Chihuahuan Desert Loamy Plains Grassland

This ecological system occurs in the northern Chihuahuan Desert and extends into limited areas of the southern Great Plains on alluvial flats, loamy plains, and basins sometimes extending up into lower piedmont slopes. Although there is some localized topography with hills and bluffs, sites are typically flat or gently sloping to moderately steep and may be somewhat mesic if they receive runoff from adjacent areas, but these are not wetlands or mesic, bottomland grassland. Soils are non-saline, finer textured loams or clay loam. Vegetation is characterized by perennial grasses and is typically dominated by Pleuraphis mutica or with Bouteloua eriopoda codominant (more historically) or Bouteloua gracilis. In degraded stands, Scleropogon brevifolius, Dasyochloa pulchella, or Aristida spp. may codominate. Pleuraphis jamesii may become important in northern stands and Bouteloua gracilis in the Great Plains and on degraded stands. If present, mesic graminoids such as Pascopyrum smithii, Panicum obtusum, Sporobolus airoides, and Sporobolus wrightii typically have low cover and are restricted to drainages and moist depressions (inclusions). Scattered shrubs such as Ephedra torreyana, Flourensia cernua, Gutierrezia sarothrae, Larrea tridentata, Cylindropuntia imbricata, Prosopis glandulosa, and Yucca spp. may be present, especially on degraded sites.

The vegetation in examples of this system is characterized by perennial grasses and is typically dominated by Pleuraphis mutica or with Bouteloua eriopoda codominant (more historically before heavy grazing reduced this over much of the range) or Bouteloua gracilis. In degraded stands, Scleropogon brevifolius, Dasyochloa pulchella (= Erioneuron pulchellum), or Aristida spp. may codominate. Pleuraphis jamesii may become important in northern stands and Bouteloua gracilis in the Great Plains and on degraded stands. If present, mesic graminoids such as Pascopyrum smithii, Panicum obtusum, Sporobolus airoides, and Sporobolus wrightii typically have low cover and are restricted to drainages and moist depressions (inclusions). Scattered shrubs such as Ephedra torreyana, Flourensia cernua, Gutierrezia sarothrae, Larrea tridentata, Cylindropuntia imbricata (= Opuntia imbricata), Prosopis glandulosa, and Yucca spp. may be present, especially on degraded sites. In west Texas, this system includes two somewhat distinct grassland types: one on loams of intermountain basins, and the other on foothills and shallow soils at the basin edges. These types are often closely juxtaposed and share graminoid composition but differ in abiotic sites, aspect, and invading shrubs. The loamy grasslands are dominated by species such as Bothriochloa barbinodis, Bothriochloa laguroides ssp. torreyana, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Dasyochloa pulchella, Pleuraphis mutica, and Scleropogon brevifolius. These grasslands occur in extensive level plains with deep soils. Prosopis glandulosa is the common shrub invader. Other shrubs present to dominant as invaders include Larrea tridentata, Flourensia cernua, and Mimosa aculeaticarpa var. biuncifera. The foothill grasslands are of similar composition with respect to grasses, but occupy rolling landscapes at slightly higher elevations and are on shallow soils. Condalia ericoides, Juniperus spp., and Acacia constricta are common invaders.

These upland grasslands occur at approximately 1150-2320 m (3500-7610 feet) elevation and are found on various sedimentary and igneous substrates, including alluvial flats, loamy plains, and desert basins sometimes extending up into lower piedmont slopes including mesatops. Sites are typically flat or gently sloping so precipitation does not run off and may be somewhat mesic if they receive runoff from adjacent areas, but these are not wetlands or bottomland grasslands described in Chihuahuan-Sonoran Desert Bottomland and Swale Grassland (CES302.746). Annual precipitation is usually from 20-40 cm (7.9-15.7 inches). Soils are non-saline, finer-textured loams or clay loam that are often derived from sedimentary parent materials but are quite variable and may include fine-textured soils derived from igneous and metamorphic rocks. These grasslands can occur on a variety of aspects and slopes ranging from flat to moderately steep. When they occur near foothill grasslands, they will be at lower elevations (Landfire 2007a). In Texas, this system occurs primarily on Quaternary alluvium but is also found on other formations at higher elevations of mountain foothills. Two somewhat distinct areas are loams of the intermountain basins, and foothill grasslands over shallow soils at the basin edges. The foothill grasslands often occupy Shallow Ecological Sites over Perdiz Conglomerate, but may also occur on gravelly sites.

Historic fire frequency in this ecosystem is not known, but is likely less frequent than other denser desert grasslands because of less fuel in this typically open grassland ecosystem (Humphrey 1963). The effects of burning tobosa-dominated grasslands is variable depending upon soil moisture and plant phenology at the time of the fire, precipitation in the months following the fire, and site characteristics that influence soil moisture availability, and fire intensity based on research in the Great Plains (Innes 2012). However, the dominant grass Pleuraphis mutica is likely to survive most fires and can sprout from rhizomes and grow quickly after top-kill by fire (Britton and Steuter 1983).

