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COOPERATIVE EXTENSION Bringing the University to You Fact Sheet-02-12 Sagebrush Regions in Nevada: Climate and Topography influence Species Composition Brad Schultz, Extension Educator, Central Area Kent McAdoo, Natural Resource Specialist, Northeast Area Introduction Sagebrush (Artemisia spp.) is the primary shrub on most Nevada rangelands at elevations above 4,500 ft to 5,000 ft. Neil West identifies two distinct sagebrush regions: the sagebrush semi-desert and the sagebrush steppe. The same species of sagebrush occur in reach region, but important differences are present. These include different climatic patterns, different ratios of shrubs to grasses and forbs (nonwoody flowering plants) in the herbaceous (grasses and forbs) understory, and different amounts of total vegetation production each year. These differences result in each sagebrush type responding differently to disturbances. Managing sagebrush-grass rangelands in Nevada requires understanding how the sagebrush species and their associated shrubs, grasses, and forbs interact with each other, disturbances, climate, topography, and soils. The potential relationships are many and complex. Sagebrush semi-desert The sagebrush semi-desert covers about 44 million acres, mostly in Nevada (Figure 1). This area generally is north of the Nevada Test Site, and south of a diagonal line that stretches from Reno toward the northeast corner of Nevada. The sagebrush semi-desert is positioned between the saltdesert shrub vegetation type and the pinyon-juniper (Pinus monophylla/Juniperus spp.) woodland. These woodlands have expanded into many areas previously classified as sagebrush semi-desert. These include the drier mountain slopes, lower foothills, and gentle slopes above the valley bottoms (i.e. alluvial fans). The most common sagebrush species are Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), basin big sagebrush (A. t. ssp. tridentata), and black sagebrush (A. nova). Less abundant sagebrushes are mountain big sagebrush (A. t. ssp. vaseyana), low sagebrush (A. arbuscula), and bud sagebrush (A. spinescens). After a mature plant community has developed (20 to 30+ years after a fire), the sagebrush semi-desert typically has a higher ratio of sagebrush to herbaceous species than does the sagebrush steppe. Many shrub species occur in the sagebrush semi-desert; however, they are not major components of most mature sagebrush communities. Associated shrubs are green Mormon tea (Ephedra viridis), rubber rabbitbrush (Chrysothamnus nauseosus), horsebrush (Tetradymia glabrata), spiny hopsage (Grayia spinosa), Prickly gilia (Leptodactylon pungens), shadscale (Atriplex confertifolia), four-wing saltbush (Atriplex canescens), and winterfat (Krascheninnikovia lanata). Toward the sagebrush semidesert’s southern boundary blackbrush (Coleogyne ramosissima) and Nevada Mormon tea (Ephedra nevadensis) are common. Mexican cliffrose (Purshia neomexicana) is common toward the southern and eastern boundary, where more summer rainfall occurs. The cover, density, and/or biomass production (pounds of growth) of the associated shrubs is usually high only on recently disturbed sites, or where the soil type prevents the presence of sagebrush. Many species of perennial grasses occur in the sagebrush semi-desert. They seldom contribute a substantial amount of biomass in mature sagebrush communities, regardless of how well the area is managed. Most, but not all, are cool season species. These include Indian ricegrass (Achnatherum hymenoides), desert needlegrass (Achnatherum speciosum), squirreltail (Elymus elymoides), Sandberg’s bluegrass (Poa secunda), needle-and-thread (Hesperostipa comata). Thurber's needlegrass (Achnatherum thurberianum), bluebunch wheatgrass (Pseudoregneria spicatum) and mutton grass (Poa fendleriana) become more common in the northern part of the sagebrush semi-desert, and at high elevations (>6,000 ft). Great Basin wildrye (Leymus cinereus) typically is limited to landscape positions where runoff accumulates. Examples include ephemeral (short-lived) streams, meadows, and bottomlands. Common warm season grasses include galleta grass (Pleruaphis jamesii), sand dropseed (Sporobolus cryptandrus) and Fendler’s three-awn (Aristida purpurea). The warm season grasses have their highest abundance in the southern and eastern part of the region, where summer rainfall is more frequent. Some are found in