Bromus tectorum L. var. nudus

Type fragment for Bromus dumetorum Lam.
Catalog Number: US 865488A
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Verification Degree: Card file verified by examination of alleged type specimen
Preparation: Pressed specimen
Collector(s): Collector unknown
Locality: France, Europe

Canada

Rounded National Status Rank: NNA - Not Applicable

United States

Rounded National Status Rank: NNA - Not Applicable

More info for the term: cover

The native range of cheatgrass encompasses much of Europe, the northern rim of Africa, and southwestern Asia [246,321]. Cheatgrass can now be found throughout most of Europe to southern Russia and western Asia. It has also been introduced to North America, temperate South America, Japan, South Africa, Australia, New Zealand, and Iceland [321,438,492].

The introduction of cheatgrass to North America probably occurred independently, several times, via ship ballast, contaminated crop seed, packing material and at least 1 deliberate introduction for a college experiment in Pullman, Washington, in 1898 [321,438]. By the early 1860s, cheatgrass had been identified in the U.S. in New York and Pennsylvania [235,492], and by the early 1900s, cheatgrass was present in much of its current range, though it was sparsely distributed [261,470]. Early infestations were commonly found near wheat (Triticum aestivum) cropland and railroads [307]. Once introduced, cheatgrass spread into adjacent rangeland in areas where it was adapted to local environmental conditions. Its expansion was especially rapid in parts of the Intermountain West, where its introduction followed a period of excessive livestock grazing in an ecosystem comprised of native plants that apparently did not evolve with heavy grazing pressure [261,307].

Cheatgrass now occurs throughout most of the U.S., Canada, Greenland, and northern Mexico [307]. It has been found in all Canadian provinces, and its range extends into Alaska and Yukon. In the eastern U.S. cheatgrass is mainly a roadside weed [438]. It is a "common invader" in the northern Great Plains [324]. Cheatgrass is most prominent and invasive in the Intermountain West, west from the Rockies to the Cascades and Sierra Nevada and north from central Utah, Nevada, and northeastern California to Canada. It is especially invasive in sagebrush (Artemisia spp.) steppe and bunchgrass regions in the Great Basin, Columbia Basin, and Snake River Plains in Nevada, Utah, Washington, Oregon, and Idaho [307,438,461]. The number and size of infestations in these regions has increased dramatically over the last 20 years [307]. Plants Database provides a distributional map of cheatgrass in the United States and Canada.

The following table presents acres of BLM-administered rangelands either infested or at risk of infestation by cheatgrass in a 5-state area as of 1992 (adapted from [337]):

*Percent values refer to the estimated composition of cheatgrass by weight in the plant community

The National Science and Technology Center mapped about 31.5 million acres (12.7 million ha) of cheatgrass in the Great Basin from 2000 satellite imagery, about 6.7 million acres (2.7 million ha) of which was in "non-vegetative" areas (i.e. agriculture, urban, water and barren) [282].

Specific information on the plant communities in which cheatgrass may be found is limited outside its primary area of occurrence, where it is found mostly on abandoned cropland, roadsides, and "waste places." The following lists reflect ecosystems and cover types in which cheatgrass is most common and invasive, and are not necessarily exhaustive or definitive.

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More info for the terms: association, climax, codominant, competition, cover, density, fire severity, fire tolerant, fire-free interval, forb, forbs, fuel, grass/fire cycle, invasive species, litter, mesic, natural, presence, series, severity, shrub, shrubs, succession, tree, wildfire

Cheatgrass is a facultative seral species, acting as both an early seral invader and as a climax dominant on many sites that historically supported perennial grass and forb communities. Everett [148] suggests that in areas where cheatgrass is dominant or codominant, the habitat type or "potential natural community" may not be useful classifications due to severity of past disturbances and irreversible changes in the plant community over time. Instead one might consider the "most probable plant community" at a given site under specified circumstances. Cheatgrass is especially prevalent in the early stages of fire succession or following other disturbances when shrubs, trees, and perennial grasses are removed [34,139,145,210,291,295,382,396]. Cheatgrass' successional status varies with plant community composition, disturbance type, and disturbance history.

