Chihuahuan-Sonoran Desert Swale Grassland

This ecological system is named based on the regions (Chihuahuan and Sonoran deserts) where it is best developed and occupies significant areas, however, it does occur well outside these regions, at least as far north and east as the Rolling Plains of Texas. The system occurs in relatively small depressions or swales and along drainages throughout the northern and central Chihuahuan Desert and adjacent Sky Islands and Sonoran Desert, as well as limited areas of the southern Great Plains on broad mesas, plains and valley bottoms that receive runoff from adjacent areas. Occupying low topographic positions, these sites generally have deep, fine-textured soils that are neutral to slightly or moderately saline/alkaline. During summer rainfall events, ponding is common. Vegetation is typically dominated by Sporobolus airoides, Sporobolus wrightii, Pleuraphis mutica (tobosa swales), or other mesic graminoids such as Pascopyrum smithii or Panicum obtusum. With tobosa swales, sand-adapted species such as Yucca elata may grow at the swale's edge in the deep sandy alluvium that is deposited there from upland slopes. Sporobolus airoides and Sporobolus wrightii are more common in alkaline soils and along drainages. Other grass species may be present, but these mesic species are diagnostic. Scattered shrubs such as Atriplex canescens, Prosopis glandulosa, Ericameria nauseosa, Fallugia paradoxa, Krascheninnikovia lanata, or Rhus microphylla may be present.

The vegetation of this grassland system is typically dominated by Sporobolus airoides, Sporobolus wrightii, Pleuraphis mutica (in tobosa swales), or other mesic graminoids such as Pascopyrum smithii or Panicum obtusum. In tobosa swales, sand-adapted species such as Yucca elata may grow at the swale's edge in the deep sandy alluvium that is deposited there from upland slopes. Sporobolus airoides and Sporobolus wrightii are more common in alkaline soils and along drainages. Other grass species may be present, but these mesic species are diagnostic. Prosopis glandulosa may be present and, in some cases, may develop into a significant canopy. Other scattered shrubs such as Atriplex canescens, Ericameria nauseosa, Fallugia paradoxa, Krascheninnikovia lanata, or Rhus microphylla may be present. Sporobolus airoides is often associated with more alkaline (to gypsic), poorly drained areas and Sporobolus wrightii with less alkaline better drained areas. Distichlis spicata, Allenrolfea occidentalis, and Suaeda spp. are characteristic of more saline and alkaline sites. In Texas, Pleuraphis mutica is generally the clear dominant (Elliott 2013).

This ecological system occurs in relatively small depressions or swales and along drainages on broad mesas, plains and valley bottoms that receive runoff from adjacent areas. These sites occupy low topographic positions and generally have deep, fine-textured soils that are neutral to slightly or moderately saline/alkaline. The system typically occurs in local topographic lows that may be associated with drainages, or may represent swales or basins, but typically receives run-off from the surrounding landscape. Soils are generally clayey, and in some cases the shrink-swell characteristics of the soil may limit the development of woody species. Stands of the system typically occur on Quaternary alluvium, but may be local in nature and mapped within various geological formations. It is generally found on local topographic lows that may be associated with a drainage or may occur as basins or swales. Soils are typically tight ones, and Clay Flat Ecological Sites are typical.

[from M087] During the last century, the area occupied by this desert grassland and steppe decreased through conversion of desert grasslands as a result of drought, overgrazing and Prosopis glandulosa seed dispersion by livestock, and/or decreases in fire frequency (Buffington and Herbel 1965, Brown and Archer 1987). It is believed that mesquite formerly occurred in relatively minor amounts and was largely confined to drainages until cattle distributed seed upland into desert grasslands (Brown and Archer 1987, 1989). Shrublands dominated by Prosopis spp. have replaced large areas of desert grasslands, especially those formerly dominated by Bouteloua eriopoda, in Trans-Pecos Texas, southern New Mexico and southeastern Arizona (York and Dick-Peddie 1969, Hennessy et al. 1983). Studies on the Jornada Experimental Range suggest that combinations of drought, overgrazing by livestock, wind and water erosion, seed dispersal by livestock, fire suppression, shifting dunes, and changes in the seasonal distribution of precipitation have caused this recent, dramatic shift in vegetation physiognomy (Buffington and Herbel 1965, Herbel et al. 1972, Humphrey 1974, McLaughlin and Bowers 1982, Gibbens et al. 1983, Hennessy et al. 1983, Schlesinger et al. 1990, McPherson 1995).

Impermeable caliche and argillic horizons are common on these sites. These layers restrict deep percolation of soil water and may favor the shallower rooted grasses over more deeply rooted shrubs such as Larrea tridentata and Prosopis spp. (McAuliffe 1995). Pleuraphis mutica is relatively tolerant of livestock grazing. In west-central Arizona, livestock have nearly eliminated all native grasses except Pleuraphis mutica from semi-desert grassland (Brown 1982a). Stands codominated by Scleropogon brevifolius are characteristic of sites with past heavy grazing by livestock (Whitfield and Anderson 1938).

In gypsophilous grassland Sporobolus nealleyi is dominant with Tiquilia hispidissima and Opuntia polyacantha on crusted gypsum ridges, but not on unstable gypsum dunes (Burgess and Northington 1977). The eolian processes and sand substrate on gypsum dunes may be as important ecologically as the chemical properties of the gypsum parent material as seen by presence of sand-loving plant species such as Achnatherum hymenoides, Andropogon hallii, Artemisia filifolia, Muhlenbergia pungens, and Psorothamnus scoparius on gypsum dunes.

[from M087] These native mixed semi-desert grasslands are the dominant grassland type and range from open grasslands with low shrub canopy cover (less than 10% cover) to denser grassland with higher shrub and succulent 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). These grasslands were historically maintained as open grasslands with low shrub cover by fire-return intervals of 2.5 to 10 years (Wright 1980, Robinett 1994, McPherson 1995, Brown and Archer 1999). Both drought and livestock grazing interact with grass cover and fire-return intervals can affect the rate of shrub increase (Wright 1980, Robinett 1994, McPherson 1995, Brown and Archer 1999). Gori and Enquist (2003) found that 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). Although there is debate of causes of these hydrologic changes (arroyo formation), Cooke and Reeves (1976) found strong evidence that arroyo formation was initiated by building ditches, canals, roads and embankments along channels that altered valley floor hydrology.

The introduction of the 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). Anable et al. (1992) and Cable (1971) found Eragrostis lehmanniana is a particularly aggressive invader and alters ecosystem processes, vegetation composition, and species diversity.

Conversion of this type has commonly comes from urban and exurban development near cities such as Sierra Vista, Arizona, altered hydrological regimes (water developments/reservoirs) (Cooke and Reeves 1976), and irrigated agriculture especially hay meadows dominated by non-native forage grasses. Fire suppression has allowed 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 and threats include fragmentation from housing and water developments, 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 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 system is found in the central and northern Chihuahuan Desert and adjacent Sky Islands and Sonoran Desert, as well as limited areas of the southern Great Plains.