Cheatgrass outcompetes tumblemustard and other broadleaf plants when sufficient litter has accumulated on the soil surface to allow for cheatgrass germination [ , ]. On some sites, cheatgrass may be pre-empted by foxtail fescue Vulpia myuros , a native annual grass [ ]. 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 [ , ]. Medusahead can replace cheatgrass on some sites [ , , ], especially moist sites [ ] and those with fine-textured soils [ , ]. Over the past 40 years medusahead has replaced cheatgrass over extensive areas in the sagebrush zone in California, Idaho, Oregon, and Washington [ ].
Medusahead litter impedes cheatgrass establishment, and medusahead may do better in low-nitrogen environments than cheatgrass [ , ]. 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. 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 [ , ]. 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.
Or, cheatgrass establishes on a site and increases in density with "improperly timed" grazing or other disturbance [ ]. Following fire, native species cover is typically reduced, and cheatgrass cover may increase or decrease, depending on prefire cheatgrass density and seed availability [ , , ]. 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 [ , , , ]. 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 [ ].
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 [ , , ].
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 [ , ]. The response of perennial forbs varies with season of burning, and most are more tolerant of fire in late summer [ ].
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. Although rabbitbrush may initially increase with fire, it is killed when the fire-free interval decreases to 5 years or less [ , , ]. Big sagebrush is the dominant species in vast areas of the Intermountain landscape, and none of the subspecies sprout after burning [ , ]. To allow establishment and persistence of many sagebrush species, the fire-free interval must be greater than 20 to 50 years [ ].
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 [ , , ] 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 [ ].
According to state-and-transition models for sagebrush steppe presented by Laycock [ ], 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 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 [ ].
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 , , , ]. 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 [ ]. Cheatgrass may remain a minor component of later successional stages on these sites, occupying the interspaces between perennial plants [ ].
Salt-desert shrubland: In many salt-desert shrubland sites dominated by species such as saltbush Atriplex spp. 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 , , ]. The demise of dominant shrub species contributes to cheatgrass dominance [ , ]. 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 [ ] and convert desert plant communities to cheatgrass indefinitely [ ]. Generally, most native plant species in the deserts of North America are poorly adapted to survive fire [ 48 , 69 , ]. 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 [ ]. 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 [ ] 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 [ ].
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 [ , , , , ]. 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 years after fire , trees with understory shrubs to years after fire and mature pinyon-juniper years after fire until the next disturbance [ ].
As pinyon-juniper stands increase in density and approach crown closure, herbaceous cover [ ] and seed production [ , ] decline. See Goodrich [ ] 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. 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 [ ].
Nonnative annuals may subsequently prevent perennials from establishing [ 77 , , , ]. 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 [ ]. It has been suggested that with seeding, a perennial community can be well developed within 2 years even with a strong presence of cheatgrass [ ]. Johnswort [ ]. Cheatgrass is present in early successional stages after logging and burning in grand fir series in the Wallowa Mountains of Washington [ ].
Research by Pierson and Mack [ , ] 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 [ ]. 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 [ ].
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 [ ]. Cheatgrass roots often continue to develop during winter, while aboveground growth is minimal [ 46 , , , , , ]. In mild winter weather, cheatgrass plants can tiller and produce many adventitious roots [ ].
Harris [ ] provides a detailed examination of root phenological development in cheatgrass, as compared with medusahead and bluebunch wheatgrass.
He concludes that fall germination and winter growth, even at near-freezing soil temperatures, allows annuals to increase number and length of roots during winter so that by spring, they are in control of the site and exhaust upper profile moisture supplies to the detriment of bluebunch wheatgrass. Studies in Washington and Idaho suggest the following sequence of development in cheatgrass: Increase in cheatgrass root length occurs between mid-December and mid-March, and may reach 36 inches 90 cm by mid-March [ ], with additional growth between mid-March and mid-April.
