Regional Interpretation - Intermountain West
Great Plains | Intermountain West | Southwest | Texas and Oklahoma | Other
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Overview
The Intermountain West includes the Columbia River Basin and Snake River Plateau in the northwest, the Great Basin in Nevada and western Utah, and the Colorado Plateau in the Four Corners area of Utah, Arizona, New Mexico and Colorado. In addition to large areas of forest, this region has the highest proportion of Federal lands. Neither Federal land nor forest land was included in the NRI rangeland on-site data collection. Consequently, results in this region should be carefully interpreted. Although they accurately reflect the results on non-Federal rangeland, they reflect the status of a small proportion of the region as a whole.
Much of this region is characterized by ecosystems that were historically dominated by shrubs and bunchgrasses, with various species of sagebrush being the most dominant shrub. Interspaces between the shrubs are interspersed with bunchgrasses, forbs and biological soil crusts (lichens, mosses and cyanobacteria at the soil surface). Dramatic soil variability, driven by geology (soil parent material) and subsequent landscape formation, contribute to large differences in potential plant community composition. Soil-driven differences in plant communities are particularly evident in many parts of Utah, where salt-affected soils cover large areas (e.g., Bonneville Salt Flats). Large precipitation gradients and differences in potential evaporation and transpiration associated with aspect (lower on north-facing slopes and higher on south- and west-facing slopes) also contribute to variability in ecological potential in this region. There are some significant localized areas of irrigated agriculture. Where fields have been abandoned, they revert to rangeland.
Soil and Site Stability
Soil and site stability shows moderate departure from reference condition on non-Federal lands in most of the southern part of this region (Figure 1). The southern areas typically have lower precipitation and higher evaporation and transpiration. There are also fairly large areas of saline soils. Although shrub species such as greasewood (Sarcobatus vermiculatus (Hook.) Torr.) and saltbush (Atriplex spp.) tolerate these soils, there is typically little understory and interspace vegetation. All of these factors may contribute to lower resistance and resilience to degradation. These same factors, together with the greater departure from expected conditions, explain the high percentage of bare ground (Figure 2) and low soil aggregate stability (Figure 3) measured throughout the southern part of this region. The higher concentrations of bare ground also tend to be located in relatively large intercanopy gaps (Figures 4-7). Shrub clusters and coppice dunes often accumulate windblown soil and litter and are associated with high rates of infiltration; these areas can collect runoff water from surrounding lands. Large intercanopy gaps devoid of vegetation and microbiotic crusts are highly susceptible to wind erosion, particularly following disturbance (e.g., by animals, vehicles, or machinery). Invasion and increase of juniper causes a decline in sagebrush (Artemisia spp.) species. As the amount of bare ground increases and patches of bare ground coalesce, the resulting areas become highly susceptible to sheet erosion. Water flow paths become eroded between mature shrubs and can develop into rills and gullies. Soil loss down to bedrock is common.
Hydrologic Function
The pattern of hydrologic function (Figure 8) is nearly identical to that for soil and site stability, and for virtually identical reasons. A loss of herbaceous understory and associated increases of bare ground lead to reduced infiltration capacity and increased runoff. Where bare ground is concentrated in large intercanopy gaps, the effect is even more pronounced. In this region, when soil and site stability and hydrologic function are compromised by excessive soil loss, thresholds are crossed and recovery to original historical ecological states is unlikely.
Biotic Integrity
Biotic integrity in much of this region has been reduced (Figure 9) by the replacement of native grasses with non-native annual grasses (e.g., medusahead (Taeniatherum caput-medusae (L.) Nevski), wildrye, and invasive brome grasses such as cheatgrass (Bromus tectorum L.) and forb species such as the knapweed complex (Centaurea spp.), and by the associated loss of native perennial grass species (Figures 10-12). Sagebrush stands have been reduced by the invasion of juniper and the occurrence and frequency of wildfires, which then perpetuate invasion of non-native annual grasses. Rehabilitation of invasive plant-dominated lands has often involved the seeding of non-native introduced forage grasses, such as crested wheatgrass (Agropyron cristatum (L.) Gaertn.), that may dominate plant communities and reduce the native plant diversity. A number of studies have demonstrated that shifts in species composition to either invasive plants or competitive introduced forage grasses may have significant effects on nutrient cycling, hydrologic function, and wildlife habitat, both directly and through its effects on the fire regimes (fire intensity and frequency often increases with an increase in invasive plant species).
Figures 10-12. Non-Federal rangeland where non-native medusahead, annual brome grasses and Centaurea species are present (Source: Non-Native Plant Species Table 3)
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