Sagebrush ecosystems cover vast stretches of western North America and cover more area than any other type of rangelands on this continent. Though the appearance and composition of sagebrush communities vary greatly across the west, the one thing they all have in common is an overstory of sagebrush plants of the genus Artemisia. The sagebrush steppe and shrubland communities are iconic rangeland types characterized by miles and miles of native sagebrush and perennial grasses. A variety of animals such as pronghorn antelope, black-tailed jackrabbits and sage-grouse call sagebrush ecosystems home. The greatest modern challenges to managing sagebrush ecosystems include invasion by native and exotic plants and changes in historic fire regimes.
There are two potential natural vegetation types in the Intermountain West in which sagebrush is dominant: sagebrush steppe and sagebrush shrubland (often referred to as Great Basin sagebrush. Sagebrush steppe, once occupying 44.8 million ha (112 million acres), occurs predominantly in the upper portion of the Intermountain West, with its southern boundary in the northern Great Basin. In this community type, grass and forb species more or less co-dominate with sagebrush. Sagebrush shrubland is found to the south of sagebrush steppe, potentially occupying 17.9 million ha (44.8 million acres) of the Great Basin, Colorado Plateau, and adjacent areas. In the sagebrush shrubland type, sagebrush dominates, and is accompanied by few grasses and forbs, even in late seral conditions. Most of the sagebrush shrubland occurs under more arid conditions than found in the sagebrush steppe. Even though the dividing lines between sagebrush steppe and sagebrush shrubland have often been viewed as somewhat arbitrary, the two sagebrush types will be considered individually, in terms of ecological structure and function and response to management practices, when appropriate.
The climate for sagebrush steppe and sagebrush shrubland vegetation is semiarid, temperate and continental. On the western boundary of the Intermountain West, the Sierra Nevada and Cascade Mountains intercept moist air masses originating over the Pacific Ocean, especially in winter; and weather fronts developing in the Great Plains are often blocked by the Rocky Mountains to the east. The majority of precipitation falls as snow between November and March, with some rain April through June. Summers are typically dry, except for monsoonal storms in August and September in the southern portion of the region. Average annual precipitation varies across the region, depending on elevation and latitude, with 208 mm (8.3”) near Yakima, Washington, 180 mm (7.2”) near Reno, Nevada, 247 mm (9.8”) near Twin Falls, Idaho, 363 mm (14.5”) near Filmore, Utah, and 419 mm (16.8”) near Cortez, Colorado. The length of the frost-free period is approximately 120 days for both vegetation types, but can be reduced to 80 days at higher elevations. The average yearly temperature is 8.9°C (48°F), with the average warmest monthly temperature at 31.9°C (89.5°F) in July and the average coolest monthly temperature at -8.5°C (16.7°F) in January. Freezing temperatures can occur during all months at higher elevations.
Average high temperatures and average precipitation (both annual and monthly) appear to play an important role in determining whether the vegetation on sagebrush sites resembles sagebrush steppe or sagebrush shrubland. In Nevada, the mean high temperature in sagebrush steppe, from March through June (the period of peak plant growth), is 2.8-3.4°C (5-6°F) cooler than in sagebrush shrubland. And, average annual precipitation in sagebrush steppe is 80 mm (3.2 in) more than in sagebrush shrubland, with greater precipitation almost every month from November through June. More precipitation and lower evaporative losses increase the amount of soil water for plant growth, and help account for the greater diversity and productivity of shallow-rooted grasses and forbs in sagebrush steppe communities.
Topography and Soils
The Intermountain West is subdivided into three physiographic provinces. 1) The Columbia-Snake River Plateau, located in southern Idaho and eastern Washington and Oregon, has a nearly flat to rolling topography with weathered loess over horizontal layers of lava and basalt. Elevations range from 150-1,525 m (500-5,000 ft). 2) The Basin and Range, covering most of Nevada, the western half of Utah, southern Idaho, southeastern Oregon, northeastern and southeastern California, and extending south into the Sonoran Desert, is characterized by numerous north-south oriented, fault-tilted mountain ranges that are separated by broad, alluvial valleys (basins). Limestones, dolomites, shales and sandstones are the dominant outcrops along the fault escarpments. Valley floors are less than 1,525 m (5,000 ft) in elevation, with mountain peaks typically reaching 2,130-3,050 m (7,000-10,000 ft). The Great Basin, lacking exterior drainage and containing remnants of large Pleistocene lakes, comprises the northern half of the Basin and Range. 3) The Colorado Plateau, centered in the four corners region of Utah, Colorado, New Mexico and Arizona, is a vast area of nearly horizontal sedimentary formations separated by bends of strata along broad open folds interspersed with intrusive igneous features. The landscape is characterized by high plateaus and mesas, deep canyons, volcanic and dome mountains, sand deserts, and shale deserts with badlands, and ranges in elevation from about 1,525-3,350 m (5,000-11,000 feet), not including canyon bottoms.
Sagebrush steppe vegetation is found at elevations of 150-2,000 m (500-6,550 feet) on deep loessal soils on rolling topography in the Columbia-Snake River Plateau, and deep alluvial soils along mountain foothills to shallow skeletal soils on ridgetops in the Great Basin. Common soil orders (and suborders) include Aridisols (Arigids and Cambids) and Mollisols (Ustolls and Xerolls); Andisols (Xerands) derived from volcanic ash are conspicuous on the Columbia-Snake River Plateau. The sagebrush shrubland type is primarily found at elevations of 1,200-1,800 m (3,940-5900 ft) on pediments, bajadas and foothills in the Great Basin, and at 1,300-1,800 m (4,265-5,900 feet) on mesa tops, benches or pediments with a sandy-gravelly covering in the Colorado Plateau. Common soil orders include Aridisols, Alfisols and Mollisols.
Historic or Natural Potential Plant Communities
The characterization of presettlement vegetation in sagebrush steppe and sagebrush shrubland vegetation types is based on journals and diaries of the first explorers and settlers reports from early scientific and military surveys, historic landscape photographs, and remaining relict areas.
The floristic diversity of sagebrush steppe is moderate, with 13-42 species found at different sites, and the vertical and horizontal structure typically consists of a shrub-dominated overstory and grass/forb-dominated understory. There is considerable sorting of grass, forb and sagebrush taxa along gradients of temperature and moisture. Perennial herbaceous species can dominate or be co-dominant with sagebrush, depending on the last fire, insect outbreak, or climatic. The shrub overstory is primarily comprised of one or more subspecies of big sagebrush (ssp. tridentata, ssp. vaseyana, ssp. wyomingensis), along with rabbitbrushes (Ericameria nauseosus, Chrysothamnus viscidiflorus), bitterbrush (Purshia tridentata), horsebrush (Tetradymia canescens), and other sagebrushes (A. tripartita, A. longiloba). Bluebunch wheatgrass (Elymus spicatus) is probably the most widespread and important perennial grass; other principal grass species include Idaho fescue (Festuca idahoensis), Sandberg bluegrass (Poa secunda), basin wildrye (Elymus cinereus), western wheatgrass (Elymus smithii), junegrass (Koeleria macrantha), needle-and-thread (Hesperostipa comata), and bottlebrush squirreltail (Elymus elymoides). Arrowleaf balsamroot (Balsamorhiza sagitatta) is often the most widespread and abundant forb; other major forb species include hawksbeard (Crepis spp.), phlox (Phlox spp.), western yarrow (Achillea millefolium), lomatium (Lomatium spp.), lupine (Lupinus spp.), groundsel (Senecio integerrimus), and mulesears (Wyethia amplexicaulis). A microphytic crust (comprised of lichens, algae, microfungi and cyanobacteria), which fixes atmospheric nitrogen and protects against soil surface erosion, is often present in the interspaces between perennial plants.
Sagebrush shrubland is simple in floristic composition, with various sagebrushes typically making up over 70% of the relative cover and 90% of the biomass. The often widely spaced shrubs trap wind-blown soil and litter, creating “islands of fertility” that provide a more favorable microclimate for grasses and forbs than in interspaces, which are primarily occupied by microphytic crusts. The shrub overstory is usually comprised of Wyoming big sagebrush (A. tridentata ssp.wyomingensis), with basin big sagebrush (A. tridentata ssp. tridentata) more prevalent in mesic, lower elevation sites and mountain big sagebrush (A. tridentataspp. vaseyana) more common in higher elevation sites. On drier sites, bud sagebrush (Picrothamnus desertorum), black sagebrush (A. nova), and low sagebrush (A. arbuscula) can be found at low, mid, and high elevations, respectively. Other shrub species can include yellow rabbitbrush (C. viscidiflorus), horsebrush, ephedra (Ephedra spp.), and spiny hopsage (Grayia spinosa), as well as the succulent, plains prickly pear (Opuntia polyacantha) (Figure 9). In the understory, common perennial grasses include Indian ricegrass (Stipa hymenoides), bottlebrush squirreltail, galleta (Hilaria jamesii) and sand dropseed (Sporobolus cryptandrus), and common perennial forbs include globemallow (Sphaeralcea spp.), aster (Aster spp.), phlox, and milkvetch (Astragalus spp.).