The aerial portion of cheatgrass plants resumes growth after mid-March, with no further increase in height after emergence of inflorescences around mid-May. Prior to April, cheatgrass' root system consists primarily of a single main root with short laterals. About the time the aerial portion of the plants begins regrowth, more lateral development of the root systems may be observed. By the time of spring emergence of associated perennial species, cheatgrass has its root system near fully developed [ ].
Therefore, when bluebunch wheatgrass seedlings need moisture to survive the summer, cheatgrass has already depleted moisture beyond the depth of bluebunch wheatgrass roots [ ]. Cheatgrass plants with multiple culms tend to develop a more extensive root system than those with only a single culm. Tillering is evident by April [ ]. Cheatgrass shoot growth typically occurs in late winter or early spring and continues until soil moisture is exhausted [ , ].
Growth and development is rapid, and plants may flower, develop seeds, and become fully dried within 2 to 3 months [ 46 , , , ].
Forage, Not Fuel
Greenhouse studies indicate a requirement for either vernalization or short day lengths to initiate panicle production in cheatgrass, followed by long day lengths necessary for flowering [ ]. As a winter annual, cheatgrass usually flowers in spring, from mid-April through June [ , ]. The anthers of cheatgrass florets open over about an day period. Seeds reach the dough stage in mid- to late May and are usually ripe in June or July [ 46 , , , ].
Cheatgrass seeds shatter within a week after maturity [ ]. Plants die and become dry after seeds are ripe, or after early summer drought [ 46 , , ]. This occurs by June 5 at lower elevations 2,, feet , m in southwestern Idaho, and by about June 30 in southeastern Idaho, where the season is later because of higher elevation 4,, feet 1,, m [ ]. Corresponding stages are about 6 weeks later in bluebunch wheatgrass [ ]. In hot weather, cheatgrass roots are unable to supply enough moisture to prevent a drop of leaf water potential, resulting in desiccation and death of the plant [ ].
During ripening, cheatgrass plants turn purple and then brown as they mature and senesce. A sudden drop in temperature or a sudden drought causes purple coloration that fades when growing conditions become more favorable [ ]. Seeds are viable when the fruits have barely started to turn purple and are still mostly green [ ]. Seeds begin to fall shortly after the purple stage is reached [ ]. Some viable seed is produced even when the inflorescences are clipped before any purple coloration appears [ ].
The characteristic color changes in cheatgrass while it is curing from green to purple to straw color are proposed as an indicator of impending flammability because these colors are generally correlated with progressive drying of plants.
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The onset of purple coloring may be taken as a warning that hazardous fire conditions will develop within 2 weeks [ ]. Variations in phenology of about 20 days emergence of inflorescences, development of purple color, drying, and browning were observed by Hulbert [ ] in plants grown from seed from different geographic locations, and in plants grown under different environmental conditions location, aspect, elevation. Cheatgrass is a strong competitor in the postfire environment, where it takes advantage of increased resource availability and produces an abundant seed crop [ 48 , , ]. A cheatgrass population may average around 1, plants per square foot 10, per m 2 prior to burning.
During a wildfire, most of the cheatgrass seeds beneath the canopy of sagebrush plants are killed by the heat associated with the burning of the shrub. Some cheatgrass seeds located in the interspaces among shrubs are also consumed, while those that are buried or lying in cracks in the soil will likely survive. These plants are released from competition, and have more water and nutrients available to them.
The cheatgrass plants in this sparse population can produce abundant tillers, each supporting many flowers, thus producing a large seed crop [ ]. Young and others [ ] provide an illustration of cheatgrass fire adaptations with an example from a big sagebrush ecosystem which suggests that hybridization in postfire populations contributes to the success of cheatgrass in these ecosystems. Studies by Novak e.
Fire facilitates cheatgrass dominance on some sites by interrupting successional trajectories of postfire plant communities, and cheatgrass facilitates fire and can thus shorten the interval between fires [ 48 , , , , , ].
This cycle has been documented in the Great Basin since the s [ , , , ], and has been reported in the Mojave and Sonoran deserts beginning in the early s [ 71 ]. The result is a type conversion from native shrub and perennial grasslands to annual grasslands adapted to frequent fires.
