Common (Translated Scientific) Name: Crested Wheatgrass - Cheatgrass - Tall Tumblemustard Western North American Ruderal Semi-Desert Scrub & Grassland Macrogroup
Colloquial Name: Western North American Cool Semi-Desert Ruderal Scrub & Grassland
Hierarchy Level: Macrogroup
Type Concept: This macrogroup contains disturbed grasslands and scrub found in semi-desert basins, piedmonts, and foothills throughout the western U.S. and Canada, possibly extending into lower montane zones on warm aspects. Vegetation of the macrogroup can be a monoculture of a single non-native graminoid species, or a mix of several non-native forbs and graminoids. Dominant graminoids include Agropyron cristatum (which has been purposefully seeded for forage or to prevent soil erosion, but has become naturalized), Bromus tectorum (an annual prolific seed-producer and highly invasive grass species), and Bromus arvensis. Invasive and wind- and animal-distributed non-native forb species include Descurainia sophia, Erodium cicutarium, Halogeton glomeratus, Sisymbrium altissimum, and Zygophyllum fabago. Numerous other non-native herbaceous species may be present to dominant. Invasive non-native shrublands are less common. This macrogroup can also include vegetation dominated by native ruderal species when vegetation is the result of anthropomorphic disturbance. These are dry grasslands, forb-dominated meadows or shrublands that occur in cool semi-arid climates. Stands occur on flat to moderately steep ground that can be large areas or narrow strips adjacent to roadsides or under powerlines and other disturbed areas. Soils are mostly mineral and well-drained. Soils may be compacted and eroded with biological crusts absent because of disturbance.
Diagnostic Characteristics: This widespread upland semi-desert scrub and grassland macrogroup is dominated by invasive, non-native shrubs, grasses or forbs. Widespread dominant and diagnostic herbaceous species include naturalized forage species such as Agropyron cristatum and numerous other non-native herbaceous species such as Bromus tectorum, Bromus arvensis, Halogeton glomeratus, Salsola tragus, Sisymbrium altissimum, Taeniatherum caput-medusae, Verbascum thapsus, and Zygophyllum fabago. No invasive non-native shrub species have been identified as being diagnostic to this type.
Rationale for Nominal Species or Physiognomic Features: This macrogroup may be dominated by many cool semi-desert non-native species such as Acroptilon repens, Agropyron cristatum, Bromus tectorum, Halogeton glomeratus, Salsola tragus, Sisymbrium altissimum, Taeniatherum caput-medusae, Verbascum thapsus, and Zygophyllum fabago. Agropyron cristatum, Bromus tectorum, and Sisymbrium altissimum are widespread cool semi-desert western species that were chosen to represent this macrogroup.
Classification Comments: This macrogroup may be difficult to determine from native grasslands where native species are present. The test is that the non-native species, especially invasive species, far outweigh native species in abundance, such that a well-trained observer cannot tell what the native counterpart may have been or to do so is only speculation. This macrogroup can also include vegetation dominated by native ruderal species when caused by anthropomorphic disturbance such as old fields.
Similar NVC Types:
M301 Western North American Ruderal Marsh, Wet Meadow & Shrubland, note: may overlap in transition zones where upland forbs intermix with wetter forbs and graminoids.
M493 Western North American Ruderal Grassland & Shrubland, note: may overlap where vegetation shares wide-ranging non-native species.
M498 Great Plains Ruderal Grassland & Shrubland, note: may overlap where vegetation shares wide-ranging non-native species.
M512 North American Warm Desert Ruderal Scrub & Grassland, note: "is similar but typically has more heat-tolerant or cold-sensitive species, such Bromus rubens or Eragrostis lehmanniana, dominating the vegetation."
Physiognomy and Structure: This macrogroup includes ruderal vegetation with an open to dense shrub canopy and/or an herbaceous layer dominated by annual or perennial grasses or forbs.