Current Plant Communities
Sagebrush steppe and sagebrush shrubland communities in the Great Basin were little influenced by humans before Anglo-American settlement in the mid-1800s. Since then, a variety of interacting factors, including excessive livestock grazing, conversion to agriculture, urban and exurban development, recreation activities, mining and energy development, invasive plant species, altered fire regimes, and climate change, have caused widespread changes in the structure and function of sagebrush communities. Of these factors, the greatest threats to the persistence of historical sagebrush communities in the Great Basin are the invasion of non-native annual grasses, primarily cheatgrass (Bromus tectorum), into low- and mid-elevation sagebrush (Figure 12), and the encroachment of pinyon (Pinus edulis, P. monophylla) and juniper (Juniperus osteosperma, J. occidentalis, J. scopulorum) species into mid- and high-elevation sagebrush (Figure 13). Changes in fuel dynamics associated with these invasions have resulted in altered fire regimes.
Sagebrush ecosystems provide habitat for about 250 vertebrate species. Many of the wildlife species found in sagebrush shrubland are also found in sagebrush steppe, but in greater abundance, due to a greater diversity of shrubs, an increased proportion of herbaceous species, and higher levels of productivity. Mammals common in both vegetation types include pronghorn (Antilocarpa americana), badger (Taxidea taxus), coyote (Canis latrans), black-tailed jackrabbit (Lepus californicus), Ord’s kangaroo rat (Dipodomys ordii), Townsend’s ground squirrel (Spermophilus townsendii), and Great Basin pocket mouse (Perognathus parvus). Mule deer (Odocoileus hemionus), elk (Cervus elaphus), and bighorn sheep (Ovis canadensis) primarily use sagebrush steppe as winter range (West 1983a). Free-roaming horses (Equus caballus) are present in both sagebrush types (Beever 2003). Birds typically found in sagebrush steppe and sagebrush shrubland include the golden eagle (Aquila chrysaetos), western red-tailed hawk (Buteo jamaicensis), marsh hawk (Circus cyaneus), American raven (Corvus corax), sage sparrow (Amphispiza belli), Brewer’s sparrow (Spizella breweri), sage thrasher (Oreoscoptes montanus), horned lark (Eremophila alpestris), mourning dove (Zenaida macroura), broad-tailed hummingbird (Selasphorus platycercus), and greater sage-grouse (Centrocercus urophasianus). Common reptiles include the Great Basin rattlesnake (Pituophis melanoleucas), gopher snake (Pituophis calenifer), striped racer (Coluber taeniatus), and sagebrush lizard (Sceloporus graciosus).
A considerable diversity of invertebrates is present in sagebrush ecosystems. From 220 to over 1,000 insect species have been collected at different sagebrush sites in the Great Basin. And, 237 insect species (including 52 aphids, 23 beetles, 3 moths, 12 grasshoppers, 7 cicadas, and 23 ants) and 72 spider species have been found on or in big sagebrush.
The present Great Basin landscape began to form 10-15 million years ago in the Miocene as the Earth’s crust began to stretch, initiating block faulting that created basin (valley) and range (mountain) topography. The Sierra and Cascade mountain ranges also began to rise, creating drier, cooler conditions (rain shadow effects) that favored the evolution of semidesert plants. In the mid-Pliocene, about 3-4 million years ago, there was also an immigration of several genera from Eurasia that included sagebrush, saltbush (Atriplex spp.), and Mormon tea (Ephedra spp.). Throughout most of the Pleistocene, 1.6 million to 25,000 years ago, there were at least 15 major periods of cooling (glacier formation) and warming (glacial retreat), resulting in the expansion and retreat of grasslands, shrublands, and forests. Gradual warming 25,000 to 11,000 years ago gave way to warmer and drier conditions during the Holocene (the last 10,000 years), forcing subalpine conifers to move up in elevation and allowing sagebrush and saltbush to expand in lowlands as pluvial lakes shrank. The migration of humans into the Great Basin area about 13,000 years ago may have influenced sagebrush vegetation, mainly by reducing the number of large herbivores and changing the fire regime. Evidence for long-term vegetation change comes from fossil plant parts such as pollen in lake sediments, and seeds, leaves and twigs found in woodrat (Neotoma spp.) middens.
Yearly and Seasonal Variation
Precipitation (amount and distribution) is the major driver of seasonal and yearly vegetation changes in sagebrush steppe and sagebrush shrubland communities. Under drought conditions, stress-tolerant shrubs such as big sagebrush often survive, while less tolerant perennial grasses and forbs can suffer considerable mortality. In contrast, big sagebrush and associated shrubs can suffer substantial die-off following successive years of above-average precipitation, which result in anoxic soils and increased susceptibility to fungal diseases. Availability of soil moisture on sites is strongly influenced by soil properties (texture and depth), slope and aspect. Excessive grazing pressure can impact herbaceous plant production, particularly during drought years.
In a comprehensive study of 17 sagebrush steppe sites over 10 consecutive years, total annual production at a representative site varied almost 5-fold between the driest and wettest years, and individual species such as Sandberg bluegrass produced 12 times as much herbage in the most productive year as in the least productive year.
Climate, herbivory, fire, invasive species, and several anthropogenic activities have contributed to changes in the composition, structure and dynamics of sagebrush steppe and sagebrush shrublands. Certain combinations/sequences of these disturbances, e.g., wet-dry cycles, heavy livestock grazing and invasive species, can speed up transitions to alternative states and/or result in irrevocable changes in environmental conditions and biotic potentials of ecological sites. In sagebrush steppe, succession usually moves in one of three general directions: 1) dominance of woody species and longer fire return intervals; 2) increase in exotic weeds, particularly annual grasses, and shorter fire return intervals; or, 3) return to a co-dominance of perennial grasses and forbs with shrubs. State-and-transition models have the potential to provide a framework for organizing concepts about multiple, interactive processes (including disturbances) driving ecosystems to change to alternative states and the roles that management can play in directing these processes. State-and-transition models continue to be developed for sagebrush steppe and sagebrush shrubland ecological sites.
During the past 10,000 years since the end of the Pleistocene, dominance between perennial herbaceous species and shrub species in sagebrush ecosystems has shifted, coincident with changes in climate, with grasses and forbs being favored during cooler, wetter periods and shrubs being favored during warmer, drier periods. Like most perennial plants, sagebrush greatly reduces its overall growth, i.e., leaf and stem growth, development of flowering shoots, and canopy cover, during drought. Following drought, big sagebrush can recover more rapidly than associated perennial grasses and forbs on most sagebrush steppe sites, allowing it to reoccupy the plant community at a more rapid rate.
Climate change is expected to have significant impacts on Great Basin and Intermountain West plant communities by the end of the 21st century. Projected warming for the region ranges from 2-5°C (3.6-9.0°F). While projected changes in precipitation are uncertain, two trends in precipitation (from the 1950s to date) with potentially important impacts on plant populations are likely continue into the future: 1) an increase in annual precipitation and interannual variability in precipitation, and 2) a decline in snowpack, indicating that more winter precipitation is falling as rain. A shift from a snow- to rain-dominated precipitation regime would alter the timing of soil moisture pulses in sagebrush ecosystems, potentially hurting plant species that are dependent on snowmelt, and increase frost damage due to the absence of snow cover. A shift in precipitation distribution, with the majority falling between April and July, could alter the productivity, composition, and structure of sagebrush steppe vegetation, primarily due to a reduction in the perennial forb component. Other potential impacts include: earlier arrival of spring, affecting plant phenology; shifts in animal species composition; increased weed invasibility, especially annual grass and juniper species; and more frequent and larger fires over a longer fire season.