Fire regimes: Cheatgrass expansion has dramatically changed fire regimes and plant communities over vast areas of western rangelands by creating an environment where fires are easily ignited, spread rapidly, cover large areas, and occur frequently [ ]. An estimated 80, km 2 of primarily shrubland and grassland communities in the Great Basin have fire regimes that have been seriously altered because of the presence of cheatgrass. Cheatgrass promotes more frequent fires by increasing the biomass and horizontal continuity of fine fuels that persist during the summer lightning season and by allowing fire to spread across landscapes where fire was previously restricted to isolated patches [ 37 , 46 , 48 , 71 , 78 , , , , , , ].
Fire in these habitats can have severe effects on native species of plants and animals, although the impact of fire regime changes may differ by region and ecosystem type due to differences in the composition and structure of the invaded plant communities [ , , , ] and to climatic differences such as occurrence of summer thunderstorms [ 48 , ]. A review by D'Antonio [ ] suggests that species that alter the disturbance regime of a site are those that are qualitatively different from the rest of the species in a community i. Where invaders are similar in overall life form to natives, they tend to alter primarily fuel biomass per unit area of ground.
This in turn has the potential to influence fire intensity, or slightly modify the existing fire regime, as may be the case with cheatgrass invasion in the more mesic temperate grasslands of North America [ ]. Where invaders have no functional analogues in terms of fuel characteristics in the invaded system, they have the potential to alter fire frequency and even to introduce fire to ecosystems where it had no evolutionary role, resulting in a complete alteration of that community [ , ].
This has been the case with the introduction of cheatgrass in sagebrush grasslands, desert shrublands, and pinyon-juniper woodlands over extensive areas in the Columbia and Great basins and other areas the Intermountain West. In these systems, cheatgrass fills spaces between widely spaced vegetation and dries earlier than most native species. Thus, from the time plants dry until the onset of fall rains, cheatgrass stands present a fire hazard not usually found in vegetation native to the areas where it is most invasive.
The introduction and increasing dominance of cheatgrass has changed the seasonal occurrence and increased the frequency and size of wildfires in these ecosystems, thus altering successional patterns [ 48 , , , , , ]. The degree of change and impacts on native ecosystems varies with differences in species composition and structure of invaded plant communities [ 78 , ]. Historic fire seasons in the sagebrush steppe occurred between July and September [ 1 , 15 , , ], with the middle to end of August being the period of the most extreme fire conditions [ 79 ].
Cheatgrass matures by July, while most native herbaceous species it replaces mature in late August. With cheatgrass dominant, wildfires tend to occur earlier in the season, when native perennials are more susceptible to injury by burning [ , ]. Where cheatgrass has invaded the Snake River Plains of Idaho, the natural fire cycle has shortened from years to years [ ]. Fires are larger and more uniform, with fewer patches of unburned vegetation remaining within burns [ , ].
These altered fire regimes and subsequent changes in botanical composition can occur with or without livestock grazing [ ]. Wyoming big sagebrush communities are the most xeric of the big sagebrush communities, and the subspecies is more susceptible to fire than the other big sagebrush subspecies [ ]. When Wyoming big sagebrush communities burn, resulting vegetation is generally dominated by annuals such as Russian-thistle, tumblemustard, and cheatgrass [ , ].
Fire-tolerant, sprouting shrubs e. Continued increases in fire frequency eventually remove and exclude all perennial shrubs, grasses, and forbs from these communities, and cheatgrass competition prevents their reestablishment [ , , , ]. Large areas of fire-induced annual communities occur in areas formerly occupied by the Wyoming big sagebrush cover type [ , ].
Basin big sagebrush is also very susceptible to fire. After fires in basin big sagebrush communities, annuals usually dominate, and shrubs such as rabbitbrush and horsebrush may increase. Competition from annuals cheatgrass and medusahead makes reestablishment of native grasses difficult [ , , , , ].