Floristics: Vegetation of the macrogroup can be a monoculture of a single non-native graminoid species, or a mix of several non-native forbs and graminoids. Graminoids include cool semi-arid Agropyron cristatum and other species which may have been purposefully seeded to prevent soil erosion or for forage livestock, but have become naturalized. Other invasive wind- and animal-distributed non-native species diagnostic of this macrogroup may include Bromus tectorum, Bromus arvensis (= Bromus japonicus), Descurainia sophia, Halogeton glomeratus, Hypericum perforatum, Salsola tragus, Sisymbrium altissimum, Taeniatherum caput-medusae, Verbascum thapsus, and Zygophyllum fabago. Numerous other non-native herbaceous species may be present to dominant. Invasive non-native shrublands are less common. This macrogroup can also include vegetation dominated by native ruderal species when vegetation is the result of anthropomorphic disturbance. However, ruderal native shrublands such as Gutierrezia sarothrae and Ericameria spp. are included in native macrogroups.
Dynamics: Most of the invasive diagnostic species are cool-season (C3) plants such as Agropyron cristatum and Bromus tectorum. Cheatgrass expansion has radically changed fire regimes and vegetation over large areas in the Intermountain West. Cheatgrass invades native vegetation such as big sagebrush shrubland, then produces large amounts of fine fuels that readily carry fire, increasing the number, size and frequency of burns (FRI = 3-5 year) which reduces cover of perennial vegetation and favors dominance by annual grasses (Young and Evans 1978, Zouhar 2003). Crested wheatgrass burns quickly and is therefore less susceptible to damage by fire than some native bunchgrass species that have a thick cespitose growth form. The fire may stay longer in the culms, resulting in heat transfer to the ground and the death of the plant (DePuit 1986). In crested wheatgrass, there is usually little heat transfer into the soil, so the tillers and root system are usually undamaged (DePuit 1986). Thus the more frequent fire regime caused by the introduction of Bromus tectorum also favors the maintenance of Agropyron cristatum over the establishment or survival of native bunchgrasses (S. Rust pers. comm. 2014).
Environmental Description: This ruderal macrogroup occurs in cool semi-arid areas throughout western North America and is composed of disturbed upland grasslands and scrub dominated by non-native species. Most stands occur below approximately 1500 m (5000 feet) in elevation. Generally, these are areas that have been heavily disturbed by heavy equipment, such as old plowed fields, townsites, abandoned mill sites, and livestock holding areas. It is abundant in waste areas often as abandoned pastures that are no longer irrigated, construction areas, roadside margins or other weedy places. However, it also occurs over vast acres of livestock-grazed lands in the semi-arid west, where livestock such as cows and horses have broken soil biotic crust, compacted soil and reduced native plant vigor. Sites are not mowed or otherwise maintained. Climate: Climate is cool semi-arid continental, with most of the precipitation falling during the winter and spring. Below freezing temperatures are common in the winter. Soil/substrate/hydrology: This macrogroup occurs on disturbed mesic to dry soils. The physical environmental settings are similar to both semi-desert grassland and semi-desert shrub-steppe macrogroups.
Geographic Range: This macrogroup contains disturbed grasslands and scrub found in semi-desert basins, piedmont, and foothills throughout the western U.S. and Canada, and possibly extending into lower montane zones on warm aspects.
Nations: CA, US
States/Provinces: AZ, CA, CO, ID, MT, NM, NV, OR, UT, WA?, WY
|US Forest Service Ecoregions (2007)|
Confidence Level: Moderate
Confidence Level Comments:
Synonomy: > Agropyron cristatum (Crested wheatgrass rangelands) Semi-natural Stands (Sawyer et al. 2009) [42.030.00]
> Bromus tectorum (Cheatgrass grassland) Semi-natural Stands (Sawyer et al. 2009) [42.020.00]
>< Crested Wheatgrass (614 ) (Shiflet 1994) [Represents crested wheatgrass in Northern Great Plains, but does not necessarily represent Interior West include other dominant diagnostic species.]
Concept Author(s): K.A. Schulz, in Faber-Langendoen et al. (2014)
Author of Description: K.A. Schulz
Version Date: 15Oct2014
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