Intermountain Semi-Desert Shrub-Steppe

This ecological system occurs throughout the intermountain western U.S., typically at lower elevations on alluvial fans and flats with moderate to deep soils, and extends into south-central Montana between the Pryor and Beartooth ranges where a distinct rainshadow effect occurs. This semi-arid shrub-steppe is typically dominated by graminoids (>25% cover) with an open shrub to moderately dense woody layer with a typically strong graminoid layer. The most widespread (but not dominant) species is Pseudoroegneria spicata, which occurs from the Columbia Basin to the Northern Rockies. Characteristic grasses include Achnatherum hymenoides, Bouteloua gracilis, Distichlis spicata, Poa secunda, Poa fendleriana, Sporobolus airoides, Hesperostipa comata, Pleuraphis jamesii, and Leymus salinus. The woody layer is often a mixture of shrubs and dwarf-shrubs, although it may be dominated by a single species. Characteristic species include Atriplex canescens, Artemisia tridentata, Chrysothamnus greenei, Chrysothamnus viscidiflorus, Ephedra spp., Ericameria nauseosa, Gutierrezia sarothrae, and Krascheninnikovia lanata. Artemisia tridentata or Atriplex canescens may be present but does not dominate. Annual grasses, especially the exotics Bromus arvensis and Bromus tectorum, may be present to abundant. Forbs are generally of low importance and are highly variable across the range but may be diverse in some occurrences. The general aspect of occurrences may be either open shrubland with patchy grasses or patchy open herbaceous layers. Disturbance may be important in maintaining the woody component. Microphytic crust is very important in some stands.

The plant associations in this broadly-defined system are typically semi-arid shrub-steppe dominated by graminoids (>20% cover) with an open shrub to moderately dense woody layer. The typically open woody layer is often a mixture of shrubs and dwarf-shrubs, although it may be dominated by a single species. This system also includes some open to sparse, mixed-shrub stands with sparse herbaceous layers that are frequently the result of disturbance. Characteristic woody species include Artemisia filifolia, Artemisia tridentata, Atriplex canescens, Ephedra cutleri, Ephedra nevadensis, Ephedra torreyana, Ephedra viridis, Ericameria nauseosa, Chrysothamnus viscidiflorus, Gutierrezia sarothrae, Krascheninnikovia lanata, and Sarcobatus vermiculatus. Other shrubs occasionally present include Purshia tridentata and Tetradymia canescens. Shrubs such as Artemisia filifolia, Artemisia tridentata, Atriplex canescens, or Sarcobatus vermiculatus may be present but do not dominate as they do in big sagebrush shrublands and steppes, black greasewood or mixed salt desert scrub systems. Trees are very rarely present in this system, but some individuals of Pinus ponderosa, Juniperus scopulorum, Juniperus occidentalis, or Cercocarpus ledifolius may occur. The typically moderately dense herbaceous layer is dominated by bunchgrasses which occupy patches in the shrub matrix. One of the most widespread species is Pseudoroegneria spicata, which occurs from the Columbia Basin to the Northern Rockies. Other locally dominant or important species include Sporobolus airoides, Leymus cinereus, Festuca idahoensis, Pascopyrum smithii, Bouteloua gracilis, Distichlis spicata, Pleuraphis jamesii, Elymus lanceolatus, Elymus elymoides, Koeleria macrantha, Muhlenbergia richardsonis, Hesperostipa comata, and Poa secunda. Annual grasses, especially the exotics Bromus arvensis (= Bromus japonicus) and Bromus tectorum, may be present to abundant. Forbs are generally of low importance and are highly variable across the range, but may be diverse in some occurrences. Species that often occur are Symphyotrichum ascendens (= Aster adscendens), Collinsia parviflora, Penstemon caespitosus, Achillea millefolium, Erigeron compositus, Senecio spp., and Taraxacum officinale. Other important genera include Astragalus, Oenothera, Eriogonum, and Balsamorhiza. Mosses and lichens may be important ground cover. Forbs are common on disturbed weedy sites. Weedy annual forbs may include the exotics Descurainia spp., Helianthus annuus, Halogeton glomeratus, Lactuca serriola, and Lepidium perfoliatum. The vegetation description is based on several references, including Hanson (1929), Branson et al. (1976), Barbour and Major (1977), Brown (1982), Hironaka et al. (1983), West (1983e), Knight (1994), Holland and Keil (1995), Reid et al. (1999), West and Young (2000), Tuhy et al. (2002), Barbour et al. (2007), and Sawyer et al. (2009).

