Colorado Plateau Low Sagebrush Shrubland

This ecological system occurs in the Colorado Plateau, Tavaputs Plateau and Uinta Basin in canyons, gravelly draws, hilltops, and dry flats at elevations generally below 1800 m. Soils are often rocky, shallow, and alkaline. This type extends across northern New Mexico into the southern Great Plains on limestone hills. It includes open shrublands and steppe dominated by Artemisia nova or Artemisia bigelovii sometimes with Artemisia tridentata ssp. wyomingensis codominant. Semi-arid grasses such as Achnatherum hymenoides, Aristida purpurea, Bouteloua gracilis, Hesperostipa comata, Pleuraphis jamesii, or Poa fendleriana are often present and may form a graminoid layer with over 25% cover.

These shrublands are dominated by Artemisia nova or Artemisia bigelovii, sometimes with Artemisia tridentata ssp. wyomingensis codominant. Other shrubs that may be present include Atriplex canescens, Chrysothamnus viscidiflorus, Ephedra spp., Ericameria spp., Gutierrezia sarothrae, Lycium spp., and Yucca spp. The herbaceous layer ranges from sparse to moderately dense and is composed of semi-arid grasses such as Achnatherum hymenoides, Aristida purpurea, Bouteloua gracilis, Hesperostipa comata, Pleuraphis jamesii, or Poa fendleriana forming a graminoid layer sometimes with over 25% cover. The floristic description is based on several other references, including Jameson et al. (1962), Brown (1982), West (1983a), Baker and Kennedy (1985), Francis (1986), Dick-Peddie (1993), West and Young (2000), Howard (2003), Fryer (2009), and NatureServe Explorer (2011).

This ecological system occurs in the Colorado Plateau, Tavaputs Plateau and Uinta Basin in canyons, gravelly draws, hilltops, mesatops and dry flats at elevations generally below 1800 m. This type extends across northern New Mexico into the southern Great Plains on limestone hills and sandstone breaks. Soils are often rocky, shallow and alkaline. Adjacent upland systems include Colorado Plateau Pinyon-Juniper Woodland (CES304.767) and Inter-Mountain Basins Montane Sagebrush Steppe (CES304.785) (deeper soils) at higher elevations and Inter-Mountain Basins Mixed Salt Desert Scrub (CES304.784) at lower elevations. The environmental description is based on several other references, including Jameson et al. (1962), Brown (1982), West (1983a), Baker and Kennedy (1985), Francis (1986), Dick-Peddie (1993), West and Young (2000), Howard (2003), Fryer (2009), and NatureServe Explorer (2011).

The diagnostic species of this system, Artemisia nova or Artemisia bigelovii, grow in more xeric sites than other Artemisia shrublands (Hironaka et al. 1983). This dwarf-shrubland system is associated with shallow, rocky soils which experience extreme drought in summer. The plants are low and widely spaced, which tends to decrease the risk of fire. Fire is uncommon on drier sites because of discontinuous and low fuel buildup on the generally unproductive sites (Fryer 2009). Fire effects on Artemisia bigelovii is not known but assumed to be similar to Artemisia nova (Howard 2003), with fire-return intervals (FRI) ranging from 35 to over 100 years for xeric, low-productivity sagebrush communities of the Great Basin (Fryer 2009). In general, most sites are thought to have relatively long fire-return intervals (100-200 years) according to LANDFIRE models developed by experts (Fryer 2009). Stands in the western Great Plains typically have higher herbaceous cover (Shaw et al. 1989) which may decrease FRI. These shrubs are fire-sensitive and rarely sprout after burning. They reproduce from light wind-dispersed seeds from adjacent unburned areas to disturbed areas (Howard 1999, 2003, Fryer 2009). It generally takes around 30 years for a burned Artemisia nova stand to recover to pre-fire density (Hironaka et al. 1983, Fryer 2009). Artemisia tridentata ssp. wyomingensis may be present to codominant and shares similar ecological characteristics on these relatively xeric sites (Howard 1999).

Scattered trees may be present in some stands of this system. Fire reduces sagebrush abundance in both sagebrush and pinyon-juniper systems. Where these systems are adjacent, periodic fire likely prevents establishment of juniper and pinyon trees in sagebrush stands (Wright et al. 1979). In order to maintain dominance of sagebrush, fire-return interval must be long enough to permit sagebrush stands to mature, but short enough to prevent establishment and growth of trees in these sites. Fire-return intervals of 150-250 years for stand-replacing fire will likely maintain these shrublands. Expansion and contraction of trees into sagebrush shrublands are regulated by a combination of climate, fire, and bark beetle infestations with trees seedlings establishing during wetter periods (Wright et al. 1979, Paysen et al. 2000).

