Soil and Site Stability
Soil and site stability show moderate or greater levels of departure for relatively high proportions of rangeland in this region (Figure 1). The proportion of the area showing significant departure is particularly high along the Rio Grande. An increase and dominance of mesquite (Figures 2-5), a simultaneous loss of grasses, and increased bare ground (Figures 6-10), runoff, and erosion may explain a portion of the change.
Figures 2-5. Non-Federal rangeland where native invasive mesquite species are present or make up at least 15, 30, or 50 percent of the plant cover
(Source: Native Invasive Woody Species Table 2 & Table 6)
Hydrologic function shows patterns similar to soil and site stability in this region, with even higher proportions of land showing at least moderate departure in many parts of the region (Figure 11). This is due to the sensitivity of hydrologic function to both soil degradation and, frequently, changes in the plant community associated with invasive woody plants. Mesquite (mainly Prosopis glandulosa Torr.) and juniper (Juniperus spp.) are particularly widespread in this region (Figures 12-15). While native, they can and do increase in the absence of fire. Where increased woody cover is associated with reduced grass cover, infiltration capacity can decline with increased runoff in interspace areas between shrubs. Accelerated runoff over time can result in changes of natural water flow paths and the formation of interspace rills which may develop into gullies. Soil loss can be excessive, and recovery on these sites can be slow. On shallow soils, these channels can quickly erode to bedrock. In contrast, dense grass cover and associated root mass tend to increase soil porosity, soil aggregate stability (Figure 16), and overall soil health.
Significant shifts in plant community composition associated with increased dominance of invasive species, both grasses and shrubs, are reflected in significantly reduced biotic integrity throughout most of this region (Figure 17). While many of these species (e.g. mesquite and juniper) are native throughout this region, the high biotic integrity departure ratings reflect the fact that they have invaded soils where they would not normally occur, become dominant under a natural disturbance regime, or significantly increased on soils where they are native. The potential plant communities for most of this region are grassland and savanna ecosystems.