Chihuahuan Desert Sandy Plains Grassland

This ecological system occurs across the Chihuahuan Desert and extends into the southern Great Plains where soils have a high sand content. These dry grasslands or steppe are found on sandy plains and sandy mesatops. The graminoid layer is typically dominated or codominated by Bouteloua eriopoda and Sporobolus flexuosus. Other common species are Aristida purpurea, Bouteloua gracilis, Hesperostipa neomexicana (minor), Muhlenbergia arenicola, Pleuraphis jamesii, Sporobolus airoides, Sporobolus contractus, and Sporobolus cryptandrus. Typically, there are scattered desert shrubs and stem succulents present, such as Ephedra torreyana, Ephedra trifurca, Larrea tridentata, Cylindropuntia imbricata, Prosopis glandulosa, Yucca baccata, Yucca elata, Yucca campestris, and Yucca torreyi, that are characteristic of the Chihuahuan Desert. The widespread shrub Artemisia filifolia is also frequently present along with Atriplex canescens, especially in the northern extent. In Texas, non-native species Eragrostis lehmanniana and Eragrostis barrelieri are frequently found in this system.

The graminoid layer is typically dominated or codominated by Bouteloua eriopoda and Sporobolus flexuosus. Other common species are Aristida purpurea, Bouteloua gracilis, Cenchrus spinifex, Hesperostipa neomexicana (minor), Muhlenbergia arenicola, Pleuraphis jamesii, Sporobolus airoides, Sporobolus contractus, and Sporobolus cryptandrus. Typically, there are scattered desert shrubs and stem succulents present, such as Ephedra torreyana, Ephedra trifurca, Larrea tridentata, Cylindropuntia imbricata (= Opuntia imbricata), Prosopis glandulosa, Yucca baccata, Yucca elata, Yucca campestris, and Yucca torreyi, that are characteristic of the Chihuahuan Desert. The widespread shrub Artemisia filifolia is also frequently present along with Atriplex canescens, especially in the northern extent. In Texas, non-native species Eragrostis lehmanniana and Eragrostis barrelieri are frequently found in this system.

This grassland or steppe system occurs on sandy, gently sloping, undulating piedmont slopes or plains at elevations ranging from 1065-1525 m (3500-5000 feet). Mean annual precipitation ranges from 20-27 cm (8-10.5 inches), although rainfall is highly variable ranging from 5-50 cm (2-20 inches). Half of the precipitation or more typically falls during summer monsoonal events. Annual frost-free season exceeds 200 days. Spring southwesterly winds are an important factor for soil/sand distribution (Landfire 2007a). Historically the grassland type was widespread in the northern Chihuahuan Desert occupying sandy sites and dominated by Bouteloua eriopoda and other grasses, especially Sporobolus flexuosus and Sporobolus cryptandrus. Natural spatial variation in the vegetation of this ecological system may be governed by slight variations in soil texture. For example, dropseeds may dominate on loamy sands. Variation in the depth to a restrictive horizon, such as caliche, may also drive variation in grass cover (Landfire 2007a). Frequently, mesquite shrublands have invaded former black grama grassland sites, including the development of coppice dunes (Landfire 2007a). In Texas, this system occurs on eolian sands, sometimes as a thin veneer over surrounding formations, such as caliche, and sandstone. Soils are sandy, loamy sands, and shallow sandy loams.

Wind is an important disturbance agent in this grassland system. The grassland is highly sensitive to grazing and frequent drought. Fire is relatively infrequent, but can result in a significant change of dominant vegetation (Landfire 2007a). The role of fire in New Mexico's black grama-dominated grasslands is unclear, as studies of historical records do not document fires in these grasslands (Wright 1960, Buffington and Herbal 1965). However, in contrast to other desert grasslands, fire has been shown to decrease black grama cover (Buffington and Herbel 1965, Drewa and Havstad 2000). Several other New Mexico studies have shown that black grama decreases with other disturbances, such as drought, livestock grazing, and clipping, recovering slowly, if at all, after such events (Buffington and Herbel 1965, Gibbens and Beck 1988, Gosz and Gosz 1996, Whitford et al. 1999, Drewa and Havstad 2000, Gibbens et al. 2005). While drought was a conflicting factor in many of these studies, it is important to note that studies in Arizona were also conducted during times of drought and resulted in longer recovery times, not a lack of recovery in perennial grasses (Schussman 2006a).

Bouteloua eriopoda is a key plant due to its dominance under pristine conditions, its high forage value and its consequent sensitivity to grazing. Shifts away from black grama dominance are thought to be due to overgrazing and/or multi-year periods of summer or spring drought, or due to the introduction of Prosopis glandulosa seeds with or without grazing. With continuous heavy grazing, the proportional representation of black grama declines because it is preferred by cattle over species of Sporobolus, Aristida, and Gutierrezia (Paulsen and Ares 1962). Sporobolus spp. are more palatable than Aristida spp., so dropseeds may also decline relative to threeawns and Gutierrezia spp. Under climatic conditions that are not conducive to black grama reproduction, or due to the loss of components of the soil biota, demographic limitations may lead to persistent absence of black grama, even without shrub invasion. Shrub invasion is, however, very common. Loss of soil stability and/or a reduction in black grama cover may permit either the survival or establishment of mesquite seedlings due to reduced competition or fire frequency. These grasslands have been shown to trend towards shrublands over the last 100 years (Buffington and Herbel 1965, Gibbens et al. 2005). Subsequent grazing by livestock and native herbivores, competition from shrubs, erosion, and concentration of nutrients under adult shrubs eventually lead to persistent reductions of grass cover and mesquite-dominated coppice dunes with bare or snakeweed-dominated interdunal areas. A substantial number of studies document states and potential causes of transitions. There are multiple competing and complementary explanations for individual transitions that have not been formally tested. If the operation of these mechanisms is case-contingent, it may be especially problematic to define the causes of transitions quantitatively (e.g., a threshold cover of black grama). Nonetheless, careful monitoring of black grama health should be a key feature of management. Overall, the high palatability of black grama during times of year when most other species are less palatable, coupled with the limited capacity of this grass to regenerate under current climatic conditions (Neilson 1986), leads to a relatively high probability of transition with poor range management.

As degradation continues, grasses are replaced by shrubs. Current species dominance is sand-sage and broom dalea in the northern extent and mesquite and broom snakeweed in the southern extent of these grasslands. A significant proportion of the extent of these grasslands have been converted to dune shrubland with mesquite dominance and soil redistribution by wind erosion in the southern portion. There is a lack of research regarding thresholds in response to disturbance and restoration techniques (Landfire 2007a).

Conversion of this type has commonly come with overgrazing by livestock and drought that has allowed succession and conversion to desert scrub dominated by mesquite and sometimes creosotebush (Gori and Enquist 2003). This grassland also has been invaded by non-native perennial forage grasses Eragrostis lehmanniana and Eragrostis curvula, particularly in the eastern portion of its range (Cable 1971, Anable et al. 1992, Gori and Enquist 2003). Common stressors and threats include fragmentation from housing and water developments, drought, altered fire regime from 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 contain significant rock cover. Potential climate change effects could include a reduction in the current extent of the ecosystem and conversion to desert scrub, if climate change has the predicted effect of less effective moisture with increasing mean temperature (TNC 2013).

This Chihuahuan Desert ecological system extends into the southern Great Plains where soils have a high sand content.