west-central Nevada at lower elevations. At these sites, winter and spring precipitation often is sufficient to store substantial amounts of soil moisture. Adequate soil moisture, spring rainfall, and 1 warm spring temperatures coincide with one-another to promote growth of the warm season grasses. Forbs are common, but most species are annuals. These annual forbs are almost completely absent in dry years, but are abundant in wet years. They can provide important food for wildlife, but their high variability in forage production from year-to-year may limit some wildlife populations. Forbs in the sagebrush semi-desert dry by late spring and provide almost no summer forage. The shrubs and perennial grasses provide better continuity for forage from year-to-year. The total amount of plant biomass produced each year varies from about 450 lbs/ac to 1,350 lbs/ac. This is much less than the sagebrush steppe. Sagebrush in mature communities typically accounts for about 70% of the relative plant cover, and as much as 90% of the plant biomass. Absolute cover from the woody and herbaceous plants ranges from about 10% to 40%. The sagebrush semi-desert is significantly warmer and drier of the two sagebrush regions (Figures 3 and 4). Average annual precipitation ranges from as little as 6 in to over 12 in. Most precipitation occurs in the winter and early spring (November-April). The variation among years for both total annual precipitation and winter-spring precipitation is much higher in the sagebrush semi-desert, than for the sagebrush steppe. Sagebrush steppe The sagebrush steppe covers over 11 million acres in Nevada (Figure 2). In Nevada, it occurs in two distinct areas: one loosely defined by latitude and the other by elevation. North of the sagebrush semi-desert, the sagebrush steppe occupies the mountains, hills, plateaus and mesas above the valley bottoms, where the soil often has a high salt content. Farther south, it occupies the upper elevations of mountain ranges where a sagebrush zone occurs above the pinyonjuniper woodland. This situation resembles islands of sagebrush steppe at high elevations, surrounded by rings of PJ woodland at intermediate elevations, and vast expanses of sagebrush semi-desert on the lower mountain slopes, foothills, and valley slopes. The lower elevation limit for the sagebrush steppe in extreme northern, and northwestern Nevada is about 4,500 feet to 5,000 feet. In central and eastern Nevada it typically occurs in the mountains above 6,500 feet to 7,500 feet, depending on aspect. All of the sagebrush species found in the sagebrush semi-desert also occur in the sagebrush steppe. Wyoming sagebrush is most common at the lower elevations on droughty soils. Mountain sage is typical where annual precipitation is more than about 12 inches. Low sagebrush typically inhabits shallow soils with high clay content. Lahontan sagebrush (A. arbuscula ssp. longicaulis), a subspecies of low sagebrush, is restricted to northwestern Nevada near the historic shorelines of Lake Lahontan. Soils are typically shallow and have a clay layer (horizon) at shallow depths. Black sagebrush is most common in the high elevation varient of the sagebrush steppe, where soils are shallow and calcium content is high. These areas are most common in eastern Nevada. Less common sagebrush species include silver sagebrush (A. cana), alkali sagebrush (A. arbuscula var longiloba), and three-tip sagebrush (A. tripartita). The sagebrush steppe either has, or has the potential to have a high abundance of herbaceous species in mature sagebrush communities. Roughly equal amounts of shrubs and herbaceous species occur on undisturbed and/or properly managed sites. Also, herbaceous species are common in the interspaces between sagebrush and other shrubs. In contrast with the sagebrush semi-desert, the sagebrush steppe often has a high abundance of other shrubs in mature plant communities. These include bitterbrush (Purshia tridentata), snowberry (Symphoricarpos spp.), mountain mahogany (Cercocarpus ledifolius), Douglas rabbitbrush (Chrysothamnus viscidiflorus), currant (Ribes spp.), and serviceberry (Amelanchiar spp.). These species often occupy areas that accumulate drifting snow. Common, widespread grasses include bluebunch wheatgrass, beardless wheatgrass (Pseudoregneria inerme) Sandberg=s bluegrass, Canby bluegrass (Poa canbyi = Poa secunda), Thurber=s needlegrass, Idaho fescue (Festuca idahoensis), squirreltail, western