Native large and small herbivores have been grazing/browsing sagebrush steppe and sagebrush shrublands for several million years. Pleistocene megafauna, including mammoths, camels, horses, ground sloths and bison, grazed/browsed vegetation during the growing season by following green-up from valleys to mountainous areas, and returning to valleys with the onset of snow, allowing time for grazed plants to regrow and set seed and to accumulate fuel for periodic fires. This assemblage of selective and generalist grazers/browsers would have dispersed the impacts of foraging across virtually all plant species, helping to maintain diverse plant communities. The majority of Pleistocene megafauna became extinct between 12,000 and 10,000 years ago, except for bison, elk, moose, deer, pronghorn, bighorn sheep and mountain goats, and populations of these grazers/browsers may have been low prior to Anglo-American settlement. This paucity of large herbivores over the past 10,000 years, and the dominance of bunchgrasses rather than more grazing-tolerant rhizomatous grasses in sagebrush and other rangeland ecosystems, led to the conclusion that native plant communities in much of the Intermountain West evolved without an abundance of large herbivores.
Abundant, native small herbivores, including rodents, lagomorphs, birds, insects and nematodes, may have had a larger impact on sagebrush ecosystems than the large herbivores remaining after the mass extinctions. Insect outbreaks involving grasshoppers (Melanoplus spp.,Ageneotettix spp., many other species), Mormon crickets (Anabrus simplex) and the defoliator moth (Aroga websteri) are of interest because these insects can kill or reduce the performance of sagebrush and associated species across large tracts of land (Figure 15). Non-outbreak insect herbivores, such as aphids (Obtusicauda spp., Pleotrichophorus spp., many other species), gall-forming midges (Rhopalomyia spp.) and defoliator weevils (Anthonomus spp., Apion spp.), can also substantially reduce inflorescence growth, successful flower production, and seed output of big sagebrush plants.
It is evident that herbivory was not a new process in sagebrush steppe and sagebrush shrublands when livestock were introduced in the 1860s; however, there are several differences in how livestock and native large herbivores used the forage resource. From the late 1800s to the early 1900s, large numbers of cattle and sheep usually grazed throughout the entire growing season, year after year, exerting selective grazing pressure on specific plant species, often in preferred portions of the landscape (Figure 16). Many exotic, invasive plant species were also introduced during this period. Repeated, selective grazing of perennial grasses and forbs decreased fine fuel loads and fire frequencies (Davies et al. 2010a), and placed these palatable herbaceous species at a competitive disadvantage with unpalatable, fire-sensitive, woody species, allowing sagebrush and juniper to become dominant. And, reductions in perennial grasses and forbs opened niches for the establishment of cheatgrass and other annual grasses that increased their dominance by promoting more frequent fires. Over the past several decades, managers and researchers have found, in general, that removal of livestock alone will not return woody plant-dominated or annual grass-dominated communities to the pre-grazed state; intensive management, often involving vegetation manipulation and revegetation treatments, will be required.
Prior to Anglo-American settlement in the mid-1800s, fires typically created mosaics of stands of different size in various successional states. Perennial bunchgrasses, which dominated the herbaceous vegetation in sagebrush steppe, are typically widely spaced, creating a discontinuous fuelbed that does not easily carry fire. Hence, fires tended to burn smaller areas and required hotter, drier conditions to burn. Fire return intervals ranged from decades on favorable sites (i.e., deep soils) with more abundant and continuous fine fuels to centuries on less favorable sites (i.e., shallow, coarse soils) with limited fine fuels (Miller and Heyerdahl 2008). Several studies have documented post-fire vegetation dynamics in the absence of exotic invasive species, and have found that perennial grasses and forbs, along with some resprouting shrubs (rabbitbrushes, horsebrush), generally dominate during the first 10-15+ years after fire. Big sagebrush subspecies, all non-sprouting, tend to reestablish up to 30+ years post-fire, depending on the dispersal of the small, short-lived seeds (1-2 years in soil seedbank) from adjacent, unburned plants.
Settlement in the mid-1800s initiated a series of changes in vegetation composition and structure that altered fire regimes in sagebrush ecosystems. Improper livestock grazing led to a decrease in native perennial grasses and forbs, reducing the abundance of fine fuels and increasing the abundance of shrubs (primarily sagebrush species) and conifers (juniper and pinyon pine species), which reduced fire frequency and size. Fire suppression activities also allowed these fire-sensitive woody species to expand and/or increase in density. The expansion of conifers into mid- to high-elevation sagebrush communities increased woody fuels, creating conditions for fires of greater size and intensity. The introduction and rapid expansion of exotic annual grasses (cheatgrass and medusahead) in low- to mid-elevation sagebrush communities with depleted understories increased fine fuel continuity and conditions for larger, more contiguous fires (Figure 17) with shortened fire return intervals, e.g., 3-5 years in the Snake River Plains of Idaho. This can set in motion a grass/fire cycle where invading annual grasses provide the fine fuel for the propagation of fire, and following fire, the grasses recover more rapidly than native species and cause a further increase in susceptibility to fire, eventually leading to near monospecific communities of annual grasses.
Invasive Plant Species
The invasibility of sagebrush steppe and sagebrush shrublands is governed by an array of biotic and abiotic factors, including environmental conditions, disturbance regimes and responses of native and exotic species to those regimes, as well as the biotic resistance of the resident plant community, i.e., the collective ability of the native species to limit the dominance of invading species (Chambers et al. 2007, Reisner et al. 2013). Lower elevation sagebrush communities (A. tridentata ssp. wyomingensis) located on coarser-textured soils or sites with higher potential heat loads (i.e., south- and west-facing slopes) have higher levels of water stress and lower productivity, resulting in larger gaps among perennial vegetation. These gaps are typically occupied by biological soil crusts, which reduce the amount of bare soil, and thus site availability for invasive species. Disturbances such as excessive livestock grazing can reduce native bunchgrass cover and damage biological soil crusts, increasing the amount of bare soil and freeing up water and nutrients for invasive species establishment, particularly exotic annuals like cheatgrass. Resistance to invasion is further reduced when more flammable, contiguous, annual grass fuels burn, killing foundation species such as big sagebrush, which exert disproportionate control over ecosystem processes, i.e., hydrology and nutrient cycling. The removal of big sagebrush can contribute to the expansion of cheatgrass and/or the successful establishment of invasive tap-rooted forbs (Figure 18), such as knapweeds (Centaurea spp.) and leafy spurge (Eurphorbia esula), which like big sagebrush, can use water from deeper in the soil profile than remaining native forbs and grasses. And, more frequent cheatgrass-fueled fires may promote the invasion of fire-tolerant, exotic perennial forbs such as rush skeletonweed (Chondrilla juncea), which produce large quantities of readily dispersed seeds and spread vegetatively. As mentioned in the previous Fire section, native woody species such as junipers and pinyons have invaded higher elevation sagebrush communities (Artemisia tridentata ssp. vaseyana) since the mid-1800s, due primarily to the cessation of periodic fires caused by livestock reducing fine fuels and active fire suppression by land management agencies. Thus, a spectrum of invaded sagebrush communities can exist, including near monocultures of exotic annual grasses or exotic perennial forbs, closed pinyon-juniper woodlands with little or no understory vegetation, and fairly diverse mixtures of native and invasive species.
In addition to excessive livestock grazing and altered fire regimes, the establishment and spread of invasive species in sagebrush communities are also facilitated by land development, recreation activities, and inappropriate/unsuccessful land management treatments. These anthropogenic activities disturb vegetation and soils, creating favorable sites for colonization by invaders, and they promote the dispersal of plant propagules (seeds, vegetative parts) across vast landscapes. The impacts of these activities are presented in greater detail below.
Since Anglo-American settlement 160+ years ago, a variety of human uses have resulted in the degradation, loss and fragmentation of sagebrush steppe and sagebrush shrublands. Historically, unmanaged livestock grazing reduced the perennial grass/forb component and increased shrub density/cover across large portions of these sagebrush ecosystems, leaving legacies that are still evident today. Up until the past few decades, many of these degraded areas were often manipulated to reduce or remove shrubs for greater forage production. More recently, other uses such as urban/exurban development, energy and mineral development, and motorized recreation have increased dramatically in sagebrush steppe and sagebrush shrublands. The cumulative effects of these multiple uses have stimulated a shift in management approaches toward the conservation and restoration of sagebrush ecosystems with an emphasis on biodiversity and ecosystem services.
