Present Vegetation
Originally an area of open grassland or grassland with scattered shrubs (Figure 1), much of this area today supports a mixture of shrubs and grasses with shrubs dominant. In Arizona, mesquite (Prosopis juliflora [Sw.] DC.), burroweed (Aplopappus tenuisectus Greene), and snakeweed (Gutierrezia spp.), together with acacias (Acacia spp.) and a number of cacti are the principal shrubs. In eastern Arizona, across southern New Mexico and into Texas, mesquite, creosote bush (Larrea tridentata [DC.] Coville), Spanish bayonet (Yucca spp.), tarbush (Flourensia cernua DC.), cacti, and a variety of other thorny shrubs overtop the grasses.
The increase in shrubs has been ascribed to several causes. These have included overgrazing, dissemination of the seeds by grazing animals, and fire control. There seems to be little doubt that more than one factor has been responsible in most instances. The essential cessation of grassland fires, however, and those factors related to this change, appear to have had more to do with the increase in brush than any others.
Over rather extensive areas the grasses originally dominant have largely disappeared. In others, there is a fairly good understory of many of the same grasses that were common when the country was first occupied by white men. Although there is a wide variety of grasses within the desert grassland, only a few are sufficiently abundant to be classed as association dominants. Shantz and Zon (1924) subdivides the desert grassland on a basis of dominant grasses into four communities: black grama (Bouteloua eriopoda [Torr.] Torr.), crowfoot (Rothrock) grama (B. rothrockii Vasey), curly mesquite (Hilaria belangeri [Steud.] Nash) and tobosa grass (Pleuraphis mutica Buckley). It would seem that a fifth, blue grama (Bouteloua gracilis [Willd. ex Kunth] Lag. ex Griffiths), should have been listed also. Although there have been extensive changes in the floristic composition of these communities in the 28 years that have elapsed since this classification was made, a similar breakdown would be valid if made today. In general, the black grama association is best developed in New Mexico and Texas, the Rothrock grama and curly mesquite in Arizona, the tobosa grass and the blue grama throughout the Southwest. In Arizona, the Rothrock grama community seems to have changed more down through the years than any of the others. Most of the areas at one time dominated by Rothrock grama have been taken over to a large extent by shrubs. Chief among these are burroweed, snakeweed and mesquite. Where shrub invasion has been less intensive, annual grasses are dominant today where Rothrock grama was a few years ago
Rothrock grama should not be classed as an association dominant. Although abundant on many southern Arizona ranges, this grass seems to represent a successional stage as the ranges have deteriorated or improved. In part because of its ability to set abundant seed readily, and in part because of its short life, Rothrock grama responds rapidly to favorable moisture conditions. In this respect it resembles the annual grasses with which it is usually associated. Most black grama communities support a greater variety of other grasses and shrubs than either the curly mesquite or tobosa associations. This may be because black grama has a wider tolerance, both as to soil and moisture, than curly mesquite or tobosa. Curly mesquite shows a preference for heavy clay soils on sloping, rocky sites; tobosa for poorly drained swales that may be flooded during the rainy season.