needlegrass (Achnatherum occidentale), Columbia needlegrass (Achnatherum nelsonii), Letterman=s needlegrass (Achnatherum lettermanii), mutton grass, mountain brome (Bromus marginatus) and Cusick=s blue grass (Poa cusickii). Indian ricegrass is more limited in distribution here than in the sagebrush semi-desert, often occurring where soils are sandier. Great Basin wild rye has a broader distribution than in the sagebrush semi-desert. In addition to playas and bottomland locations with deep soils, it often inhabits slopes and swales up to about 9,000 ft in elevation, particularly where snowdrifts occur. These areas provide soil moisture well into the summer. Forbs are a much more common component of the sagebrush steppe. Many are perennial and provide forage for wildlife every year. During wetter years many remain green well into the summer or fall. Important perennial forbs include Arrowleaf balsamroot (Balsamorhiza sagittata), buckwheats (Eriogonum spp.), Penstemon (Penstemon spp.), lupines (Lupinus spp) wild onions (Allium spp.), desert parsley (Lomatium spp.), phlox (Phlox spp.), loco weeds (Astragalus spp.), daisy (Erigeron spp and Townsendia spp.), evening primrose (Oenothera spp.), rosy everlasting (Antennaria rosea), false dandelion (Agoseris glauca), tapertip hawksbeard (Crepis acuminatus), and waterleaf (Phacelia spp.). Aboveground biomass production ranges from about 700 lbs/acre to over 2,200 lbs/acre. The proportion of the annual biomass that comes from grasses and forbs is variable and depends on the growing potential of the specific site, the length of time from prior burning, and grazing history. Locations where fire has recently burned (e.g. 1-15 years before present) generally have more grasses and forbs, and fewer shrubs. Site potential depends on interactions among total and seasonal precipitation, redistribution of precipitation due to winds and topography, aspect (north, south, east, west), and the soil=s water holding capacity (a function of depth, texture, and gravel/rock content). 2 Role of Climate Climate, as reflected by average high temperatures and average precipitation (annual and monthly), appears to play an important role in determining whether the vegetation on sagebrush sites resembles the sagebrush steppe or the sagebrush semi-desert. The mean high temperature in the sagebrush steppe, from March through June (i.e., period of peak plant growth), is 5°F to 6°F cooler than in the sagebrush semi-desert (Figures 3 and 4). In the sagebrush steppe, significantly lower temperatures reduce the evaporative loss of water from shallow soil depths, increasing the amount of water available for plant growth. This benefits shallow rooted grasses and forbs. Monthly and annual precipitation patterns are significantly different in the sagebrush steppe and sagebrush semi-desert. Average annual precipitation in the sagebrush steppe is about 3.25 inches more than in the sagebrush semidesert. More important is the significantly greater precipitation almost every month, from November through June. This period is important because low temperatures stop almost all plant growth, virtually eliminating the removal of soil moisture by plants. Soil moisture can re-wet the entire soil profile, assuming precipitation is adequate. Based only on precipitation, for a given soil type, locations in the sagebrush steppe potentially can store significantly more water than similar soils in the sagebrush semi-desert. Substantially more monthly precipitation in the sagebrush steppe during the spring growing season provides additional water for shallow rooted grasses and forbs. Grasses and forbs in the sagebrush semi-desert receive less water. The additional spring moisture in the sagebrush steppe helps account for that area’s greater production of grasses and forbs. Topographic and Climatic Modifications to Soil Moisture The north-south mountain ranges throughout Nevada have many east-west trending canyons. This creates a very undulating landscape that influences soil development and amount of soil moisture at any specific location. Slopes with a north or east aspect receive less direct sunlight than slopes facing south and west. Less snow is lost to evaporation, and less soil moisture evaporates from the shallowest depths. Given an equal amount of precipitation and similar soils, locations with north and east aspects usually have more soil moisture than locations with south and west aspects. The total