Domestic livestock grazing in the Intermountain West has been viewed as a “grand experiment” which revealed several limitations of sagebrush ecosystems that were not apparent to livestock operators and settlers arriving from other environments in the mid to late 1800s. Under heavy grazing pressure year after year, native bunchgrasses disappeared because they were not given a chance to recover and allow flowering and seed production for regeneration. Sagebrush species, although highly nutritious, were avoided by livestock due to high concentrations of essential oils (monoterpenoids), and greatly increased in abundance. The disastrous winter of 1889-1990 forced ranchers to recognize that forage had to be harvested and stored for wintering of livestock. Productive sagebrush/grass communities near limited water sources were converted to alfalfa and irrigated, providing conserved hay for winter and early spring feeding. Yet, many ranchers prematurely curtailed the feeding of hay, placing livestock on native ranges too early in the spring and causing damage to new growth of herbaceous species. Over time, the void left by herbaceous perennials wasn’t completely filled by sagebrush and other shrub species; it provided an opening for exotic annuals adapted to heavy grazing pressure. In addition, policy makers did not understand the limitations of sagebrush and other semiarid ecosystems when they formulated policies to encourage settlement of the West, e.g., the Homestead Acts of 1862, 1909, and 1916. Land grants under these acts were only 160-640 acres (65-260 ha) in size, too small to support an economically viable livestock operation without access to summer forage at higher elevations (forests) and winter forage in valley bottoms (salt desert shrub). This led to severe competition for forage resources in sagebrush and associated ecosystems during the period of peak cattle and sheep numbers in the late 1800s and early 1900s. Thus, within the first 40 years of domestic livestock use, much of the sagebrush steppe and sagebrush shrubland vegetation in the Great Basin had been severely damaged by overgrazing. The impacts of light to moderate grazing by livestock are less clear.
Excessive livestock grazing can have several impacts on sagebrush ecosystem components and processes. Frequent removal of, or damage to, aboveground plant tissue influences regrowth capability, seed production, interspecific competition, and ultimately plant establishment and survival. Trampling can damage biological soil crusts, compact the soil surface layer and reduce litter cover, and in combination with changes in plant community structure, can influence infiltration rates and erosion potential. Grazing influences the amount and rate of shoot and root turnover and litter accumulation, and thus alters energy flow and nutrient cycling processes. Other indirect effects of grazing include the alteration of fire return intervals and the opening of niches for invasive species. Changes in plant community composition and structure can reduce habitat suitability for many wildlife species, particularly sagebrush obligates such as sage-grouse.
Livestock management gradually improved in sagebrush and associated vegetation types with the formation of federal land management agencies, development of grazing policies, and application of research findings. Grazing permits and allotments limited livestock numbers and better controlled the season of use on lands managed by the U.S. Forest Service (starting in 1905) and the U.S. Grazing Service [starting in 1934; later becoming the Bureau of Land Management (BLM) in 1946]. Range scientists and managers developed rotational grazing systems with more appropriate stocking rates in an attempt to exert some control over the intensity, frequency, timing and duration of grazing. Improvements also resulted from water development, brush management, seeding of forage species, and invasive plant control. For several decades (1950s – 1970s), sagebrush species were viewed as undesirable, and large areas with high sagebrush cover/density were converted to stands of crested wheatgrass (Agropyron cristatum, A. desertorum) and other introduced forage grasses. Today, livestock grazing is one of many uses in sagebrush ecosystems (see below), and it can be difficult to separate the effects of livestock from those of wild horses, off-road vehicles, and other uses. A recent assessment of livestock grazing effects in sagebrush ecosystems managed by the BLM found that 70% of 1,131 allotments in the western U.S. were meeting BLM Land Health Standards. For those allotments not meeting standards, livestock were considered a causal factor in 40% of cases (about 12% of all allotments).
Historically, livestock poisoning became a significant problem as settlers moved west with excessive numbers of livestock, reducing the availability of desirable forage and forcing animals to eat poisonous plants. In spite of improved management and range conditions, losses to poisonous plants continue to occur today under situations where poisonous plants are more available or palatable than associated vegetation and conditions that cause animals to graze non-selectively (i.e., release of naïve, stressed and/or hungry animals into areas with poisonous plants).
Direct losses (effects on livestock) from poisonous plants on western rangelands include: weight loss, decreased function (due to organ damage), decreased immune response, decreased fertility, abortions, birth defects, and death. Indirect losses (management costs) include: building and maintaining fences, increased feed requirements, increased medical treatments, altered grazing programs, decreased forage availability, decreased land values, opportunity costs, and lost time to management.
Important poisonous plants in sagebrush steppe and sagebrush shrubland communities include: locoweeds (Astragalus spp., Oxytropis spp.), lupines (Lupinus spp.), foothill death camas (Zigadenus paniculatus), low larkspur (Delphinium andersonii, D. nutallianum), and broom snakeweed. Alkaloids and diterpene acids are among the toxic secondary compounds in these species.
Nearly 1,235,000 acres (500,000 ha) of land in sagebrush ecosystems in the Intermountain West were cultivated for wheat (Triticum aestivum) and other grains during the short dry-farming boom in the early 1900s. Although most cultivated lands were abandoned by the 1920s, their legacies remain in many areas today. The dry-farming process created a drastic disturbance – the land was cleared of sagebrush and other vegetation, plowed to a depth of 7-10 inches (18-25 cm), and then harrowed to pulverize the soil. The recovery of native species following abandonment can be lengthy, because annual invasive species such as cheatgrass can dominate old fields for 35-50+ years. Cheatgrass was prevalent on many abandoned cultivated areas because it was a contaminant in grain seed planted in the early 1900s. In one study where there was little evidence of cheatgrass or other exotic annuals in 90-year-old abandoned fields, there were still major differences in species composition compared to adjacent, non-cultivated sites, i.e., abandoned fields had higher cover of disturbance-adapted shrubs [rabbitbrush and greasewood (Sarcobatus vermiculatus)] than big sagebrush and lower forb cover. The reestablishment of native plant communities, or lack thereof, on abandoned fields has undoubtedly been influenced by interacting biotic constraints (limited seed banks and seed dispersal, presence of invasive species) and abiotic constraints (changes to soil structure, organic matter content, and nutrients).
Invasive plants causing significant problems in sagebrush communities are primarily non-native species from Eurasia, and include annual grasses and forbs [cheatgrass, medusahead (Taeniatherum caput-medusae), and yellow starthistle (Centaurea solstitialis)], biennial thistles [musk thistle (Carduus nutans) and Scotch thistle (Onopordum acanthium)], and perennial forbs [knapweeds (Centaurea spp.), rush skeletonweed (Chondrilla juncea), perennial pepperweed (Lepidium latifolium), leafy spurge (Euphorbia esula), and dyer’s woad (Isatis tinctoria)] (Figure 18). The expansion of certain native woody species [broom snakeweed (Gutierrezia sarothrae)], junipers (Juniperus spp.) and pinyon pines (Pinus spp.)] also negatively impacts sagebrush communities.Improper livestock grazing and other land uses (i.e., energy and mineral development, urban/exurban development, off-road vehicles) have decreased the herbaceous component of sagebrush communities and facilitated exotic annual grass invasion at lower elevations and conifer encroachment at higher elevations. These two scenarios are considered the greatest invasive species threats to the sagebrush steppe and sagebrush shrubland vegetation types.
Invasive species impacts can include: 1) reductions in plant diversity, wildlife habitat, and livestock forage; 2) changes in soil morphology, organic matter, and nitrogen dynamics, and the species composition of soil microorganisms altered fire regimes ; 4) accelerated rates of soil erosion and sediment production; and, 5) the associated socioeconomic costs at scales ranging from the individual ranch to the urban-wildland interface and extensive landscapes managed by several government agencies.
As mentioned earlier in the Disturbance Factors section, fire regimes in sagebrush steppe and sagebrush shrublands have been significantly altered since the arrival of Anglo-Americans in the mid-1800s. Increased woody fuels associated with the expansion of native juniper and pinyon species into mid- to high-elevation sagebrush communities have created conditions for larger, more intense fires. At lower elevations, increased fine fuel continuity associated with the expansion of exotic annual grasses has resulted in larger fires with shortened fire intervals (Figure 17).
These major changes in fire regimes have resulted in large-scale degradation, loss and fragmentation of sagebrush ecosystems. Associated impacts include: 1) loss of perennial native plants and spread of invasive plants; 2) loss of habitat for sagebrush obligate (sage-grouse, pygmy rabbit, pronghorn) and facultative (mule deer, elk,) wildlife species and loss of forage for livestock; 3) emission of air pollutants that can be harmful to human health; and, 4) accelerated rates of soil erosion and sediment production, degrading water quality for aquatic organisms and human use. More frequent, larger fires have dramatically increased fire suppression and rehabilitation costs and the need for fuel hazard reduction programs. As the overall livestock forage base decreases, more producers will compete for more costly, alternate sources, and they might be compelled to convert remaining sagebrush communities to introduced grasses or sell their property for exurban development to offset loss of income.
