Wyoming Basin Dwarf Sagebrush

This windswept ecological system is composed of dwarf sagebrush shrubland and shrub-steppe that forms matrix vegetation and large patches on the margins of high-elevation basins in central and southern Wyoming. Typical sites are gently rolling hills and long, gently sloping pediments and fans. These sites are very windy and have shallow, often rocky soils (Artemisia nova and Artemisia tripartita ssp. rupicola) or have shallow, poorly drained, fine-textured soils (Artemisia arbuscula). The distinguishing feature of this system is a short-shrub stratum in which dwarf-shrubs (<30 cm tall) contribute at least two-thirds of the woody canopy. Four sagebrush taxa may dominate the shrub stratum: Artemisia tripartita ssp. rupicola, Artemisia nova, Artemisia arbuscula ssp. longiloba, and wind-dwarfed Artemisia tridentata ssp. wyomingensis. Two or more of these sagebrushes often codominate, but any of them may occur alone. Where graminoids are common and tall, the vegetation often has the appearance of grassland without shrubs; the presence of shrubs is obvious only when the vegetation is viewed up close. Where graminoids contribute less cover, the vegetation is a compact shrubland. The herbaceous component of the vegetation includes both rhizomatous and bunch-form graminoids, cushion plants, and other low-growing forbs. Bouteloua gracilis, a common species of Inter-Mountain Basins Big Sagebrush Steppe (CES304.778) in Wyoming, is absent.

This system includes dwarf-shrublands dominated by one of four species: Artemisia tripartita ssp. rupicola, Artemisia nova, Artemisia arbuscula ssp. longiloba, and wind-dwarfed Artemisia tridentata ssp. wyomingensis that characterize different sites. Two or more of these sagebrushes often codominate, but any of them may occur alone. Other often wind-dwarfed shrubs, such as Artemisia frigida, Chrysothamnus viscidiflorus, Ericameria nauseosa, Gutierrezia sarothrae, or Purshia tridentata, may be present and occasionally abundant. Where graminoids are common and tall, the vegetation often has the appearance of grassland without shrubs; the presence of shrubs is obvious only when the vegetation is viewed up close. Where graminoids contribute less cover, the vegetation is open to a moderately dense canopy of compact shrubs <30 cm tall. The herbaceous component of the vegetation typically includes cool-season rhizomatous and bunch-form graminoids, cushion plants, and other low-growing forbs. Common graminoids species include Achnatherum hymenoides (= Oryzopsis hymenoides), Achnatherum speciosum (= Stipa speciosa), Achnatherum thurberianum (= Stipa thurberiana), Elymus elymoides, Elymus lanceolatus, Festuca idahoensis, Hesperostipa comata (= Stipa comata), Koeleria macrantha, Leucopoa kingii, Pascopyrum smithii, Poa fendleriana, Poa secunda, and Pseudoroegneria spicata. Forb cover is typically minor and includes Antennaria microphylla, Cerastium arvense, Heterotheca villosa, Packera multilobata (= Senecio multilobatus), Phlox hoodii, Senecio integerrimus, Sphaeralcea coccinea, and Stenotus armerioides.

Climate: Climate is semi-arid with 20-30 (45) cm of annual precipitation. The temperature regime is continental, with cold winters, warm summers, large diurnal ranges, and a short frost-free season.

Physiography/landform: This windswept ecological system of dwarf sagebrush shrubland and shrub-steppe occurs from 1500 to 3200 m elevation. These sites are very windy, gently rolling hills and long, gently sloping pediments and fans, broad ridgetops, the ridges of low mountains and the margins of high-elevation basins.

Soil/substrate/hydrology: Soils are variable but are often shallow and rocky. Artemisia nova generally occupies medium- to coarse-textured soils, often with a large volume of rock fragments and frequently calcareous. Artemisia arbuscula-dominated stands have poorly drained, very heavy, montmorillonite (smectite) clay soils with some coarse fragments, usually effectively very shallow to a hard clay pan, not deep enough to support either big sagebrush or deep-rooted grasses. Those two sagebrushes do grow together sometimes. Artemisia tripartita ssp. rupicola-dominated stands have coarse-textured (gravelly), well-drained shallow soils.

The key ecological factors for this system are the harsh, windswept, semi-arid climate with a short growing season and shallow soils. Artemisia nova and Artemisia tripartita ssp. rupicola dwarf-shrublands are associated with shallow, rocky soils which experience extreme drought in summer, whereas Artemisia arbuscula-dominated stands occur on shallow, poorly drained, fine-textured soils.

Fire is not important in this ecosystem, because it occurs very infrequently. Plants are low and widely spaced so there is little fuel to carry a fire. Replacement fire is predicted to occur every 300 years (LANDFIRE 2007a). Fire effects are variable depending on dominant species. Artemisia arbuscula ssp. longiloba, Artemisia nova, and Artemisia tridentata ssp. wyomingensis are generally killed by burning and do not resprout, so fire impacts can be severe (Young 1983, Howard 1999, Steinberg 2002a, Fryer 2009). However, Artemisia tripartita ssp. rupicola shrubs can sprout from the stump after being top-killed by fire and will reproduce both by seed and by layering (Tirmenstein 1999k). Hironaka et al. (1983) notes that some populations may have variation in this ability.

LANDFIRE developed a VDDT model for this system which has two classes (LANDFIRE 2007a, BpS 2210720):
A) Early Development 1 All Structures (herbaceous-dominated-30% of type in this stage): Grass-and-forb-dominated site for approximately 125 years. Black/low sagebrush seedlings are young and begin to establish towards the end of this seral period. Replacement fire occurs every 300 years

B) Late Development 1 Open (shrub-dominated-70% of type in this stage): Black/low sagebrush with mid-height late-seral grasses (150 or more years).

Soil erosion caused by native ungulates sometimes can occur in these stands when they trail across them, especially in spring and fall when the sites are wet. The sites are resilient and resistant to trampling in summer and winter, when they are dry or frozen (LANDFIRE 2007a).

The primary threats that alter the natural processes of this system are poor 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. While grazing appears to have little effect on shrub densities, it does tend to decrease the abundance of tall bunchgrasses and increase the cover of forbs such as Arenaria congesta (Johnston 2001). Shrubs are favored in overgrazed ranges because heavy grazing may deplete the perennial graminoid layer leaving only a shrub layer that may increase at the expense of grass cover (Hironaka et al. 1983). Grazing also favors non-native, grazing-tolerant grass species such as Poa bulbosa and Poa pratensis.

Excessive grazing also 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 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).

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 Wyoming. High- and low-density urban and industrial developments 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 or oil and gas operations can drastically impact natural vegetation. Large-scale wind power development is expanding in this system, fragmenting the habitat and facilitating establishment of invasive species. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

This system occurs throughout the basins of central and southern Wyoming, extending south into adjacent portions of Colorado. It also occurs on the eastern side of the Continental Divide in Montana, where Artemisia nova shrublands are found on calcareous substrates.