Chihuahuan Salt Flat Shrubland

This ecological system includes extensive open-canopied shrublands of typically saline basins in the Chihuahuan Desert. Stands often occur on alluvial flats and around playas, as well as in floodplains along the Rio Grande and Pecos River, possibly also extending into the San Simon of southeastern Arizona. Substrates are generally fine-textured, saline soils. Vegetation is typically composed of one or more Atriplex species, such as Atriplex canescens, Atriplex obovata, or Atriplex polycarpa, along with species of Allenrolfea, Flourensia, Salicornia, Suaeda, or other halophytic plants. Graminoid species may include Sporobolus airoides, Pleuraphis mutica, or Distichlis spicata at varying densities.

Vegetation is typically composed of one or more Atriplex species, such as Atriplex canescens, Atriplex obovata, or Atriplex polycarpa, along with species of Allenrolfea, Flourensia, Salicornia, Suaeda, or other halophytic plants. Graminoid species may include Sporobolus airoides, Sporobolus wrightii, Pleuraphis mutica, or Distichlis spicata at varying densities. Occasional riparian species may be present near watercourses, such as Prosopis pubescens or Populus deltoides ssp. wislizeni. In Texas, species making up the often relatively sparse vegetative cover include Allenrolfea occidentalis, Atriplex acanthocarpa, Atriplex canescens, Flourensia cernua, Isocoma pluriflora, Koeberlinia spinosa, Cylindropuntia leptocaulis, Prosopis glandulosa var. torreyana, Sesuvium verrucosum, Suaeda suffrutescens, and Ziziphus obtusifolia. Non-native halophiles such as Salsola tragus, Alhagi maurorum, Peganum harmala, and Tamarix spp. are commonly encountered to dominant. Graminoids commonly found, and sometimes constituting significant cover, include Trichloris crinita, Distichlis spicata, Pappophorum bicolor, Pleuraphis mutica, Scleropogon brevifolius, Sporobolus airoides, and Sporobolus wrightii.

This system includes extensive open-canopied shrublands of typically saline basins in the Chihuahuan Desert. Stands often occur on alluvial flats, around playas and floodplains of the Rio Grande and Pecos River, possibly also extending into the San Simon of southeastern Arizona. Sites are flat to gently sloping with slopes up to 3%. Elevation ranges from 1000-1300 m (3300-4300 feet). Substrates are generally fine-textured, saline soils but may include moderately coarse-textured alluvium in the floodplains. In Texas, this system is associated with Salty desert grassland, Salty Clay Fan, and Salty Bottomland Ecological Sites. Water tables are generally shallow but fluctuate within reach of deep-rooted plants, and in most places are high enough that salts accumulate on the surface of the soil.

[from M086] In the U.S., much of this desert scrubland is thought to be a result of recent expansion of Larrea tridentata and Prosopis glandulosa into former desert grasslands and steppe in the last 150 years as a result of a combination of drought, overgrazing by livestock, wind and water erosion, and/or decreases in fire over the last 70-250 years from fire suppression and fine-fuel removal by livestock, and changes in the seasonal distribution of precipitation (Buffington and Herbel 1965, Herbel et al. 1972, Humphrey 1974, Ahlstrand 1979, McLaughlin and Bowers 1982, Gibbens et al. 1983, Hennessy et al. 1983, Donart 1984, Brown and Archer 1987, Schlesinger et al. 1990, Dick-Peddie 1993, McPherson 1995, Gibbens et al. 2005). Seed dispersion by livestock is an additional factor in the increase of Prosopis glandulosa (Brown and Archer 1987). It is believed that Prosopis glandulosa stands formerly occurred in relatively minor amounts and were largely confined to drainages until cattle distributed seed upland from the bosques into desert grasslands (Brown and Archer 1987, 1989). This macrogroup also includes invasive Flourensia cernua shrublands that occur in former (degraded) tobosa (Pleuraphis mutica) flats and loamy plains. Presence of Scleropogon brevifolius is common in these invasive stands. Dick-Peddie (1993) suggested that absence of Flourensia cernua as codominant and presence of Dasyochloa pulchella, Acourtia nana, and Yucca elata may be indicators of recent conversion of desert grasslands into desertscrub, but more research is needed. Conversely, sparse understory Larrea tridentata shrublands on remnant early Holocene erosional surfaces often with shallow calcareous soils and desert pavement may indicate pre-historic distributions of Larrea tridentata desertscrub in the Chihuahuan Desert (Stein and Ludwig 1979, Muldavin et al. 2000b).

Historical natural-ignition fires were relatively small, probably 10-15 acres in size. Repeated fire is thought to help maintain a general mosaic pattern between open grassland and shrub-dominated areas (Johnston 1963). Wright et al. (1976) found that Prosopis glandulosa is very fire-tolerant when only 3 years old. Most plants resprout after being top-killed by fire. Thus, prior to livestock grazing reducing fire frequency, repeated grassland fires probably maintained lower stature of shrubs and prevented new establishment by killing seedlings.

Drought is a relatively common occurrence in this desertscrub, generally occurring every 10-15 years and lasting 2-3 years with occasional long-term drought periods (10-15 years duration). Prosopis spp. and other shrubs have extensive root systems that allow them to exploit deep-soil water that is unavailable to shallower rooted grasses and cacti (Burgess 1995). This strategy works well, especially during drought. However, on sites that have well-developed argillic or calcic soil horizons that limit infiltration and storage of winter moisture in the deeper soil layers, Prosopis spp. invasion can be limited to a few, small individuals (McAuliffe 1995). This has implications in plant geography and desert grassland restoration work in the southwestern United States.

On sandsheet and dune sites, Prosopis glandulosa is more common on warmer, drier sites on sandsheets with subsoils composed of clays or carbonate substrates, whereas Artemisia filifolia is more common on relatively cooler/moisture sites with coarse, deep sand (S. Yanoff pers. comm. 2007). These sites are also more susceptible to grazing pressure.

This ecological system occurs in saline basins in the Chihuahuan Desert. Stands often occur around playas and on alluvial flats, as well as in floodplains along the Rio Grande and Pecos River, possibly also extending into the San Simon of southeastern Arizona.