Cheatgrass can outcompete many other competitive nonnative plant species including halogeton (Halogeton glomeratus), tumblemustard (Sisymbrium altissimum), and Russian-thistle (Salsola kali) [120,344]. On some sites cheatgrass may be replaced by other highly competitive, nonnnative species such as forage kochia (Kochia prostrata) [90,271,305,331,476], and nonnative invasives such as common St. Johnswort (Hypericum perforatum), Dalmatian toadflax (Linaria dalmatica), yellow starthistle (Centaurea solstitialis), spotted knapweed (C. stoebe ssp. micranthos), diffuse knapweed (C. diffusa), squarrose knapweed (C. virgata ssp. squarrosa), rush skeletonweed (Chondrilla juncea), and leafy spurge (Euphorbia esula) [187,374]. Northam and Callihan [318] also suggest that cheatgrass may potentially be replaced by other nonnative, annual grasses (e.g., interrupted windgrass (Apera interrupta), corn brome (Bromus squarrous), little lovegrass (Eragrostis minor), poverty grass (Sporobolus vaginiflorus), and ventenata (Ventenata dubia)) that are capable of invading and establishing in cheatgrass-infested areas.

Artificial seeding of nonnative (and sometimes invasive) perennial species such as crested wheatgrass (Agropyron cristatum) is commonly carried out following wildfire on managed lands to preclude the development of undesirable cheatgrass stands and to meet other management objectives [144,260,329]. A study in Utah followed vegetation changes for 3 years after wildfire and artificial seeding of desirable perennial species (mostly nonnative). Cheatgrass density increased steadily for 3 years following burning and seeding, unlike the seeded species [329].

Sagebrush steppe: Cheatgrass is most invasive and persistent in sagebrush steppe and cold desert regions of the Intermountain West. Cheatgrass initially established in the Intermountain area with the introduction of livestock, which dramatically changed the balance between herbaceous understory and woody overstory species when intense grazing removed native bunchgrasses. There is a lack of native annual grasses, so cheatgrass fills that niche, and has truncated succession on many sites [111,159]. Results presented by Young and Evans [482] suggest that as long as there is a seed source and a suitable seedbed, cheatgrass will dominate on big sagebrush sites after removal of shrub overstory. They did not encounter an assemblage of native annual plants that was capable of preventing cheatgrass dominance on big sagebrush sites.

Cheatgrass can dominate the 1st or 2nd year after disturbance, and has dominated some sites for 40 to 80 years, even in the absence of further disturbance [48,93,115,167,236]. This suggests that communities dominated by cheatgrass are a permanent and widespread feature of the landscape in some areas [236]. Piemeisel [344] quantified the steps in succession from bare ground to cheatgrass dominance and reported that even rodent disturbance was sufficient to maintain cheatgrass dominance. On sites where a major shrub and/or bunchgrass component has not been eliminated by cultivation, fire, grazing, or herbicides, a shrub/cheatgrass or bunchgrass/cheatgrass climax community may occur [96,111,159]. When excessive grazing causes replacement of most perennial herbs by cheatgrass and other annuals, and when fire eliminates shrubs like big sagebrush and antelope bitterbrush, the percentage of rangeland dominated entirely by annuals progressively increases [111,159]. On some sites essentially pure cheatgrass stands may be found [461]. A combination of burning and grazing can also result in annual rangeland dominated by cheatgrass and rubber rabbitbrush [116,159].

A common pattern of succession after disturbance in cropped and abandoned fields in the Great Basin and Snake River Plains begins with Russian-thistle and/or flixweed tansymustard (Descurainia sophia) and tumblemustard, followed by cheatgrass dominance within about 5 years [111,146,159,343,344,483,488]. Cheatgrass outcompetes tumblemustard and other broadleaf plants when sufficient litter has accumulated on the soil surface to allow for cheatgrass germination [146,344]. On some sites, cheatgrass may be pre-empted by foxtail fescue (Vulpia myuros), a native annual grass [483]. Abandoned fields in the bluebunch wheatgrass/Sandberg bluegrass habitat type quickly develop a dense stand of cheatgrass that may dominate, along with rubber rabbitbrush, for more than 40 years [111,115]. On heavily grazed or abandoned farmland in the common snowberry (Symphoricarpos albus)/Idaho fescue and Nootka rose (Rosa nutkana)/Idaho fescue zones, cheatgrass will ultimately give way to Kentucky bluegrass, and on some sites, medusahead [159]. In the sagebrush steppe in northeastern California, Russian-thistle, tumblemustard, and cheatgrass form a seral continuum that closes many sagebrush communities to the establishment of perennial seedlings. Medusahead has extended the seral continuum by replacing cheatgrass on some low sagebrush (Artemisia arbuscula) sites on the Modoc Plateau [309,481].

