Columbia Plateau Low Sagebrush Steppe

This ecological system can form the matrix of the landscape and is composed of sagebrush dwarf-shrub-steppe that occurs in a variety of shallow-soil habitats throughout eastern Oregon, northern Nevada and southern Idaho. Artemisia arbuscula ssp. arbuscula and close relatives (Artemisia arbuscula ssp. longiloba and occasionally Artemisia nova) form stands that typically occur on mountain ridges and flanks and broad terraces, ranging from 1000 to 3000 m in elevation. Substrates are shallow, fine-textured soils, poorly drained clays that occur in thin-soil areas and are frequently very stony. Other shrubs and dwarf-shrubs present may include Purshia tridentata, Eriogonum spp., and other species of Artemisia. Common graminoids include Festuca idahoensis, Koeleria macrantha, Poa secunda, and Pseudoroegneria spicata. Many forbs also occur and may dominate the herbaceous vegetation, especially at the higher elevations. Isolated individuals of Juniperus occidentalis and Cercocarpus ledifolius can often be found in this system. This ecological system is closely related to the concept of shallow-dry sagebrush in the resistance-resilience framework.

Artemisia arbuscula ssp. arbuscula and close relatives (Artemisia arbuscula ssp. longiloba and occasionally Artemisia nova) form stands. Other shrubs and dwarf-shrubs present may include Purshia tridentata, Eriogonum spp., and other species of Artemisia. Common graminoids include Festuca idahoensis, Koeleria macrantha, Pseudoroegneria spicata, and Poa secunda. Many forbs also occur and may dominate the herbaceous vegetation, especially at the higher elevations. Isolated individuals of Juniperus occidentalis (western juniper) and Cercocarpus ledifolius (mountain-mahogany) can often be found in this system.

This system occurs on shallow-soil habitats, ranging from 1000 to 3000 m in elevation.

Climate: Climate is semi-arid with a large proportion of the 20-30 cm of annual precipitation falling as winter snow. The temperature regime is continental, with cold winters, warm summers, a large diurnal temperature range, and a short frost-free season.

Physiography/landform: Stands typically occur on mountain ridges and flanks and broad terraces, but may be associated with flats, depressions, and slopes with soils that are either very shallow or quite poorly drained. In the Columbia River Basin, the vegetation in this system occupies the driest habitats of all the Artemisia-dominated stands.

Soil/substrate/hydrology: Substrates are generally fine-textured, usually poorly drained clays that occur in shallow-soiled areas, which are almost always very stony and characterized by recent rhyolite or basalt. Beetle and Johnson (1982) report that Artemisia arbuscula ssp. arbuscula grows in soils with a high volume of gravel (even though soil may be in clay textural class or contain a clay-rich layer that impedes drainage), and that Artemisia arbuscula ssp. longiloba grows in clay soils, often alkaline, that contain no gravels. Soils dominated by Artemisia nova are typically alkaline and calcareous.

The diagnostic species of this system, Artemisia arbuscula ssp. arbuscula, Artemisia arbuscula ssp. longiloba, or Artemisia nova, grow in more xeric sites than other Artemisia shrubs (Hironaka et al. 1983), and are highly drought-tolerant. Artemisia arbuscula tends to grow where claypan layers exist in the soil profile and soils are often saturated during a portion of the year, while Artemisia nova tends to grow where there is a root-limiting layer in the soil profile (LANDFIRE 2007a). This 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 (Chappell et al. 1997).

Fire influences the density and distribution of shrubs. In general, fire increases the abundance of herbaceous perennials and decreases the abundance of woody plants (WNHP 2011). The fire interval for this system is 110 years (LANDFIRE 2007a). Anecdotal observations indicate that these patches often are not burned during surrounding forest fires. Fire is uncommon because of discontinuous and low fuel buildup on the generally unproductive sites (Young and Palmquist 1992, Fryer 2009, Sawyer et al. 2009). Most sites are thought to have relatively long fire-return intervals (100-200 years) according to LANDFIRE models developed by experts (LANDFIRE 2007a). These shrubs are fire-sensitive and rarely sprout after burning.

The dominant shrub species can easily colonize burns via wind-dispersed seeds from adjacent unburned areas into disturbed areas (Howard 1999, Steinberg 2002a, Fryer 2009). It generally takes around 30 years for a burned stand to recover to pre-fire shrub density (Zamora and Tueller 1973, Hironaka et al. 1983, Howard 1999, Steinberg 2002a, Fryer 2009). However, recovery of this system after fire may take up to 325-450 years (Baker 2006).

Grazing by wild ungulates occurs in this shrubland system. Native browsing tends to open the canopy cover of shrubs but does not often change the successional stage (LANDFIRE 2007a).

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 miles. 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 VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 0811240 and BpS 0911240). Dominant shrub is Artemisia arbuscula. Dominant herbaceous species are Poa secunda and Pseudoroegneria spicata.

A) Early Development 1 All Structures (10% of type in this stage): Zero to 1% low sagebrush cover. Herbaceous cover of bunchgrasses and forbs would fill to about 20-30% cover within a few years.

B) Mid Development 1 Open (40% of type in this stage): Dominant lifeform is herb. Minimum cover = 20%, maximum cover = 40%. Minimum height for herbs is 0.6 m. Scattered and usually small low sagebrush is present, but perennial grasses and forbs continue to dominate. The general formation is that of a shrub savanna. Sagebrush cover is usually 1-5% in this stage.

C) Late Development 1 Open (50% of type in this stage): Sagebrush is codominant with perennial grasses and forbs. Sagebrush and herbaceous cover can be variable depending on site productivity. Bare ground and rock in the interspaces increase on less productive sites. The general formation is that of a shrubland. Expected composition is 50-60% grass; 5-10% forbs; 20-40% shrubs. Windswept ridges with thinner soils may be still more open.

The primary land uses that alter the natural processes of this system are associated with livestock practices, annual exotic species invasion, fire regime alteration, direct soil surface disturbance, and fragmentation. Barbour and Major (1988) report that Artemisia nova is utilized by livestock to a much greater degree than other species of Artemisia, resulting in low, pruned plants (West 1983a). Both Artemisia arbuscula and Artemisia nova are considered a valuable browse plant during the spring, fall, and winter months and are often grazed by native ungulates (elk and mule deer) and domestic livestock. Prolonged livestock use can cause a decrease in the abundance of native, perennial bunchgrasses and increase in the cover of shrubs and non-native grass species, such as Poa bulbosa and Poa pratensis.

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 madritensis, Bromus tectorum, Schismus spp., and other exotic annual grasses. 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 nova (Fryer 2009, Sawyer et al. 2009).

Direct and indirect fire suppression are a threat to this system where stands are adjacent to pinyon-juniper woodlands. Over the long term, heavy grazing by livestock removes the fine fuels that carry fire that indirectly leads to a reduction in fire frequencies, which can lead to pinyon-juniper encroachment with subsequent loss of shrub and herbaceous understory (LANDFIRE 2007a).

Human development has impacted many locations throughout the range of this ecological system. 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 indirect 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.

This system is found throughout the basins of eastern Oregon and southern Idaho, south into northern Nevada and northeastern California.