amount of soil moisture available, however, is modified by how the precipitation occurs (e.g., snow, rain) and the strength and direction of local winds during precipitation events. Rainfall on a given location either evaporates, infiltrates, ponds, or runs-off down gradient. Snowfall is often redistributed by winds, followed by melting and infiltration at a later time (often months). Throughout both sagebrush regions, most precipitation occurs during the winter and spring, often as snow. Most storms have strong winds from the southwest or west, which blow substantial amounts of snow off south and west facing slopes (particularly steep slopes or slopes with short vegetation), and increase snow accumulations on adjacent north and/or east facing slopes, and/or in nearby concave depressions. This partially explains why north-facing slopes can have abundant native bunchgrasses and forbs, and small amounts of sagebrush; and nearby south-facing slopes have mostly sagebrush, few perennial grasses and forbs, more bareground, and large amounts of cheatgrass. On the north and east facing slopes, more effective precipitation, less direct sunlight, and slower snowmelt increase soil moisture at shallow depths for a longer period. In both sagebrush regions, this process results in more diverse plant communities on north and east facing slopes, with proportionately greater amounts of perennial grasses and forbs. Species Interactions Prior to settlement in the mid to late 1800's, the primary disturbance in the sagebrush steppe was fire. Fire would remove all of the sagebrush for several years. Some of the other shrubs (e.g., rabbitbrush) can return quickly because they grow from sprouts at the root crown, but sagebrush must originate from seed. Healthy sagebrush produces large amounts of seed during most years, but the seed is short-lived. Seed banks are absent or small, and if present are largely destroyed by the fire, because most seed is in the plant litter directly below the mother plant. Sagebrush seed disperses only short distances (several to tens-of-feet) from mother plants; therefore, sagebrush can take many years to re-establish sagebrush. Forbs often establish in mass after a fire, for at least three reasons. First, many produce large amounts of seed. Second, the seed is often viable for several years or more. Third, many seeds from forbs have a hard seed coat that allows better survival during fires. Perennial grasses generally produce small to moderate amounts of seed per plant during wet years, and little or no seed during dry years. Also, most of their seed does not survive more than one-year; thus seedbanks are small or absent. The initial response from grasses is largely from existing plants that survived the fire. Existing plants provide seed, which if the amount and timing of precipitation is adequate, can result in many new grass plants. Perennial grasses increase less rapidly than forbs, but quicker than sagebrush and most other shrubs that re-establish from seed. With time, sagebrush increases. When sagebrush canopy cover reaches about 15%, the perennial grasses and forbs start to decline. Initially, grasses and forbs produce less biomass and seed per plant. The older grasses and forbs eventually die, but the combination of fewer viable seeds and strong competition from shrubs reduces or ends seedling recruitment. At sagebrush canopy cover levels of 30% to 40%, virtually all of the herbaceous species are lost from the community. If this state persists for many years the seedbank for most species is lost, sharply reducing their potential for rapid recovery after a fire. The near absence of perennial grasses and very low seed production per plant prevents their ability to rapidly recover. Decadent sagebrush sites are prone to invasion of introduced annual weeds with or without the presence of fire, or other intense disturbances. Once a few weeds are established, and desired perennial grasses and forbs are absent, large and/or intense disturbances accelerate the complete takeover of the site by the undesired annual weeds. 