed on dissected mountain slopes, largely without bedrock structural control. Playa lakes are common following periods of rains, but are ephemeral in the hot, dry climate prevalent in this Section.

Disturbance Regimes. Drought has probably been the principal historical source of disturbance.

Land Use. Land use includes range for cattle grazing on about 90 percent of the area.

Cultural Ecology. The Basin and Range Section is a physiographically diverse area characterized by expansive playas and open grassland basins cut by steep, rugged mountain, mesa, and canyon terrain. Humans have been utilizing the area for 8,000 to 10,000 years, although evidence of occupation prior to 7,000 B.C. remains scarce and scattered. Paleo-Indian materials are especially prevalent, however, from the foothills of the Tularosa Mountains. The area was widely utilized by Cochise and Oshara Tradition Archaic populations between 7,000 B.C. and 200 A.D. Site distribution points to a highly mobile hunting and gathering nomadic subsistence pattern initially, followed by use of increasingly smaller areas and a seasonal cycle of upland and lowland exploitation. Puebloan use and occupation were most prevalent between 200 and 1150 A.D. in the south and 200 and 1400 A.D. in the north. Southern basin, range, and mountain areas supported the Mogollon culture, while more northern mountain areas also included the southern fringe of the Anasazi tradition. Puebloan settlement reflected gradual movement toward major drainages and waterways over time. Basin and range deserts were widely used for wild plant procurement, agriculture, and settlement.

References to the Apache appear in 16th century Spanish documents and later historic accounts. Spanish expeditions passed through the area, but major settlements were restricted to the Rio Grande and the area east of the Mogollon and Tularosa Mountains. Livestock ranching and mining gained prominence in the 1800's. Gold, silver, copper, and turquoise were mined in the Mogollon, Burro, and Black Range Mountains of New Mexico. Introduction of the railroad in the 1800's witnessed an influx of European settlement along the Rio Grande, the southern Burro Mountains (Deming, Lordsburg, and Silver City, New Mexico) and more northern reaches of the Mogollon Mountains. In more northern, remote mountain areas, small ranching, mining, and timber-related settlements were established along major rivers and ephemeral drainages. Ranching and tourism flourish in the area today, and both Anglo and Hispanic cultures influence contemporary life.

Compiled by Southwestern Region.


Section 321B--Stockton Plateau

Geomorphology. This Section is in the Great Plains geomorphic province. The predominant landform consists of open high hills with smaller areas of tablelands. These landform were formed by fluvial sedimentation of continental erosional products from adjacent mountain ranges, which was followed by sheet erosion and transport. These processes resulted in a region of shallow dissection. Elevation ranges from 2,600 to 4,500 ft (800 to 1,300 m). Local relief in most of the Section ranges from 500 to 1,000 ft. Relief in a small area of tablelands ranges from 300 to 500 ft.

Lithology and Stratigraphy. Rocks were formed during Paleozoic (35 percent), Mesozoic (40 percent), and Cenozoic (25 percent) Eras. Paleozoic strata consist of Pennsylvanian marine deposits. Mesozoic strata consist of nondifferentiated mixture of Lower and Upper Cretaceous marine deposits (limestone, and sandstone). Cenozoic strata consist of lower Tertiary volcanic rocks of high alkalic content.

Soil Taxa. Soils are Argids and Orthids. Haplargids, Paleargids, and Calciorthids are on uplands, piedmont plains, and dissected terraces. Calciorthids, Ustolls, and Torriorthents are on uplands with shallow depths to bedrock. Paleorthids are on mesas and terraces. Gypsiorthids are in closed basins. Natragids and Torrerts are on basin floors. Torrifluvents are on flood plains and Torripsamments are on sandy uplands. These soils have a thermic temperature regime, aridic moisture regime, and mixed or carbonatic mineralogy. Soils are well drained, shallow to deep, and medium textured. Soil moisture is limited for use by vegetation during most of the growing season.

Potential Natural Vegetation. K\"uchler classified vegetation as trans-Pecos shrub savanna ({\it Flourensia-Larrea}); with juniper and redcedar woodlands. The predominant vegetation form is short to mid height grasslands with sparse cover of drought-deciduous and scale-leaved shrubs and small trees. Species include desert shrubs in association with short to mid height grasses and oak savannas.

Fauna. Typical large to medium size herbivores and carnivores include pronghorn, coyote, swift fox, ringtail, hooded skunk, ocelot, and collared peccary. Smaller herbivores include desert shrew, desert cottontail, Mexican ground squirrel, yellow-faced pocket gopher, Nelson's pocket mouse, and Merriam's kangaroo rat. Several bats, western mastiff and yuma myotis, are present here. Birds of grasslands include bronzed cowbird, Baird's sparrow, and white-necked raven. Birds of thickets include black-capped vireo, scaled quail, Harris' hawk, Inca dove, cave swallow, golden-fronted woodpecker, and pyrrhuloxia. Amphibians include Couche's spadefoot toad, western spadefoot toad, Rio Grande leopard frog, Great Plains toad, red-spotted toad, spotted chirping frog, and Mexican mud turtle. Reptiles include Texas banded gecko, Big Bend gecko, desert spring lizard, canyon lizard, crevice spiny lizard, gray checkered whiptail, little striped whiptail, plateau spotted whiptail, checkered whiptail, Texas-Pecos rat snake, gray-banded kingsnake, Big Bend patch-nosed snake, Mexican black-nosed snake, Big Bend black-headed snake, rock rattlesnake, and black-tailed rattlesnake.

Climate. Precipitation ranges from 8 to 13 in (200 to 320 mm). Temperature ranges from 55 to 64 oF (13 to 18 oC). The growing season lasts 200 to 240 days.

Surface Water Characteristics.This section has a low density of intermittent streams that originate in nearby mountainous areas and flow mainly following rains. Major river systems include the Rio Grande and Big Canyon. Flow rates are low except during periods of heavy rain, when large amounts of surface runoff can occur. Dendritic drainage pattern has developed. Playa-type lakes are present following rains but quickly dry up, leaving high salt concentrations.

Disturbance Regimes. This section is part of the Chihuahuan Desert and drought has been the principal disturbance.

Land Use. Cattle grazing occurs on about 90 percent of the area.

Cultural Ecology. Reserved.