Regional Interpretation - Southwest
Great Plains | Intermountain West | Southwest | Texas and Oklahoma | Other
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The Southwest region includes the Sonoron Desert of Arizona, the Mojave Desert of southern California and Nevada, and the Chihuahuan Desert of southern New Mexico and west Texas. It also includes the southern Rocky Mountains of south-central Colorado and north-central New Mexico. This region includes the most arid areas of the United States. Strong precipitation and temperature gradients associated with latitude, longitude, and elevation largely determine general patterns of potential vegetation and plant production in the region, with local differences associated with differences in soils and landscape position. Water redistribution by runoff is an important factor in determining landscape patterns, which are reinforced by land degradation that increases runoff. Potential plant communities in most ecosystems include a significant shrub component. For those that do not, such as the Chihuahuan Desert grasslands, shrubs are often invasive. The Sonoran Desert is characterized by a high proportion of succulent species, where survival depends on the infrequency of sub-freezing temperatures. Like the Intermountain West region, the Southwest includes large areas of non-surveyed public lands interspersed with non-Federal lands. The Mojave Desert, in particular, has very small proportions of non-Federal land. There are also significant areas of forest in the higher elevations, particularly in west-central New Mexico and east-central Arizona.
Soil and Site Stability
Soil and site stability shows moderate departure from reference condition on at least 10 percent of the non-Federal land in most of this region, and on over 25 percent in parts of southern Arizona (Figure 1). As in the southern Intermountain region, aridity contributes to lower resistance and resilience of these areas. Invasion of grasslands by persistent shrubs, resulting in increased bare ground and, more significantly, increased proportion of the soil surface exposed in intercanopy gaps, explains much of the increased departure from reference conditions for soil stability in southern New Mexico and West Texas. High levels of bare ground occur naturally, however, particularly in the extremely arid parts of southwestern Arizona. This helps explain why bare ground is higher in southwestern than in southeastern Arizona (Figure 2), but soil and site stability shows greater departure from reference conditions in southeastern Arizona.
The pattern of hydrologic function (Figure 3) is similar to that for soil and site stability, and for virtually identical reasons. A loss of herbaceous cover associated with replacement of grasses by shrubs leads to increased bare ground, and a higher proportion of the bare ground in large intercanopy gaps. Accelerated runoff and soil loss in the intercanopy gaps is common.
The reduction in biotic integrity in much of this region (Figure 4) is due to the invasion of native, rather than non-native species. Mesquite (Figure 5) and creosote bush (Larrea spp.) are both highly invasive on many soils in the Chihuahuan and Sonoran Deserts. Juniper species (Figure 6) are also highly invasive throughout this region. In addition, there are significant effects of non-native species including buffelgrass (Pennisetum ciliare (L.) Link) and Lehmann lovegrass (Eragrostis lehmanniana Nees) in southern Arizona. This shift in species composition negatively impacts nutrient cycling and the quality of wildlife habitat, both directly and through its effects on the fire regime (Fire intensity and frequency often increases with higher densities of certain invasive plant species). This shift also affects soil surface and soil-plant-water relations, which affects soil and site stability. These feedbacks occur in all regions, but are particularly important in the Southwest and Intermountain West regions.
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