These grasslands are prone to flooding during high precipitation events because of slow infiltration. This may result in overland flow and erosion of topsoil and some short-term loss of vegetative cover. Landfire (2007a) modeled this system and predicted that during a >500-year flooding event in a swale or stream channel, sites could downcut, thus lowering the water table, and favor woody species in an altered state. Drought cycles likely resulted in a reduction in vegetative cover and production of these sites (Landfire 2007a). Annual growth of woody vegetation depends on annual rainfall; drought negatively affected woody species. Cyclic drought impacts vegetation growth two to three years out of every 10 years, and vegetation-killing drought has a mean return interval of 100 years (Landfire 2007a).

Some grasslands with deep argillic horizons in the San Rafael valley in Arizona and Animas valley in New Mexico have not shown shrub or tree encroachment and/or conversion in the absence of fire or presence of livestock grazing (McAuliffe 1995, Muldavin et al. 2012c). These deep-soil systems have maintained open grassland characteristics despite fire suppression, drought, and livestock grazing. However, there are other valley bottom areas that once supported grasslands, such as the San Simon valley, that have been converted to shrublands due to soil erosion. It is unclear exactly what mechanisms are responsible for the resilience seen in some areas and not in others. McAuliffe (1995) highlighted research on the Santa Rita Experimental Range in Arizona that shows sites of the mid-Pleistocene fan remnants with strongly developed argillic horizons that have not been significantly invaded by deep-rooted shrubs when compared to nearby younger substrates with weakly developed or absent argillic horizons. McAuliffe (1995) suggested these impermeable argillic layers restrict deep percolation of soil-water and may favor the shallower-rooted grasses like tobosa. These soil - water - vegetation relationships may apply to these grasslands in the Chihuahuan Desert.

These native semi-desert grasslands are a dominant grassland type within this ecoregion and range from open to moderately dense grasslands sometimes with low-shrub canopy cover (less than 10% cover). Over time without fire or other disturbance, stands become dominated by woody vegetation and convert to shrublands or woodlands (Gori and Enquist 2003). Conversion to juniper woodlands or mesquite shrublands is common when trees or mesquite exceed 15% cover (Gori and Enquist 2003). Gori and Enquist (2003) found after grassland conversion to shrubland there is a loss of perennial grasses and increases of bare ground. If not protected by surface rock, topsoil erosion can occur changing the site to be less suitable for grass recolonization (McAuliffe 1995).

Hydrological alterations also occurred in many semi-desert grasslands during early Anglo-American settlement time with a period of arroyo formation from 1865 to 1915 (Cooke and Reeves 1976). During this time many broad valley bottom drainages were incised, lowering water tables. This resulted in changes to more xeric vegetation because of decreased water availability, as well as increased sediment movement, altered hydrologic relationships, and loss of productive land (Cooke and Reeves 1976). There is debate of causes of these hydrologic changes. Cooke and Reeves (1976) found strong evidence that arroyo formation in this ecoregion was initiated by building ditches, canals, roads and embankments along channels that altered valley floor hydrology.

The introduction of invasive non-native, perennial grasses Eragrostis lehmanniana and Eragrostis curvula has greatly impacted many semi-desert grasslands in this ecoregion (Cable 1971, Anable et al. 1992, Gori and Enquist 2003). Cable (1971) and Anable et al. (1992) found that Eragrostis lehmanniana is a particularly aggressive invader and alters ecosystem processes, vegetation composition, and species diversity.

Conversion of this type has commonly come from overgrazing by livestock and drought. Fire suppression may have contributed to succession and conversion to shrublands, desert scrub and woodlands especially from oak, pinyon or juniper tree invasion (Gori and Enquist 2003). This grassland has also converted to invasive non-native, perennial forage grasses Eragrostis lehmanniana and Eragrostis curvula (Cable 1971, Anable et al. 1992, Gori and Enquist 2003).

Common stressors and threats include fragmentation from housing and water developments, altered fire regime from direct fire suppression and indirect fire suppression from livestock grazing and fragmentation, introduction of invasive non-native species, and overgrazing by livestock which can lead to severe soil compaction and reduce vegetation cover exposing soils to erosion of topsoil, especially if soil surface does not significant rock cover. Some of these sites are impacted by head-cutting of drainages that decreased functionality of systems. Potential climate change effects could include a reduction in the current extent of the ecosystem and conversion to desert scrub or expanding woodlands, if climate change has the predicted effect of less effective moisture with increasing mean temperature (TNC 2013).

This grassland system is found from the northern to central Chihuahuan Desert and extends across the Trans-Pecos and into areas of the southwestern Great Plains. It extends from western Texas across New Mexico and into southeastern Arizona. Stands are described from Jornada del Muerto Basin, Marfa grasslands and Marathon Basin, south to central Chihuahua and Coahuila, Mexico.