Physiognomy and Structure: This macrogroup includes ruderal vegetation with an open to dense shrub canopy and/or an herbaceous layer dominated by annual or perennial grasses or forbs.

Floristics: Vegetation of the macrogroup can be a monoculture of a single non-native graminoid species, or a mix of several non-native forbs and graminoids. Perennial graminoids include Eragrostis curvula, Eragrostis lehmanniana, Pennisetum ciliare, Pennisetum setaceum, Sorghum halepense (mesic sites), and several other species (which have been purposefully seeded to prevent soil erosion or for livestock forage), and/or invasive non-native annual species such as Bromus rubens, Schismus arabicus, and Schismus barbatus. Numerous other non-native herbaceous species may be present to dominant, including Brassica tournefortii. Dense stands of native ruderal species such as Amaranthus palmeri or Solanum elaeagnifolium resulting from anthropomorphic disturbance are also included in this macrogroup. Invasive non-native shrublands are less common. This macrogroup includes upland desert scrub strongly dominated by invasive native species (Prosopis glandulosa and Prosopis velutina) with >95% relative cover and >10% absolute shrub cover. Prosopis spp.-dominated stands that occur naturally (non-ruderal) in desert lowlands, drainages, washes and riparian areas (bosque) are excluded from this ruderal type. It also includes any desert scrub with an exotic species-dominated understory (>90% relative cover) in the herbaceous layer.

Dynamics:  Historically, fire was very rare in the southwestern deserts. The fine fuels needed to carry fire were not present. However, because of recent expansion of non-native grasses such as Bromus rubens and Pennisetum ciliare into desert scrub, fine fuels have become available and fire frequency, size and severity have increased (Brooks and Matchett 2006). Native desert scrub vegetation, such as Carnegiea gigantea, Larrea tridentata, and Yucca brevifolia, did not adapt tolerance to fire, is generally killed when burned and does not readily re-establish or does so very slowly for a variety of reasons such as harsh climate (Esque et al. 2004, Brooks et al. 2007, Abella 2009, 2010).

During the last century, the area occupied by this ruderal desert thornscrub group has increased through conversion of desert grasslands as a result of drought, overgrazing and Prosopis glandulosa seed dispersion by livestock, and/or decreases in fire frequency (Brown and Archer 1987). It is believed that this group formerly occurred in relatively minor amounts and was largely confined to drainages until cattle distributed seed upland from the bosques 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).