This ecological system occurs throughout the intermountain western U.S., from the western Great Basin to the northern Rocky Mountains and Colorado Plateau and extends into south-central Montana between the Pryor and Beartooth ranges where a distinct rainshadow effect occurs. Elevation ranges from 300 m up to 2500 m. The climate where this system occurs is generally hot in summers and cold in winters with low annual precipitation, ranging from 18-40 cm and high inter-annual variation. Much of the precipitation falls as snow, and growing-season drought is characteristic. Temperatures are continental with large annual and diurnal variations. Sites are generally alluvial fans and flats with moderate to deep soils. Some sites can be flat, poorly drained and intermittently flooded with a shallow or perched water table often within 1 m depth (West 1983e). Substrates are generally shallow, calcareous, fine-textured soils (clays to silt loams), derived from alluvium; or deep, fine to medium-textured alluvial soils with some source of subirrigation during the summer season. Soils may be alkaline and typically moderately saline (West 1983e). Some occurrences are found on deep, sandy loam soils, or soils that are highly calcareous, but not deep sand with active dune fields (Hironaka et al. 1983). The environmental description is based on several references, including Hanson (1929), Branson et al. (1976), Barbour and Major (1977), Brown (1982), Hironaka et al. (1983), West (1983e), Knight (1994), Holland and Keil (1995), Reid et al. (1999), West and Young (2000), Tuhy et al. (2002), Barbour et al. (2007), Sawyer et al. (2009), and NatureServe Explorer (2011).

Disturbance dynamics in this system are variable because of variation in the compositions; however, most are dominated by short- to long-lived, deciduous shrubs that are adapted to low- to medium-frequency, medium- to large-sized and low- to medium-intensity fire (Carey 1995, Tirmenstein 1999b, 1999f, 1999g, Anderson 2001b, 2004b, Scher 2001, Sawyer et al. 2009). Some shrubs, such as Chrysothamnus viscidiflorus, Ephedra torreyana, Ephedra viridis, Ericameria nauseosa, Sarcobatus vermiculatus, and Tetradymia canescens, are generally top-killed in burns, but then vigorously resprout from rootcrowns unless the fire was very severe (Tirmenstein 1999b, 1999f, 1999g, Anderson 2001b, 2004b, Scher 2001, Sawyer et al. 2009). Other shrubs, such a Gutierrezia sarothrae and Krascheninnikovia lanata, are more typically killed by fire and only weakly sprout post-fire, if at all (Carey 1995, Tirmenstein 1999g). However, in most cases, reestablishment generally proceeds rapidly through light wind-dispersed seeds from adjacent unburned areas, and in some case, these shrub species aggressively invade disturbed open sites, then decline after 15 years to be replaced by longer-lived species such as Artemisia tridentata or Sarcobatus vermiculatus (Carey 1995, Tirmenstein 1999b, 1999f, 1999g, Anderson 2001b, 2004b, Scher 2001, Sawyer et al. 2009). Some stands, such as those dominated by Ericameria parryi, are too sparse to carry fire (Sawyer et al. 2009). Many stands have a lush herbaceous layer that dries to fine fuels that readily carry fire regardless of shrub density.

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 2003).

Chrysothamnus viscidiflorus plants are relatively short-lived (approximately 12-13 years). Infestation in some densely populated stands by larvae of the beetle Acamaeodera pulchella is related to senescence and death. In shrubland stands were Chrysothamnus viscidiflorus is scattered, there are lower levels of infestation (Young and Evans 1974).

The primary land uses that alter the natural processes of this system are associated with livestock grazing and introduction of exotic annual grasses. Increases in fine fuels from alien annual grasses, such as Bromus madritensis, Bromus tectorum, and Schismus spp., can substantially increase the fire frequency. Shrubs species, such as Chrysothamnus viscidiflorus, Ephedra torreyana, Ephedra viridis, Ericameria nauseosa, Sarcobatus vermiculatus, and Tetradymia canescens, that vigorously resprout from rootcrowns after burning become more common and replace longer-lived and more fire-sensitive species such as Artemisia tridentata, Artemisia bigelovii, Artemisia nova, and Coleogyne ramosissima (Tirmenstein 1999a, 1999b, Sawyer et al. 2009). This system is expected to increase in range with increased fire-return interval (FRI) (Tirmenstein 1999a, 1999b, Sawyer et al. 2009).

Both Chrysothamnus viscidiflorus and Ericameria nauseosa are considered important forage for livestock on depleted ranges (Tirmenstein 1999a, 1999b). Palatability of these shrubs to both livestock and wildlife is variable depending subspecies, with Chrysothamnus viscidiflorus ssp. lanceolatus, Chrysothamnus viscidiflorus ssp. puberulus, Ericameria nauseosa var. speciosa (= Chrysothamnus nauseosus ssp. albicaulis), Ericameria nauseosa var. hololeuca, and Ericameria nauseosa var. salicifolia noted as favored subspecies (Tirmenstein 1999a, 1999b). Over time, heavy grazing by livestock will decrease abundance of desirable perennial grasses such as Achnatherum hymenoides, Bouteloua gracilis, Hesperostipa comata, Pleuraphis jamesii, or Pseudoroegneria spicata.

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 the 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.

This system occurs throughout the intermountain western U.S., typically at lower elevations, and extends into Wyoming and Montana across the Great Divide Basin. It barely gets as far north into north-central Montana (mapzone 20) but is unlikely to be mapped.