Physiognomy and Structure: Vegetation in this alliance has a sparse to moderately dense herbaceous layer is dominated by perennial bunch grasses. A sparse to moderately dense forb layer is often present. Annuals may be seasonally abundant.

Floristics: The vegetation is characterized by an herbaceous layer with sparse to moderately dense cover of perennial grasses that is usually dominated or codominated by Bouteloua eriopoda, Bouteloua gracilis, Pleuraphis jamesii, or Sporobolus airoides. Other common perennial grasses may include Achnatherum hymenoides (= Oryzopsis hymenoides), Aristida purpurea, Bouteloua curtipendula, Bouteloua dactyloides (= Buchloe dactyloides), Elymus elymoides, Hesperostipa neomexicana (= Stipa neomexicana), Muhlenbergia porteri, Muhlenbergia torreyi, Pascopyrum smithii, and Sporobolus cryptandrus. The sparse forb layer may include Artemisia carruthii, Artemisia dracunculus, Astragalus spp., Chaenactis stevioides, Cryptantha sp., Cymopterus newberryi, Grindelia squarrosa, Lappula occidentalis, Machaeranthera pinnatifida, Plantago patagonica, Ratibida spp., Scleropogon brevifolius, Sphaeralcea coccinea, and Zinnia grandiflora. Occasional shrubs and dwarf-shrubs, such as Artemisia bigelovii, Atriplex canescens, Atriplex confertifolia, Atriplex obovata, Ericameria nauseosa, Ephedra spp., Gutierrezia sarothrae, Krascheninnikovia lanata, Opuntia spp., or Yucca spp., may occur with less than 10% total cover.

Dynamics:  Pleuraphis jamesii is both drought- and grazing-resistant (USFS 1937, Weaver and Albertson 1956, West et al. 1972). In parts of its range it increases under grazing, and in others parts it decreases. The grass is favored in mixedgrass stands because it is only moderately palatable to livestock, but decreases when heavily grazed during drought and in the more arid portions of its range where it is the dominant grass (West et al. 1972). This grass reproduces extensively from scaly rhizomes. These rhizomes make the plant resistant to trampling by livestock and have good soil-binding properties (USFS 1937, Weaver and Albertson 1956, West et al. 1972). The abundance of Bouteloua eriopoda-dominated grasslands has declined significantly in the last 50 years (Nelson 1934, Gardner 1950, Buffington and Herbel 1965, Herbel et al. 1972, Hennessy et al. 1983). These grasslands have been replaced largely by shrublands dominated by Prosopis glandulosa in Trans-Pecos Texas, southern New Mexico, and southeastern Arizona. 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). 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, except on sites that have well-developed argillic or calcic soil horizons that limit infiltration and storage of winter moisture in the deeper soil layers (McAuliffe 1995). McAuliffe (1995) found Prosopis spp. invasion on these sites to be limited to a few small individuals. This has implications in plant geography and grassland revegetation work in the southwestern United States. Bouteloua gracilis is an extremely drought- and grazing-tolerant shortgrass species. It is one of the most widely distributed grasses in the western U.S. and is present in many different grassland, shrubland and woodland communities. It evolved with grazing by large herbivores and generally forms a short sod. However, in some stands ungrazed plants develop the upright physiognomy of a bunchgrass.