Medusahead can replace cheatgrass on some sites [295,381,475], especially moist sites [196] and those with fine-textured soils [187,198]. Over the past 40 years medusahead has replaced cheatgrass over extensive areas in the sagebrush zone in California, Idaho, Oregon, and Washington [196]. Medusahead litter impedes cheatgrass establishment, and medusahead may do better in low-nitrogen environments than cheatgrass [187,189]. Coexistence of cheatgrass and medusahead is most likely in habitats low in nitrogen and phosphorus. In more fertile habitats, cheatgrass is likely to have the competitive advantage unless other environmental factors (e.g. high clay content) favor medusahead [109].

Soils in sagebrush steppe habitats tend to be low in organic matter, low in available phosphorus and nitrogen, and have limited water availability; therefore, mycorrhizae can be important to native plants for acquisition of water and nutrients. Invasion by either nonmycorrhizal or facultative mycorrhizal plants such as cheatgrass can reduce populations of mycorrhizae, thus indirectly affecting successional dynamics in semiarid lands [171,464].

Postfire succession in sagebrush steppe: Grazing and agricultural practices have disturbed many habitats, but each year, more sagebrush rangeland is converted to annual grass rangeland due to wildfires. The successional trajectory following fire depends on prefire plant community and seed bank composition, site fire history, fire severity, fire return interval, and livestock grazing practices before and after fire. With the many possible permutations of these variables, successional patterns are very site specific, although generalized patterns of the grass/fire cycle have been described as follows: 1st, dominant native perennial grasses and forbs are reduced by grazing, and sagebrush and rabbitbrush increase forming a dense canopy with little understory vegetation [340,461]. Or, cheatgrass establishes on a site and increases in density with "improperly timed" grazing or other disturbance [342]. Fire is carried through the community within the canopy or via cheatgrass fuels in the spaces between shrubs and/or bunchgrasses. Following fire, native species cover is typically reduced, and cheatgrass cover may increase or decrease, depending on prefire cheatgrass density and seed availability [340,342,461]. By the 2nd or 3rd postfire year, given sufficient moisture, cheatgrass cover may increase to the point where the site is closed to seedlings of perennial grasses [340,476,477,484]. It has also been suggested that an increase in intraspecific diversity in cheatgrass populations after fire can increase its adaptability and improve its chances for site dominance [484]. As cheatgrass dominance increases, so does the likelihood of fire, and within 3 to 6 years following the initial fire, the amount and continuity of fuels is usually sufficient (in the absence of grazing) to carry a 2nd fire. Successive fires become common, and each fire reduces the surviving shrub cover and native seed bank [340,342,461].

Associated native perennial species respond differently to fire. Native perennial seedlings are more likely to establish in wet years, as is cheatgrass. Bottlebrush squirreltail is more fire tolerant than the fescues or wheatgrasses (Triticeae) [340,494]. The response of perennial forbs varies with season of burning, and most are more tolerant of fire in late summer [484]. Recovery of shrubs tends to be slow, and those present in the early stages of succession are primarily those that can sprout. These include diverse species and subspecies of rabbitbrush (Chrysothamnus spp.), and species of horsebrush (Tetradymia spp.), ephedra (Ephedra spp.), and Prunus [46,48,473,475]. Although rabbitbrush may initially increase with fire, it is killed when the fire-free interval decreases to 5 years or less [340,342,461]. Big sagebrush is the dominant species in vast areas of the Intermountain landscape, and none of the subspecies sprout after burning [473,475]. To allow establishment and persistence of many sagebrush species, the fire-free interval must be greater than 20 to 50 years [340]. This varies between the subspecies of big sagebrush, with Wyoming big sagebrush being the most fire sensitive, followed by basin big sagebrush, and mountain big sagebrush being the most fire resilient [425,426,427] (See Fire Ecology or FEIS reviews on individual subspecies for more detail.) In annual grass dominated communities, the fire-free interval is likely to be about 10 to 12 years or less. With each successive fire, annual grass dominance is enhanced, and the fire-free interval is decreased. This results in a more homogenous landscape, decreased species diversity, and larger and more continuous burns [340]. According to state-and-transition models for sagebrush steppe presented by Laycock [251], fire, grazing, and annual invasion can lead to a threshold beyond which the steady state becomes an annual grassland. After such a threshold has been crossed, intensive human intervention may be necessary to bring the system to a state containing desirable perennial species.