Energy and Mineral Development
Historic and contemporary development of energy and mineral resources has had significant impacts on sagebrush and other semiarid and arid rangeland ecosystems in the western U.S., and even greater development will occur in the future. Energy resources include coal, oil (also shale and tar sands), natural gas, wind, solar, and geothermal; mineral resources include metals such as gold, silver, copper, and molybdenum, and industrial minerals such as sand, gravel, barite, clay, gypsum, dolomite, limestone, and phosphate. The development and use of these resources requires considerable infrastructure, i.e., handling facilities, roads, power plants, transmission lines, pipelines, etc. Even when best practices and advanced technologies are used, there are still significant impacts to vegetation, soils, wildlife, water and air.
A case study, using remote sensing and geographic information system technologies to analyze disturbances, documented the effects of natural gas development on sagebrush habitat quality and quantity in western Wyoming from1985 to 2006. During this 22-year period, gas development increased nearly 50-fold, directly impacting 2.7% [1,750 ha (4, 325 ac)] of the original sagebrush habitat via well pad and road construction. However, indirect impacts such as increased traffic on roads (noise, vehicle – animal collisions), spread of invasive species, and altered predator and disease dynamics, degraded the quality of 58.5% [37,920 ha (93,700 ac)] of the original sagebrush habitat. Although the direct impacts of gas development affect a relatively small area, the cumulative effects of the direct and indirect impacts have altered a more substantial amount of sagebrush habitat for migrating big game species and sagebrush obligate bird species.
Urban and Exurban Development
Recent, dramatic increases in human population growth in the western U.S. have resulted in the marked expansion of urban/suburban areas into sagebrush and other arid and semiarid ecosystems. These ecosystems have also been greatly impacted by low-density exurban development in rural areas. For example, in the Great Basin region from 1990 to 2004, the population grew from 2.9 to 4.9 million people, with an associated reduction in uninhabited land from [9 million to 1.2 million ha (22.2 million to 2.9 million ac)]. A major concern is that urban/suburban expansion and small exurban properties [0.7 – 16.2 ha (2 – 40 ac) per housing unit] could replace larger traditional ranches that protected native plant and animal communities and safeguarded ecosystem services. Conservation easements on ranches have been effective at reducing development and maintaining biodiversity in several sagebrush ecosystems in Wyoming.
Motorized recreation is considered one of several major threats jeopardizing the health of public and private lands in the western U.S., including large areas of sagebrush steppe and sagebrush shrublands. Use of off-highway vehicles (OHVs) has been one of the fastest-growing forms of outdoor recreation, e.g., in Idaho, the number of OHVs increased from ~8,000 in 1987 to ~117,000 in 2007. Thousands of miles of unpaved roads and trails are open to OHVs and millions of acres are open to cross-country travel, which leads to the proliferation of unauthorized trails, associated resource damage, and difficult management challenges.
Ecological effects of unmanaged OHVs are often interrelated, and include: 1) increased soil compaction and erosion, and disrupted hydrologic function; 2) impacts on plants (crushed plants, exposed roots, diminished seed germination and seedling establishment, decreased soil moisture and nutrients, and increased spread of invasive plants); 3) impacts on wildlife (increased levels of stress influencing breeding, foraging, dispersal and survival, and easier access for hunting and poaching); and, 4) increased pollution levels (noise, air and water). Noise and intrusion associated with OHVs also create conflicts with non-motorized recreationists using the same areas.
Although horses arose and diversified in North America during the Miocene (56-34 million years ago), they were one of several large mammal species to go extinct at the end of the Pleistocene (12,000-10,000 years ago), due to prehistoric human hunting pressure, climate change, or a combination of the two. Present-day, free-roaming (wild) horses in sagebrush and other arid and semiarid ecosystems are descendants of domestic horses introduced to the southwestern U.S. by Spanish explorers in the late 16th century. Wild horse numbers increased dramatically to 2-7 million during the 19th century, due primarily to predator control and increased availability of water, and declined to about 150,000 by the mid-20th century, because of persecution and removal facilitated by the Taylor Grazing Act of 1934. After passage of the Wild and Free-Roaming Horses and Burros Act of 1971, which made it a crime to harass or kill these animals on federal lands, wild horse and burro numbers rose sharply. As of February 2013, the Bureau of Land Management (BLM) estimated that 40,605 wild horses and burros (33,780 horses and 6,825 burros) were roaming on BLM – managed lands in the 10 western states (BLM 2013). This exceeds the maximum appropriate management level (AML) of 26,677 animals for these lands by nearly 14,000 animals (BLM 2013). When an AML for a herd management area is exceeded, the excess animals are gathered and prepared for adoption or sent to long-term pastures. As of July 2013, there were 49,050 wild horses and burros fed and cared for at short-term corrals in western states and long-term pastures in Oklahoma, Kansas, and South Dakota.
As with other large herbivores, large populations of horses can alter sagebrush ecosystems directly through selective plant consumption, trampling of plants and surface soil horizons, and redistribution of nutrients via ingestion and subsequent excretion. These direct impacts on vegetation and soils can alter competitive interactions among plants, alter soil erosion patterns, and create openings for invasive species, all of which reduce the functionality of habitats for sagebrush-obligate animal species. In a synthesis of data from 19 sagebrush sites across the Great Basin, sites with wild horses removed for 10-14 years exhibited greater shrub cover, total plant cover, plant species richness (2-12 more species), and cover and frequency of native grasses, and fewer grazing-resistant forb and exotic plant species (particularly cheatgrass) than did horse-occupied sites. Surface soil hardness was also greater at horse-occupied than at horse-removed sites.
The cumulative effects of vegetation degradation, fragmentation and conversion, loss of watershed functioning, and loss of native plant and animal species have placed sagebrush ecosystems, particularly those in the Great Basin, among the most endangered in the United States. A recent assessment in the region identified 133 plant species, 63 vertebrates and 11 invertebrates of conservation concern that were associated with sagebrush habitats. All of the plant species of concern, and all but one of the invertebrates of concern, were restricted to small areas. As populations of a species become smaller and more isolated, it becomes more difficult for them to adapt to changing environmental conditions and land use practices. For example, slickspot peppergrass (Lepidium papilliferum), a federally listed threatened species, is limited to small-scale sites of water accumulation (“slick spots”) in sagebrush steppe communities in the southwestern Snake River Plains of Idaho. There are only 42 populations known to exist, and many are very small. Slickspot peppergrass has the highest documented extinction rate of any of Idaho’s rare plant taxa, due to a variety of threats, including weed invasion, wildfire, off-road vehicle traffic, livestock grazing, and land conversion. Many management practices after wildfires, i.e. use of preemergent herbicides, seeding equipment that disrupts the soil surface, and exotic revegetation species such as forage kochia (Kochia prostrata), can also have negative impacts on this rare forb.
There are several sagebrush-obligate vertebrate species of conservation concern, including lizards, snakes, raptors, owls, passerine birds, rodents, rabbits, and the pronghorn, that have broad home ranges across sagebrush ecosystems in the Great Basin (Figure 14) and elsewhere in western North America. Probably the most prominent of these species is the greater sage-grouse, which has been placed on the federal list of candidate species under the Endangered Species Act of 1973. Sage-grouse have declined significantly across their range due to habitat loss and fragmentation caused by many of the previously described threats (management issues), as well as parasites, diseases, and predation. They depend on large areas of contiguous sagebrush that provide a variety of seasonal habitats for breeding, nesting, brood-rearing, and wintering. Since adults have high site fidelity, they rarely switch from these habitats once they have been selected, limiting their ability to respond to changes in their local environments. Federal land management agencies and many state and local governments are working to develop regulatory mechanisms and conservation plans to address these threats to sage-grouse and their habitats. The conservation and restoration of sage-grouse habitat may benefit other sagebrush-obligate species, thus the sage-grouse has been proposed as an indicator or umbrella species for other species of concern. However, when the spatial overlap in habitats for sage-grouse and other species was accounted for, only a few species, including the pygmy rabbit and Brewer’s sparrow, might receive substantial conservation benefits if habitats for sage-grouse were the focus of management planning and restoration.
Grazing Management Practices
Livestock producers in the Intermountain West try to meet the nutritional requirements of animals by using seasonal ranges when environmental conditions are most conducive and forage quantity and quality are as high as possible. Low-elevation desert shrub ranges, when available, are used in winter (early November to mid-April), mid-elevation foothill ranges are used in the spring (mid-April to early July) and occasionally in the fall (early October to early November), and high-elevation mountain ranges are used during summer and early fall (early July to early October). In general, forage “bottlenecks” occur during winter and in early spring when plant growth is limited on sagebrush steppe and sagebrush shrublands, and hay must be fed sparingly to reduce costs. Spring grazing on these foothill ranges is a critical period in the livestock management cycle, i.e. the high nutritional demands of cows that calve in March must be met so calves receive enough milk for rapid growth and cows are physiologically ready for rebreeding in June or July. The slow growth of native perennial grasses in early spring is often inadequate for lactating cows. And, as the season progresses, native grasses grow rapidly for a short time, but forage quantity and quality decrease as plants mature. With the dominant native grasses on foothill ranges being satisfactory only from early May to mid-June, other seeded forages, primarily introduced grasses such crested wheatgrass and Russian wildrye (Psathyrostachys juncea), are often used during April and later in the summer before cattle are moved to mountain ranges. If livestock are not properly managed, overgrazing is likely to occur on foothill ranges at this time.