Mountain big sagebrush generally has a higher capacity for recovery following disturbance than Wyoming and basin big sagebrush, with a high degree of variability between sites. Cheatgrass increases with grazing in mountain big sagebrush communities, but does not dominate to the extent that it does in drier sagebrush types. Mountain big sagebrush is easily killed by fire, but reestablishes readily from seed and tends to form dense stands after fire [ , ]. Mountain big sagebrush stands may recover within 15 to 20 years after fire, while stands of Wyoming big sagebrush may not be fully recovered after 50 to 75 years [ 60 , 77 , 78 ].
Work by Miller and Heyerdahl [ ] indicates a high degree of spatial variability in historic fire regimes in mountain big sagebrush. Salt-desert shrubland: Fires were historically infrequent in salt-desert shrublands. Desert shrublands usually lack sufficient fine fuels to carry fire, with widely spaced shrubs and bunchgrasses and relatively bare interspaces [ 69 , 71 , , , ]. Historic fire return intervals in these ecosystems dominated by saltbush, greasewood, creosote, and blackbrush are thought to average between 35 and years or more [ ].
Most native plant species in the deserts of North America are poorly adapted to survive fire [ 48 , 69 ] and references therein. Landscapes dominated by alien annual grasses, especially annual bromes Bromus spp. Brooks [ 69 ] suggests several possible reasons for this, including: a higher surface-to-volume ratio of grasses compared to forbs; more continuous vegetative cover; and the ability of alien annual grasses to remain rooted and upright longer than native forbs, allowing them to persist as flammable fuels into the summer, when the threat of fire is highest [ 69 ].
Thick layers of annual plant litter accumulate quickly and decompose very slowly in desert regions [ 69 , ]. Following 2 or more years with above-average precipitation, sufficient fuel may be present to sustain a wildfire [ ] and convert the plant community to cheatgrass or other nonnative annual grasses indefinitely [ ]. In experimental fires in the Mojave Desert, accumulations of litter led to particularly hot temperatures, long flame residence times, and continuous burn patterns [ 69 ].
Postfire plant communities in the Mojave and Sonoran deserts are typically dominated by nonnative annual grasses [ 69 ] and references therein , so burned areas are likely to be more susceptible to fire than unburned areas. Brooks and Pyke [ 71 ] note that fire regimes in the Mojave and Sonoran deserts are beginning to shift toward short-return intervals. Repeated fires stress and kill native perennials. Eventually wind and water erosion may occur, removing and diluting soil organic matter and attendant nutrient concentrations and safe sites around shrubs.
After fire has eliminated native perennials, essential mycorrhizae may also be eliminated [ ]. Biological soil crusts are also killed by severe fire, and the unusually large, frequent fires associated with cheatgrass dominance can preclude crust species recolonization and succession [ 41 ].
West [ ] gives some specific examples of fire effects on salt desert shrub ecosystems in Utah and Nevada. Pinyon-juniper: Pinyon-juniper woodlands are characterized by a large number of diverse habitat types that vary in tree and herbaceous species composition and density, and fire regime characteristics.
Fire severity and frequency vary, depending largely on site productivity. On less productive sites with discontinuous grass cover, fires were probably infrequent, small, and patchy [ ]. Fire intervals were probably greater than years in these areas, but did occur more frequently under extreme conditions [ ].panicni-poremecaj.com.ba/components/werafevap/3594.php
Cheatgrass: Fire and Forage on the Range
On more productive sites where grass cover was more continuous, fire intervals may have been 10 years or less, maintaining more open stands. Historical fire regimes in dense stands were a mixture of surface and crown fires, with surface fires at intervals of years and crown fires at intervals of years or longer. Fire susceptibility in pinyon-juniper communities also depends on the stage of stand development. In young open stands, shrubs and herbaceous cover may be sufficient to carry fire, but as the stand approaches crown closure, herbaceous cover declines and eventually becomes too sparse to carry fire [ ].
A dramatic increase in fire size and frequency has been observed in pinyon-juniper woodlands as cover of nonnative annuals such as cheatgrass increases [ , ]. Where fires have burned in singleleaf pinyon-Utah juniper woodland invaded by cheatgrass in Nevada, the woodland is being replaced by great expanses of annual grassland dominated by cheatgrass [ 48 ]. Cooler and more mesic woodlands seem to be less susceptible to invasion and complete dominance by introduced annuals; however, more information is needed regarding factors that influence pinyon-juniper woodlands susceptibility to invasion [ ].