Successional trends are not always predictable. An 11-year study on a Wyoming big sagebrush semidesert site in central Utah found that Wyoming big sagebrush was reduced and perennial bunchgrass cover increased on all burned plots. Cheatgrass cover increased for 2 years following fire, followed by a 2-year reduction in cover, and 3 years of considerable year-to-year fluctuation in cheatgrass cover. The final years showed a negligible presence of cheatgrass with and without livestock grazing. Lower-than-average rainfall during the last 4 years of the study may have played a part in the decline of cheatgrass, and the lack of repeated fire may have been important in the maintenance of perennial grasses [206]. On a sagebrush steppe site in Nevada that was ungrazed for 30 years, cheatgrass cover increased by 38% over those 30 years. Thurber needlegrass and bottlebrush squirreltail also increased (726% and 182%, respectively), and Sandberg bluegrass increased 8%. Perennial grasses as a whole increased 72%, and bluebunch wheatgrass was reestablishing naturally in favored spots [372].

Cheatgrass is less invasive in mesic environments, where it does not compete as effectively with established perennial grasses. It may be dominant only in early successional stages, and is eventually replaced by perennial species [40,277,307,448]. When mountain big sagebrush (the most mesic of the big sagebrush subspecies) is replaced by cheatgrass after fire, successional trends may be toward bottlebrush squirreltail and later bluebunch wheatgrass [378]. Cheatgrass may remain a minor component of later successional stages on these sites, occupying the interspaces between perennial plants [307].

Salt-desert shrubland: In many salt-desert shrubland sites dominated by species such as saltbush (Atriplex spp.), winterfat (Krascheninnikovia lanata), black greasewood, creosotebush (Larrea tridentata), and blackbrush, cheatgrass and other nonnative annual grasses (particularly red brome) have become dominant and altered successional pathways during the past few decades, primarily by changing FIRE REGIMES (See Fire Ecology for more details). Populations of cheatgrass in these arid ecosystems are ephemeral and tend to follow precipitation patterns such that dense populations arise during the spring following a year with high precipitation [29,214,238]. The demise of dominant shrub species contributes to cheatgrass dominance [124,491].

Just as dominance of cheatgrass promotes fire in sagebrush steppe ecosystems, salt-desert ecosystems dominated by nonnative annual grasses are more flammable than those dominated by native species [69]. Following 2 or more years with above-average precipitation, sufficient fine fuel may be present to sustain a wildfire [238] and convert desert plant communities to cheatgrass indefinitely [339]. Generally, most native plant species in the deserts of North America are poorly adapted to survive fire [48,69,137]. In experimental fires in the Mojave, flames fueled by annual bromes were sufficient to consume small shrubs such as white bursage (Ambrosia dumosa), winterfat, white burrobrush (Hymenoclea salsola), and Anderson wolfberry (Lycium andersonii), but were usually insufficient to ignite larger shrubs such as creosotebush, unless the shrubs had large accumulations of grass litter and dead shrub stems in the subcanopy [69]. Species such as shadscale and budsage do not sprout following fire. Winterfat, saltbush, gray molly, and black greasewood do sprout, but plants appear less vigorous after fire [455]. Postfire dominance of annual grasses sets the stage for the grass/fire cycle to continue, with large areas converted to annual grasslands that may persist indefinitely.

Callison and others [83] studied 8 blackbrush sites in southwestern Utah that had been burned under prescription to remove blackbrush and "increase forage production" between 1 and 37 years previous. They found that sites were dominated by forbs 1 year after fire, dominated by annual grasses (cheatgrass or red brome) 2 to 17 years after fire, and dominated by shrubs 37 years after fire. Blackbrush showed no signs of recovery after 37 years [83]. Similarly, research by Matchett and Brooks [268] indicates that nonnative annual grasses (cheatgrass and red brome) have persisted as the dominant vegetation type, along with early successional shrubs, for up to 60 years after fire in some blackbrush communities. Successional trends are difficult to predict, however, as indicated by some sites where blackbrush has recovered to prefire conditions during the same time interval [268].