Good grazing management is based on several principles, including: proper stocking rates and grazing intensity, proper kind(s) of grazing animal(s), proper season of grazing, and proper distribution of animals. Selection of the appropriate stocking rate is the most important of all grazing management decisions from the standpoint of vegetation, livestock, and economic return, regardless of the grazing system used. When light, moderate, and heavy stocking rates were used in combination with four grazing systems on seeded foothill range in central Utah, heavy stocking resulted in the lowest daily animal gains, a decline in forage production, little or no seed production, excessive trampling of plants and soil, and accelerated invasion of undesirable shrubs and exotic annual weeds. Light stocking essentially left the most forage for fall grazing. Moderate stocking resulted in the most favorable combination of forage yield, animal performance, availability of forage for fall grazing, and resistance to shrub reinvasion. Similar results have been reported for grazing systems using light, heavy, and moderate stocking rates on native sagebrush-grass ranges. Cattle are the primary grazers in sagebrush ecosystems, and can damage bunchgrasses during the flowering period, so plants should be grazed earlier in the spring and be allowed to complete seed set every second year. Heavy and even moderate stocking of sheep in the spring can damage actively growing perennial forbs; however, sheep consume sagebrush and don’t utilize dormant herbaceous species when grazing in late fall, allowing forbs to increase. Managers can improve the distribution of livestock on sagebrush-grass ranges by developing additional water sources, fencing, strategically placing salt and/or low-moisture molasses blocks, providing shade and windbreaks, developing trails on steeper slopes, and improving forage quality/quantity through burning or seeding.
Climate, topography, life history traits of key forage species, livestock performance, wildlife needs, watershed protection, ease and cost of implementation and operation, and flexibility under changing conditions are important considerations involved in grazing system selection. Common grazing systems that have been used in sagebrush steppe and sagebrush shrublands include: 1) continuous or season-long grazing, where livestock are allowed free access to any part of a range throughout the grazing season, and use follows the same general pattern each year; 2) deferred-rotation grazing, which involves rotating the deferment (delay of grazing during the growing season) of typically three pastures each successive year to promote plant reproduction and improve vigor; and 3) rest-rotation grazing, which involves rotating the rest (full year of nonuse) of typically four pastures each successive year to give plants a longer period to recover from past grazing influences. Time-controlled grazing systems have been used to a limited extent, and include: 1) short duration grazing, where very short periods of intensive grazing (1-3 days) are followed by short non-used periods (2-3 weeks) in several pastures during the grazing season; and 2) high-intensity low-frequency grazing, where short periods of intensive grazing (7-10 days) are followed by long rest periods (≥ 1 year) in numerous pastures. Research has shown that specialized grazing systems generally give either modest or no increase in grazing capacity, animal performance, and financial returns over continuous or season-long grazing, particularly when animals are well-distributed at a moderate stocking rate.
There is not one best grazing system that can be applied across sagebrush ecosystems. Deferred-rotation, rest-rotation, and high-intensity low-frequency systems have traditionally been viewed as conservation practices favoring native perennial grasses. However, periods of complete rest or deferment until seed set can also favor cheatgrass as much as or more than perennial grasses when it is present in pastures. Rest-rotation and high-intensity low-frequency systems, with their extended rest periods, integrate well with wildlife use (i.e., nesting, rearing of young) and facilitate the implementation of vegetation management treatments (i.e., burning, chemical and mechanical treatments, seeding). Short-duration grazing systems have not been widely used on sagebrush-grass ranges because the relatively short growing season (60-90 days), small size of most ranches [800-1,200 ha (2,000-3,000 ac)], and fencing costs don’t make it a viable option. One example where high-intensity low-frequency grazing has been successful in predominantly sagebrush-grass rangeland is on a 81,000 ha (200,000 ac) ranch in northern Utah that supports 5,000 cattle, 3,000 sheep, and abundant wildlife species on 60+ large pastures. The managers of this large ranch have the flexibility to view livestock grazing schedules and stocking rates as variables to be integrated adaptively with other management practices to meet several objectives, including livestock production, wildlife abundance and diversity, recreation, and land health.
Several manipulation practices have been used to modify sagebrush steppe and sagebrush shrubland communities to meet management objectives, which include improved forage production for livestock, enhanced habitat for wildlife, control of invasive species, and restoration/rehabilitation of areas that have been disturbed by wildfires, cultivation, and energy and mineral development. Management practices can be grouped into five general categories (prescribed fire, mechanical, chemical, biological, and revegetation), and they are often integrated in an attempt to reach management objectives.
Livestock Forage Production
A common problem in degraded sagebrush steppe and sagebrush shrubland communities is the increase in density, cover, and size of sagebrush and other shrubs and the concomitant decrease in perennial grasses and forbs, i.e., livestock forage. If degradation has not progressed very far, it may be possible to use previously described rotational grazing systems to gradually improve forage production and grazing capacity. When degradation has progressed too far, manipulation practices (prescribed burning, herbicide application, mechanical treatments), often followed by revegetation, will be necessary to reduce sagebrush dominance and increase herbaceous forage species. The suitability of a particular practice depends upon a variety of factors, including the density, height, and age of sagebrush and other shrubs, the amount and kinds of grasses and forbs in the understory, the topography and rockiness of the area, the type of soil and it’s susceptibility to erosion, the size of the area to be treated, the availability of equipment, and treatment costs. During the first 5-6 years after treatment, big sagebrush control can increase forage production 2-3-fold compared to untreated areas; however, follow-up treatments will probably be required at intervals of ≤ 20 years, since sagebrush can recover after manipulation under most conditions.
Prescribed burning can be successful when there are sufficient perennial grasses and forbs to provide understory fuels to carry the fire and to respond to increased resource availability and the lack of sagebrush competition after the fire. Big sagebrush does not resprout following fire, allowing several years of increased forage production before it reenters and gradually regains control of the community. If present, other shrubs such as rabbitbrushes and horsebrush resprout from basal stem buds, increasing in abundance after fire, and may require a follow-up herbicide treatment. Burning is usually done in the fall when fine fuels (dormant perennial grasses and forbs) and sagebrush canopies are drier and environmental conditions (air temperature, relative humidity, and wind) are more suitable for carrying a fire. During the spring, there are fewer days with favorable environmental conditions for burning, and increased fuel moisture usually limits burn size. Burned areas are typically rested from livestock grazing for at least one growing season after treatment.
Selective herbicides have been used for decades to control sagebrush and other shrubs and release perennial grasses (from competition) to increase forage production. 2,4-D, a translocated, growth-regulator herbicide, has generally been the most effective and economical chemical for sagebrush control. Aerially application of 2,4-D in late spring, near the end of the effective spraying period for sagebrush, will also control some associated rabbitbrush plants. Though this selective herbicide does not damage perennial grasses, it can severely damage perennial forbs such as arrowleaf balsamroot and milkvetch (Astragalus stenophyllus), which are important forage species. Thus, sites should be assessed for species composition before spraying to determine the extent of damage that may occur, and if seeding may be necessary to increase forage production and discourage replacement by undesirable species. Sprayed areas are usually rested from livestock grazing for at least the remainder of the year in which they are sprayed.
Several mechanical methods are available for controlling sagebrush and increasing herbaceous forage production. Sagebrush plants can be cut off near the base with a heavy-duty mower, broken off or uprooted with a pipe harrow, and crushed with an aerator (large rolling cylinder), causing limited disturbance to desirable understory species (Figure 26). Two passes with a pipe harrow or aerator will be required for a higher level of sagebrush control. If needed, forage species can be seeded at the time of the treatment. Where there is not an adequate understory of desirable perennial grasses and forbs, plowing or disking will kill sagebrush and prepare a seedbed for revegetation. Mechanical treatments are typically implemented in the fall, and livestock are usually excluded from treated areas the following growing season.