Prolonged livestock grazing and fire suppression have contributed to a decline of perennial grasses and an increase in shrubs and trees at many pinyon-juniper sites [ , ]. A subsequent increase in the number of large, high-severity fires following invasion by nonnative annuals such as cheatgrass has resulted in a loss of these shrubs and trees [ ].
When cheatgrass is present in the understory with little or no perennial vegetation, removing pinyon and juniper trees usually leads to cheatgrass dominance [ ]. Many dry temperate conifer forests have become susceptible to severe wildfires because of the dense forest structure that results from a century of fire exclusion and past management practices e.
Fires in these ecosystems, especially fires of high severity, can lead to invasion and dominance of cheatgrass. At Sequoia-Kings Canyon National Park, prescribed burning in ponderosa pine in the Cedar Grove section appears to have promoted vigorous invasion of cheatgrass [ ]. Cheatgrass had higher cover on severely burned sites, compared to less severely burned sites, in ponderosa pine in Arizona [ ].
The presence of cheatgrass-dominated ecosystems adjacent to these dense forests is also likely to cause larger, more frequent, and more severe wildfires [ ]. Cheatgrass fueled a large wildfire in the ponderosa pine forest type in Oregon as early as [ ]. Fire effects on many species, and the effects of invasives on disturbance regimes in temperate and boreal forests, are still poorly understood [ ].
In temperate grasslands of North America, fire has historically been an important selective force, and native communities are well adapted to frequent fires in most cases. Cheatgrass is more commonly found in the northern portion of these temperate grasslands. In more arid habitats with low natural fire frequencies cheatgrass is able to replace native species.
In mesic grasslands, however, cheatgrass does not compete as successfully against native perennial grasses, and it does not appear to pose as great a threat to native communities. A review by Grace and others [ ] suggests that cheatgrass is favored by occasional burning at study sites within shortgrass steppe and mixed-grass prairies. Smith and Knapp [ ] provide evidence that cheatgrass and other nonnative species are less frequent on tallgrass prairie sites at Konza Prairie, Kansas, that are annually burned than they are on unburned sites. Across the broad range of conditions and circumstances that occur in temperate grasslands, a complex interplay of contemporary and historical factors will ultimately determine how fire interrelates with invasive species [ ].
Cheatgrass fire regime: Cheatgrass often dominates postfire plant communities, and once established, cheatgrass-dominated grasslands greatly increase the potential and recurrence of wildfires. Cheatgrass fires tend to burn fast and cover large areas, with a fire season from 1 to 3 months longer than that of native rangeland [ , ]. The average fire-return interval for cheatgrass-dominated stands is less than 10 years [ ], and is about 3 to 6 years on the Snake River Plain as reported by Whisenant [ ] and Peters and Bunting [ ].
This adaptation to and promotion of frequent fires is what gives cheatgrass its greatest competitive advantage in ecosystems that evolved with less frequent fires. The cheatgrass-fire cycle is self-promoting, as it reduces the ability of many perennial grasses and shrubs to re-establish and furthers the dominance of cheatgrass [ , ].
Moisture availability can affect cheatgrass productivity and thus affect fuel loads on a site. Drought years may reduce the dominance of cheatgrass in both recently burned and unburned areas, thus decreasing fuel loads and the chance of fire [ ]. The following table provides some fire return intervals for ecosystems in which cheatgrass may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".
Cheatgrass seeds are also susceptible to heat kill, but can survive fires of low-severity if the entire litter layer is not consumed or if seeds are buried deeply enough to be insulated from the heat [ ]. The amount of litter or ash left on a site is a good indicator of the amount of cheatgrass seed surviving on that site [ ].