Pinyon-juniper: In pinyon-juniper ecosystems in the Great Basin, cheatgrass most commonly occurs and has its highest cover in early to mid-successional stages [145,206,244,329,484]. Cheatgrass also frequently occurs in mature pinyon-juniper communities at low densities. Succession in Colorado pinyon-Utah juniper in the Green River corridor in Utah generally proceeds in the following pattern: grasses and forbs dominate early successional stages followed by shrubs (up to 50 years after fire), shrubs with open trees (60 to 100 years after fire), trees with understory shrubs (100 to 200 years after fire) and mature pinyon-juniper (200 years after fire until the next disturbance) [166]. As pinyon-juniper stands increase in density and approach crown closure, herbaceous cover [416] and seed production [150,245] decline. See Goodrich [166] for further descriptions of crown cover, stand structure, plant composition, and ground cover attributes that are representative of each stage.

Postfire succession in pinyon-juniper woodlands is largely dependent on the degree of crown closure of the overstory before disturbance. Recovery of native communities can be rapid following fire in the perennial grass/forb through the shrub/open tree stages. If burned before crown closure has eliminated the understory, the onset of precipitation and warm temperatures encourages native woody species to sprout and native seeds to germinate [166]. Because cheatgrass is nearly ubiquitous in these woodlands and native species are lacking in the understory and seed bank of mature stands, cheatgrass and other nonnative species are likely to invade and/or dominate early successional stages following disturbance in mature pinyon-juniper stands if sites are not artificially seeded [166,351,393]. Nonnative annuals may subsequently prevent perennials from establishing [77,148,149,166]. Annual cover, often dominated by cheatgrass, can increase rapidly, achieving ground cover closure of 60-80% in 5 to 10 years. This stage can persist for 20 years or longer, and may persist until pinyon and juniper return as dominants, or may be perpetuated by frequent fires fueled by cheatgrass. On some Colorado pinyon-Utah juniper sites with south aspects in the Green River corridor, cheatgrass has dominated for 80 years [167]. It has been suggested that with seeding, a perennial community can be well developed within 2 years even with a strong presence of cheatgrass [166].

Ponderosa pine, Douglas-fir, grand fir, and western redcedar forests: Grazing in ponderosa pine/Idaho fescue associations in northeastern Washington may cause an irreversible shift to understory dominance by cheatgrass and other nonnative invasive species such as Dalmatian toadflax and common St. Johnswort [114]. Cheatgrass is also an early seral invader after logging and grazing in the Douglas-fir/ninebark association on sites in northern Idaho, although it appears to decrease in cover as succession progresses and shade increases [87,88]. Cheatgrass is present in early successional stages after logging and burning in grand fir series in the Wallowa Mountains of Washington [159]. In Douglas-fir in south-central Idaho, cheatgrass established after fire and remained at 4-8% cover for 5 years and then increased to 20% cover, 6 and 7 years after fire [259].

Research by Pierson and Mack [345,346] in mature ponderosa pine, Douglas-fir, grand fir, and western redcedar stands suggests that cheatgrass is unlikely to spread and persist in these forest habitats unless the scale and incidence of disturbance is severe and frequent enough to prevent canopy closure. Establishment of cheatgrass in low-elevation ponderosa pine and Douglas-fir forests can be enhanced by disturbance that opens the understory, removes litter, or both. Cheatgrass is unlikely to establish in grand fir and western redcedar habitat types without simultaneous opening of the overstory and understory [346]. Surviving cheatgrass plants in these forest types tend to be small and produce few, if any, seeds. Cheatgrass appears to persist within these forest zones on open sites where temperatures rise sooner in spring and light is not limiting [345]. At least in part, cheatgrass is largely restricted to forest gaps because of its intolerance of shade. Shading cheatgrass reduces its rate of growth, number of tillers, and ability to replace leaf area lost to herbivory. These responses, in turn, intensify the effects of competition and defoliation on cheatgrass in forests [347].