Revegetation is necessary when perennial grasses and forbs are lacking, or when a manager chooses to plant improved forage species. Drill seeders with multiple seed boxes and metering devices, and depth bands on disks, can dispense multiple species with varying seed sizes and shapes and seeding depth requirements. Ground broadcast seeding can be coupled with mechanical treatments, and is often used to seed areas that are inappropriate for drill seeding, such as rocky, rough, or steep terrain, areas with large amounts of debris, and small, irregularly shaped areas. Aerial broadcast seeding with aircraft is used to distribute seeds over large areas. Early efforts in the 1950s and 1960s were oriented toward establishing productive, monotypic stands of introduced, perennial grasses such as crested wheatgrass, intermediate wheatgrass (Thinopyrum intermedium), and Russian wildrye. Since then more emphasis has been given to mixtures of perennial herbaceous species that provide livestock forage and enhance wildlife habitat. Seedings are most often conducted in the fall, and livestock are excluded from seeded areas for two growing seasons.
Many poisonous plants are toxic at all times, whereas others are toxic only under certain conditions. Careful managers can reduce the risk of livestock poisoning and even allow some poisonous plants to be used as nutritious forage at select times. The following management practices can help protect livestock from poisoning: 1) learn to identify poisonous plants and the conditions under which they can be dangerous to livestock; 2) develop a grazing plan to improve rangeland and prevent poisoning; 3) do not allow stressed or hungry animals to graze in areas infested with poisonous plants; 4) provide adequate water for livestock; 5) be especially careful when grazing animals unfamiliar to an area; 6) train animals to avoid poisonous plants by implementing conditioned-taste aversions where animals associate the taste of the plant with the stomach illness caused by an administered drug (lithium chloride); and 7) control poisonous plants with herbicides and other manipulation treatments where feasible.
A change in grazing management significantly reduced cattle losses to white locoweed (Oxytropis sericea) at a high elevation sagebrush area in northern Utah. For many years, ranchers used a rest rotation grazing system, with 3 pastures grazed in sequence, and 1 pasture rested each summer. Range condition improved, but annual losses to locoweed exceeded 20%. Based on observations that most consumption of locoweed occurred after flowering during August, the grazing season was reduced from 71 to 47 days, cattle numbers were increased, and the grazing method was changed to a 3-herd, 4 pasture system. Most of the highly toxic young fruit pods were avoided, and yearly losses to locoweed poisoning declined to about 3%.
After mature broom snakeweed plants in sagebrush and other semiarid plant communities are killed by herbicides, prescribed burning or targeted grazing, snakeweed seedlings have difficulty establishing in the presence of established cool-season, perennial grasses due to resource competition. If perennial grasses are not abundant following snakeweed control, species such as crested wheatgrass should be seeded in an attempt to develop a grass stand that will prevent snakeweed establishment.
Ecological processes involved in the natural restoration of abandoned cultivated lands in sagebrush ecosystems may take over a century to recover on their own, and many historically dry-farmed sites can be altered to the point that the potential natural vegetation state is no longer attainable. However, since the passage of the 1985 Farm Bill, millions of hectares of actively cultivated lands in sagebrush steppe and sagebrush shrubland areas in the Intermountain West have been planted to perennial grasses, forbs, and shrubs under the Conservation Reserve Program (CRP). This voluntary program, administered by the U.S. Department of Agriculture, allows farmers to take agricultural lands (particularly environmentally sensitive lands) out of production for 10-15 years to achieve conservation objectives, including reduced soil erosion and improvement of wildlife habitat. The program provides the farmers with annual rental payments for non-use and cost-share assistance (up to 50%) for approved conservation practices. A recent study in eastern Washington found that CRP fields were used by a variety of passerine birds, and they were important as nesting, brood-rearing, and wintering habitat for sage-grouse.
Invasive Species and Wildfires
Invasive plants by themselves, and especially when they alter fire regimes, create complex management problems in sagebrush and other arid and semiarid ecosystems. Many invasions have expanded to large landscape scales, and considerable environmental variability defines windows of management opportunities in space and time, which are more limited in lower elevation Wyoming big sagebrush communities than in higher elevation mountain big sagebrush communities. Thus, one of the main challenges for managers is to determine the ecosystem properties that define existing windows and/or determine how to create new windows of opportunity. This challenge has been addressed, in part, with the development of decision-making frameworks such as ecologically based invasive plant management (EBIPM), which identifies the causes of succession and associated ecological processes influencing invasion, uses ecological principles to link processes in need of repair with appropriate vegetation management strategies and tools, and provides a science-based method for adaptive management planning. EBIPM and similar integrated weed management approaches have improved the control of invasive forbs such as leafy spurge, diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea stoebe), and EBIPM has recently been implemented on cheatgrass-dominated rangelands. Since the management of invasive junipers in higher elevation sagebrush communities is covered in detail in a companion module, the focus here is on the management of invasive annual grasses in low- to mid-elevation sagebrush communities.
Management of sagebrush communities can take two forms: passive management for intact communities dominated by desirable species and active management for invaded communities dominated by exotic annual grasses. Passive management involves efforts to maintain or improve community resistance to invasion and reduce the propagule pressure of annual grasses. A common approach is to change livestock management, i.e. make adjustments in animal numbers, grazing seasons, duration of grazing, and/or distribution, in order to increase the diversity and abundance of desirable species, particularly perennial grasses. Moderate levels of livestock grazing can play an important role in reducing the risk and severity of wildfires by decreasing fine fuel loads and continuity. Reductions in the propagule pressure of annual grasses can be accomplished in different ways. When crested wheatgrass was established as a vegetation barrier at the interface of medusahead infestations and sagebrush-grassland communities, the cover and density of medusahead were 42- and 47-fold less, respectively, in protected communities than in unprotected communities. The taller-statured crested wheatgrass plants reduced the spread of propagules by physically intercepting them, and increasing the distance medusahead plants had to disperse propagules to find suitable sites for establishment in adjacent sagebrush-grassland communities. Cheatgrass seed production can be reduced significantly when plants are intensively grazed just before emergence of the inflorescence, decreasing the potential for propagules to spread from an infested area to an adjacent sagebrush-grassland community.
Active management is undertaken when desired species or functional groups are absent or poorly represented in invaded communities. Annual grass-dominated areas can be modified with several practices in an attempt to reduce catastrophic wildfires, repair ecological processes, and improve ecosystem structure, function and productivity. These practices are most effective when sequenced appropriately in an integrated management approach. Control of medusahead and cheatgrass is greatest when prescribed burning in fall is closely followed by the application of imazapic, a preemergence herbicide. Burning removes annual grass litter that can intercept imazapic, allowing greater herbicide activity for killing fall-germinating cheatgrass and medusahead. Desirable species are usually drill seeded after imazapic application in late fall or early winter. Imazapic residual activity the following spring can injure developing seedlings of sagebrush and some seeded perennial grasses when the herbicide is applied at higher rates for better annual grass control. Even when annual grasses are successfully controlled and imazapic residual activity is not a problem, native species often fail to establish due to environmental conditions and annual grasses can rapidly reinvade. Thus, managers often use non-native revegetation species such as crested wheatgrass, Siberian wheatgrass (Agropyron fragile), Russian wildrye, and forage kochia (Kochia prostrata), which cost less, are more available, and have better establishment characteristics than most natives. When compared to native sagebrush communities, these simplified non-native communities provide limited habitat for wildlife, but there are no substantive differences in ecological processes, and they can be resistant to invasive species and resilient to fires. Seeding of native forbs, grasses and shrubs into established crested wheatgrass stands has not been successful to date; however, sagebrush seedlings have been transplanted and successfully established in crested wheatgrass stands. Efforts have been made to reduce management costs by burning and applying herbicide in areas that have remaining native vegetation and may not need revegetation, taking advantage of naturally occurring wildfires as burn treatments to be followed by herbicide application and seeding, and using a single-entry approach, where the herbicide and seeds are applied simultaneously.
Large, often catastrophic wildfires on public rangelands require stabilization within 1 year of containment to prevent resource degradation, and rehabilitation within 3 years of containment to repair or improve lands unlikely to recover naturally to management-approved conditions. Integrated approaches and native species are used whenever possible; however, treatment costs, equipment and seed availability, and topographic constraints can modify planning, e.g., eliminating herbicide application and using aerial broadcast seeding of mostly introduced species on large areas of rough terrain.
