Physiognomy and Structure: The physiognomy of this group includes open to dense shrublands and shrub-steppe with a sparse to dense, annual- or perennial-dominated herbaceous layer.

Floristics: This broadly defined ruderal group includes all desert scrub with an exotic species-dominated understory (>90% relative cover) in the herbaceous layer as well as exotic-dominated herbaceous stands. These open to dense grasslands and forblands are composed of either exotic annual or biennial grasses or forbs with low cover of perennial species (<10% absolute cover) or stands with a significant perennial herbaceous layer (>10% absolute cover) strongly dominated by exotics (>90% relative cover) with or without annuals and biennials present to dominant. This group 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. that occur naturally (non-ruderal) in desert lowlands, drainages, washes and riparian areas (bosque) are excluded from this ruderal type. Characteristic understory species include Brassica nigra, Brassica tournefortii, Bromus madritensis, Bromus rubens, Cynodon dactylon, Eragrostis lehmanniana, Erodium cicutarium, Panicum antidotale, Schismus arabicus, Schismus barbatus, and Sorghum halepense with little native composition remaining. Other woody species may include Calliandra eriophylla, Gutierrezia sarothrae, or Isocoma tenuisecta. Gutierrezia microcephala is abundant in some stands. Remnant desert grasses include Aristida ternipes, Bouteloua chondrosioides, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua rothrockii, Digitaria californica, and Eragrostis intermedia. Other common herbaceous species include Allionia incarnata, Ambrosia confertiflora, Boerhavia erecta, Mollugo verticillata, Cylindropuntia versicolor (= Opuntia versicolor), Panicum hirticaule, Polygala barbeyana, Proboscidea parviflora, and Phemeranthus aurantiacus (= Talinum aurantiacum). Other associated species such as Chenopodium berlandieri, Chloris virgata, Eragrostis cilianensis, Eragrostis pectinacea, Eriochloa acuminata, Ipomoea spp., Kallstroemia grandiflora, Leptochloa panicea ssp. brachiata (= Leptochloa filiformis), Salsola kali, and Solanum elaeagnifolium are often present. The ruderal native annual forb Amaranthus palmeri often dominates or codominates disturbed stands.

Dynamics:  During the last century, the area occupied by the ruderal upland mesquite-dominated 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).

For sandy site desert scrub it is important to differentiate between (1) coppice dunes / associated interdune and (2) sandsheets. Invasive mesquite dominates on coppice dunes, especially where the interdune contains an argillic horizon layer with increased clay content. Mesquite produces large taproots and long lateral roots which enable it to extract moisture from deeper depths and the associated interdune. On sandsheets, as noted by Steven Yanoff (pers. comm.), sandsage dominates. These soils are typically deeper and coarser-textured (sand and loamy sand). The coarse texture allows rapid infiltration and helps decrease wicking of soil moisture to the surface via capillary rise. Common associations on sandsheets dominated by Artemisia filifolia, Psorothamnus scoparius, and Rhus microphylla occurred historically and are not included in this group with invasive Prosopis glandulosa unless there is an herbaceous understory that is dominated by exotic species.