Low density of cheatgrass immediately following fire [ ] indicates either low numbers of cheatgrass seed in the seed bank, or poor survival of seeds during fire [ ]. If fire occurs when seed remains in panicles above ground, most seeds will be killed and cheatgrass density will decline immediately following fire [ 70 , 71 , ]. The chances of seed surviving fire are enhanced once they have dispersed onto or beneath the soil surface [ 70 , ].
In sagebrush communities, most of the litter and cheatgrass seeds are found under the canopies of sagebrush plants [ ]. The woody biomass of the shrub, plus litter accumulations, provide sufficient fuel to elevate temperatures high enough for a long enough period to consume cheatgrass seeds on these microsites. Some cheatgrass seeds in the interspace zones are also consumed by fire, but many survive even though the cheatgrass herbage is completely consumed [ ].
Fire from herbaceous fuel alone is not usually hot enough to consume cheatgrass seeds [ ]. Densities of cheatgrass seeds were higher on a low-severity burn compared with a high-severity burn. Nonetheless, the seed bank recovered to preburn levels after l growing season, even on the more severely burned site [ ].
Cheatgrass may also invade recently burned sites where it does not usually dominate or did not previously occur e. For example, pinyon-juniper woodlands with large, continuous tree canopies limit herbaceous understory and facilitate severe summer fires that promote invasion of nonnative species such as cheatgrass and red brome [ ].
Koniak and Everett [ ] observed that most of the seed bank in a mature singleleaf pinyon stand in California consisted of annuals, many of which were not present in the community as mature plants. There are some examples in the literature reporting decreased density of cheatgrass in the 1st postfire year e.
Others report increased cover of cheatgrass the 1st postfire year in ponderosa pine [ 17 , ], sagebrush [ 5 ], shadscale [ ], bluebunch wheatgrass [ , ], and cheatgrass communities [ 89 ]. This requires reintroducing native plant species that compete well with cheatgrass, reducing fire risk in areas where cheatgrass is likely to invade, carefully managing livestock grazing and human disturbances such as off-highway vehicle use, and replanting to native species following fire. Although present in Nevada for little over a century, cheatgrass has become a familiar part of the rangeland landscape.
Yet many of our significant environmental problems stem from this exotic species. Cheatgrass poses a huge risk not only to our remaining intact rangeland ecosystems but also to human safety, particularly if climate change portends increased drought and hence more frequent fire. Nevada Humanities produces and supports dynamic educational and cultural programs that enrich our lives and encourage us to explore challenging ideas. Nevada Humanities unites us through our history and heritage.
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Photo courtesy of P. Geographic Area:. Article Locations. Related Articles Bitterbrush. Seedling roots continue to grow throughout the winter! How did cheatgrass get here? Cheatgrass was introduced to North America through contaminated grain seed, straw packing material, and soil used as ballast in ships sailing from Eurasia. During this time, abusive use of rangelands, coupled with drought, left many Great Basin rangelands in poor condition. Cheatgrass was able to occupy areas where the native vegetation had been reduced, beginning its persistent march across the landscape.
It can now be found across the landscape from the bottoms of desert valleys to mountain peaks as high as 13, feet. The plant communities most affected by cheatgrass invasion are those below feet in elevation. What are its characteristics? As a winter annual , cheatgrass seeds germinate at low fall temperatures. Seedling roots continue to grow throughout the winter, and by spring, are capable of out-competing native species for water and nutrients because most native vegetation is just getting started. Cheatgrass completes its life cycle quickly and can become dry by mid-June.
Perennial grasses like bottlebrush squirreltail and Basin wildrye still contain about 65 percent of their moisture at that time. Cheatgrass is a prolific seed producer, and large seedbanks can develop. How can we fight this weed? Eradication of cheatgrass from large areas is not a reasonable goal. Efforts should focus on reducing cheatgrass dominance and increasing perennial vegetation. Increased livestock grazing in early spring helps lower seed production and reduce fuel for fires, but it is doubtful that this alone will help restore more productive species.
It is important to remember to remove grazing pressure as native plants begin to flower. Herbicides easily kill cheatgrass, but are not normally cost effective.
Related Cheatgrass: Fire and Forage on the Range
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