Active management practices have also been used to create fuel breaks to protect revegetated areas and intact sagebrush communities. When cheatgrass is intensively grazed by cattle just prior to emergence of the inflorescence in the spring, and 80-90% of the biomass is removed, there is a significant reduction in flame length and rate of fire spread during the subsequent fire season (Figure 28). Due to the short grazing window and large number of animals required, this treatment would be best applied in a strip instead of a large block. A longer-term fuel break can be created by green stripping, where flammable cheatgrass is replaced by vegetation that is less likely to ignite and carry a fire. Strips up to 100 m (325 ft) wide are either disked or treated with herbicide (glyphosate, imazapic) to control cheatgrass and then seeded with species that maintain a higher moisture content during the growing season, i.e., crested wheatgrass and forage kochia.
Energy and Mineral Development
The drastic disturbances associated with oil and gas development, and surface mining for coal, minerals, and metals require intensive reclamation practices to repair the damage to vegetation, soils, and hydrologic processes in sagebrush and other arid/semiarid rangeland ecosystems. The components of a successful surface mine reclamation plan include: 1) determining the pre-disturbance ecosystem functions that must be repaired following disturbance; 2) conducting a pre-disturbance inventory of vegetation, soil, topographic, hydrologic, wildlife, archeological, and historical resources; 3) salvaging, separating, and stockpiling soil; 4) backfilling, grading to original or approximate contour, redistributing salvaged soil, and preparing a seedbed; 5) revegetating with native or appropriate species; 6) managing for post-mining uses such as livestock grazing and/or wildlife habitat; and, 7) long-term monitoring for revegetation success, invasive weeds, erosion, wildlife diversity and abundance, and livestock production. There are similar components for oil and gas development reclamation plans; however, the disturbances for well pads and reserve pits are much smaller in size [< 1ha (1-2 ac.)] and distributed across landscapes [1 per 4-16 ha (10-40 ac.)] connected by a network of roads and pipelines. During interim reclamation, the reserve pit and portions of the well pad not needed for operational purposes are recontoured, topsoiled and revegetated; during final reclamation (following well plugging and abandonment), the well site and roads are recontoured, topsoiled, and revegetated.
As mentioned earlier, large-scale sagebrush treatments conducted from the 1950s through the 1970s were focused primarily on increasing forage production for livestock; however, in the last few decades, treatments have been increasingly applied to enhance habitat conditions for wildlife species using sagebrush habitats. Treatments are typically planned to emulate disturbances (i.e., wildfires, insects/pathogens) that maintain a mix of seral stages in a mosaic or patchy pattern with different age classes and canopy covers of sagebrush (and other shrubs) and a diversity of grasses and forbs. In addition to maintaining or improving existing sagebrush communities, efforts are also aimed at mitigating habitat losses due to fragmentation caused by land development, invasive species, and catastrophic fires. The management of sagebrush habitat for two important wildlife species, mule deer and the greater sage-grouse, is described below.
Management efforts for mule deer habitat have typically been directed toward protecting and enhancing sagebrush and other important browse species on winter ranges. These ranges are primarily limited by browse/forage quality and quantity due to the aging of native plants and the increase in abundance of undesirable plants. Thus, habitat improvement practices such as prescribed burning, mechanical treatments, herbicides, and seeding have been used to replace undesirable species with desirable species and to replace overly mature desirable plants with younger, more nutritious plants of the same species. Several small treated areas [2-10 ha (5-25 ac)] with irregular edges dispersed in a mosaic pattern within an untreated matrix of sagebrush provide a high edge-treated area ratio with nearby cover for security and thermal regulation. In drier, lower elevation Wyoming big sagebrush communities, managers should use extreme caution or not treat stands where cheatgrass or other invasive species are present. If Wyoming big sagebrush communities are treated, it is imperative to seed manipulated areas with a diverse seed mix to prevent/reduce weed invasion. Seeding is often recommended following treatments in more mesic, higher elevation, mountain big sagebrush communities.
Sage-grouse require a variety of plant communities across large landscapes for breeding, nesting, brood-rearing, and wintering. Sparsely vegetated areas (leks) are used for breeding. Nesting habitat should have 15-25% sagebrush canopy cover of medium height [40-80 cm (16-32 in)] and tall [> 18 cm (7.5 in)] bunchgrasses for screening cover. Brood-rearing habitat should have an abundance of forbs and insects, which are associated with lower shrub canopy cover (< 20%). Wintering habitat is comprised of medium height and taller sagebrush that remains 25-35 cm (10-14 in) above the snow on south and west facing slopes. Previously described threats resulting in the loss and fragmentation of sagebrush communities, and changes in the structure and composition of remaining communities have significantly altered these sage-grouse habitats, creating a complex management challenge at a regional scale. A landscape triage approach has been proposed for identifying and prioritizing lands for vegetation manipulation treatments. Lands are grouped into three categories – one that needs no immediate treatment, one that is significantly damaged and will not recover even with intervention, and one that is damaged but has the potential to recover with intervention – based on rangeland health assessments and state-and-transition models associated with ecological site descriptions. State-and-transition models also aid in determining if passive or active management should be used to achieve desirable habitat changes.
Passive management can be achieved by changing current management practices to promote the recovery of desired plant species or vegetation structure. Due to the prevalence of livestock grazing as a land use, changes in livestock management, i.e., kinds and numbers of animals, season of grazing, and/or duration of grazing, could positively impact large expanses of sage-grouse habitat. Switching from season-long to time-controlled (high-intensity low-frequency) cattle grazing, described earlier in the “Grazing Management” section, prevented an increase in shrub dominance and promoted an increase in herbaceous plant diversity and abundance, improving sage-grouse habitat in higher elevation Wyoming big sagebrush communities in northern Utah. Intensive sheep grazing in fall significantly reduced sagebrush cover and increased perennial forb and grass cover, improving brood-rearing habitat at a mountain big sagebrush site in south-central Utah.
Active management, using prescribed fire, chemical and mechanical treatments, and revegetation, is necessary when desirable species have been eliminated from sites or vegetation structure cannot be modified by passive management. These vegetation manipulation practices, described earlier in the Livestock Forage Production section, have advantages and disadvantages that should be considered before applying them to modify sage-grouse habitats. Prescribed fire should not be used where sagebrush cover is the limiting factor for sage-grouse, i.e., nesting and wintering habitats, or where introduced annuals have invaded sagebrush communities. Six years after burning a Wyoming big sagebrush area in southeastern Oregon, habitat cover was about 50% lower than the unburned control area because of the loss of sagebrush, there was no increase in the yield or nutritional quality of forb species, and there was a decrease in the abundance of insects. Based on concerns about reduced forb abundance following applications of 2,4-D to reduce sagebrush cover, emphasis has shifted to using tebuthiuron, a soil-applied herbicide that can selectively control sagebrush and increase grass and forb production when applied at low rates. Tebuthiuron effectively reduced shrub canopy cover and increased forb cover to meet recommended guidelines for sage-grouse brood-rearing habitat in a mountain sagebrush community in south-central Utah, as did a Dixie-harrow mechanical treatment. Prescribed fire, tebuthiuron and mechanical treatments, in conjunction with seeding, have reduced shrub cover and increased herbaceous species diversity and abundance, and enhanced sage-grouse habitat in higher elevation Wyoming big sagebrush communities in northern Utah. Most literature, however, indicates that habitat management programs that emphasize implementing these treatments in drier, low elevation Wyoming big sagebrush communities are not supported with respect to positive responses by sage-grouse habitats or populations.
The management of wild horses (and burros) is unique compared to other large herbivores on western rangelands because of their morphological and physiological characteristics, and the policies enacted to protect them. Being cecal digesters, horses consume a wider variety and larger amount of forage species compared to ruminant digesters of equivalent body mass. Also, an elongated head, more flexible lips, and the presence of upper front incisors allows horses to remove vegetation more closely to the ground than cattle. Horses (and burros) are not managed in the same manner as traditional wildlife species, whose numbers are controlled by hunters and natural predators, or domestic livestock, which are more intensively managed by grazing permits and rotational grazing systems. Management has focused primarily on the periodic removal of animals to try to maintain a balance between horse (and burro) populations, rangeland health, and other uses. Removed animals that aren’t adopted by private individuals are placed in long-term pastures in Midwestern states for the remainder of their lives. The BLM is investigating the logistics and efficacy of administering a contraceptive agent, porcine zona pellucida, to mares that are captured and released back onto the range.
Management challenges arise when values and perspectives of stakeholders concerning types of recreation and their impacts on the environment are very different. Federal and state agencies have attempted to manage OHV use through resource or travel management plans. Planning involves input and cooperation from interested stakeholders, as well as a variety of collaborative actions, including development and distribution of educational materials (signs, maps, brochures), recommendations for use limits and trail closures, volunteer patrols, and adopt-a-trail programs. Effective management is challenging because agencies typically do not receive enough funding for all planning actions, as well as for enforcement of regulations, repair of damaged resources and monitoring.