Insects are an important component of many shrub-steppe and grassland systems. Mormon crickets and grasshoppers are natural components of many rangeland systems (USDA-APHIS 2003, 2010). There are almost 400 species of grasshoppers that inhabit the western United States with 15-45 species occurring in a given rangeland system (USDA-APHIS 2003). Mormon crickets are also present in many western rangelands and, although flightless, are highly mobile and can migrate large distances consuming much of the forage while travelling in wide bands (USDA-APHIS 2010). Following a high population year for grasshoppers or Mormon crickets and under relatively warm dry spring environmental conditions that favor egg hatching and grasshopper and Mormon cricket survival, there may be large population outbreaks that can utilize 80% or more of the forage in areas as large as 2000 square mile. Conversely, relatively cool and wet spring weather can limit the potential for outbreaks. These outbreaks are naturally occurring cycles and, especially during drought, can denude an area of vegetation leaving it exposed to increased erosion rates from wind and water (USDA-APHIS 2003).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2310640). These are summarized as:

A) Early Development 1 All Structures (shrub-dominated - 10% of type in this stage): Early-seral community dominated by herbaceous vegetation; less than 6% sagebrush canopy cover; up to 24 years post-disturbance. Replacement fire occurs every 250 years on average. Succession to class B after 24 years.

B) Late Development 1 Open (shrub-dominated - 70% of type in this stage): Shrub cover is 0-10%. Mid-seral community with a mixture of herbaceous and shrub vegetation; 6-10% sagebrush canopy cover present; between 20-59 years post-disturbance. Replacement fire (FRI of 240 years) causes a transition to class A, whereas mixed-severity fire (FRI of 100 years) maintains the site in its present condition. In the absence of fire for 120 years, the site will follow an alternative succession path to class C. Otherwise, succession and mixed-severity fire keeps site in class B.

C) Late Development 1 Open (conifer-dominated - 20% of type in this stage): Shrub cover is 10-30%. Late-seral community with a mixture of herbaceous and shrub vegetation; >10% sagebrush canopy cover present; 75+ years post-disturbance. Replacement fire is every 200 years on average (transition to class A), whereas mixed-severity fire happens on average every 140 years due to a diminished herbaceous component compared to class B. Mixed-severity fire causes a transition to class B. Succession will keep the site in class C without fire.

Black sagebrush generally supports more fire than other dwarf sagebrushes. This type generally burns with mixed severity (average FRI of 100-140 years) due to relatively low fuel loads and herbaceous cover. Bare ground acts as a micro-barrier to fire between low-statured shrubs. Oils and resins present in the foliage and stems of sagebrush allow fire to spread. Stand-replacing fires (average FRI of 200-240 years) can occur in this type when successive years of above-average precipitation are followed by an average or dry year. Stand-replacement fires dominate in the late-succession class where the herbaceous component has diminished. Fires may or may not be wind-driven and only cover small areas. This type fits into Fire Regime Groups IV and III LANDFIRE 2007a, BpS 1210310).

Grazing by wild ungulates occurs in this type due to the high palatability of Artemisia nova compared to other browse. Native browsing tends to open up the canopy cover of shrubs but does not often change the succession stage (LANDFIRE 2007a, BpS 1210310).

Prolonged drought may reduce the foliar and basal covers of graminoids but not that of shrubs. Reduced foliar cover of graminoids will affect fire behavior. This effect is assumed minor and not included in the model (LANDFIRE 2007a, BpS 1210310).

The primary land uses that alter the natural processes of this system are associated with livestock grazing and introduction of exotic annual grasses. Artemisia bigelovii and Artemisia nova are utilized by livestock to a much greater degree than other species of Artemisia, resulting in low, pruned plants (West 1983a, Howard 2003d, Fryer 2009). Excessive grazing stresses the system through soil disturbance, diminishing or eliminating the biological soil crust, altering the composition of perennial species, and increasing the establishment of native disturbance-increasers and annual grasses, particularly Bromus tectorum and other exotic annuals. The introduction of exotic annual grasses has altered many stands by increasing the amount of fine fuels present that can substantially increase fire frequency and intensity which reduces the cover of fire-sensitive shrubs such as Artemisia bigelovii and Artemisia nova (Howard 2003d, Fryer 2009).

When grasshopper and Mormon cricket populations reach outbreak levels, they cause significant economic losses for ranchers and livestock producers, especially when accompanied by a drought (USDA-APHIS 2003, 2010). Both rangeland forage and cultivated crops can be consumed by grasshoppers. The U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) is the Federal agency responsible for controlling economic infestations of grasshoppers on western rangelands with a cooperative suppression program. They work with federal land managing agencies to conduct grasshopper suppression. The goal of APHIS's grasshopper program is not to eradicate them but to reduce outbreak populations to less economically damaging levels (USDA-APHIS 2003). This APHIS effort dampens the natural ecological outbreak cycles of grasshoppers and Mormon crickets but does not eradicate the species.

Human development has impacted many locations throughout the ecoregion. High- and low-density urban and industrial developments also have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

Occurs in the Colorado Plateau, Tavaputs Plateau and Uinta Basin in canyons, gravelly draws, hilltops, and dry flats at elevations generally below 1800 m.