3 Livestock grazing can have substantial impacts on interactions between shrubs and herbaceous species. The population of any species that is grazed heavily and repeatedly during either its growth, and/or reproductive phases generally declines. Populations of species grazed during dormancy generally increase or remain static. Studies in both sagebrush regions have shown that grazing sagebrush communities in the fall when sagebrush is producing seeds, and the grasses and forbs are dormant, has decreased the amount of sagebrush and increased the amount of perennial grasses and forbs. For sagebrush, both plant density and average plant size (biomass production) can decline with fall grazing. The decrease in shrubs and associated increase in herbaceous species with fall grazing can occur faster than on adjacent ungrazed sites. Species and Landscape Interactions The ratio of woody to herbaceous vegetation at any point in time is a function of site potential (i.e., precipitation and soil quality); how precipitation may be redistributed; disturbance history (e.g., fire, drought, insects, wildlife use, flooding, erosion, and deposition); and the type, intensity, timing, and duration of land uses (e.g., livestock grazing, farming, mineral exploration, and recreation). These factors and processes interact to determine the current species composition at any location, and the suite of alternative vegetative states (i.e., species compositions) that could occur under different management actions. Within each sagebrush region, landscape locations with soils that store more water for longer periods will have more production of perennial herbaceous species. These sites may be more resilient to vegetation change induced by herbivores, in part because better soil moisture during and after the grazing period allows the grazed plants to restore both their leaf area and carbohydrates in their roots. Landscape locations with higher soil moisture levels are more likely to endure fire and other types of disturbances better. Bibliography Christensen, E. M. 1959. A comparative study of the climates of mountain brush, pinyon-juniper, and sagebrush communities in Utah. Proceedings Utah Academy of Science 36:174-175. Laycock, W. A. 1967. How heavy grazing and protection affect sagebrush-grass ranges. Journal of Range Management 20:206-213. Perryman, B. L., A. M. Maier, A. L. Hild, and R. Olson. 2001. Demographic characteristics of 3 Artemisia tridentata Nutt subspecies. Journal of Range Management 54:160-170. Ludwig, J., D. Tongway, D. Freudenberger, J. Noble, and K. Hodgkinson (editors)ndscape Ecology. Function and Management. Principles from Australia’s Rangelands. Lyneham Canberra, Australia. 158 pages. Perryman, B. L., A. M. Maier, A. L. Hild, and R. Olson. 2001. Demographic characteristics of 3 Artemisia tridentata Nutt subspecies. Journal of Range Management 54:160170. Robertson, J. H. 1947. Responses of range grasses to different intensities of competition with sagebrush (Artemisia tridentata Nutt). Ecology 28:1-16. West, N.E. 1983. Great Basin-Colorado Plateau Sagebrush Semi-Desert. Pages 331-349. In: N. E. West (ed). Ecosystems of the World 5. Temperate Deserts and Semi-Deserts. Elsevier Scientific Publishing Company. New York, New York. 522 pp. West, N. E. 1983. Western Intermountain Sagebrush Steppe. Pages 351-374. In: N. E. West (ed). Ecosystems of the World 5. Temperate Deserts and Semi-Deserts. Elsevier Scientific Publishing Company. New York, New York. 522 pp West, N. E., K. H. Rea, and R. O. Harniss. 1979. Plant demographic studies in sagebrush-grass communities of southeastern Idaho. Ecology 60:376-388. The University of Nevada, Reno is an Equal Opportunity/ Affirmative Action employer and does not discriminate on the basis of race, color, religion, sex, age, creed, national origin, veteran status, physical or mental disability, or sexual orientation in any program activity it operates. The University of Nevada employs only United States citizens and aliens lawfully authorized to work in the United States. 4 Figure 1. Area of sagebrush semi-desert. From West 1983a. Figure 2. Area of sagebrush steppe. From West 1983b. 2 MEAN MONTHLY AND ANNUAL HIGH TEMPERATURE 100 90 TEMPERATURE (F°) 80 70 60 50 40 30 20 10 0 JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC ANNUAL MONTH Sagebrush Steppe Sagebrush Semi-Desert Figure 3. Comparison of mean monthly and mean annual high temperatures at nine locations in both the sagebrush steppe and sagebrush semi-desert MEAN MONTHLY AND ANNUAL PRECIPITATION PRECIPITATION (in) 14 12 10 8 6 4 2 L N U A A N D E C V N O C T O P T S E G A U JU L N JU A Y M A P R M A R E B F JA N 0 MONTH Sagebrush Steppe Sagebrush Semi-Desert Figure 4. Mean monthly and annual precipitation at nine locations in both the sagebrush steppe and sagebrush semi-desert.