Sagebrush-Steppe Ecosystem Ecology
Introduction
When you think of a “Sagebrush Steppe Ecosystem” what do you think of? Do you think of sagebrush? Do you think of endless miles/acres of nothing but gray? Do you think of desert? Do you think of rangeland? Do you think of some of the wildlife observed there – pronghorn, sage grouse, jackrabbits? Do you think of vast, open space with distant mountains in the background? Do you think of public land – BLM, Idaho Endowment Land, Forest Service? Do you think of recreational opportunities – off-road vehicle use, hunting? Do you think of a wasteland in need of management for more productive uses? Writers tell us that these descriptors, and even more, were used by Europeans pioneers as they trekked across the sagebrush steppe, and then began to settle and eke out livelihoods in the deserts of western United States. However, sagebrush steppe ecosystems are much more than any of these or even all of them combined.
When we talk about an “Ecosystem,” we are talking about more than just the plants or wildlife that occur on an area. We are talking about a “system” where all of the living organisms are interacting with and sustaining one another in conjunction with the soil and climate. As with all systems – mechanical, political, chemical, etc., there is a continuous flux of the different parts as they strive to maintain equilibrium. Ecosystems are not static – they are continually changing in both time and space. Ecosystem changes are in response to both endogenous (factors occurring within the system) and exogenous (factors from outside the system) factors.
There is much variability in the soils, climate, elevation, species, and production across the range where the sagebrush steppe occurs. There is also much variability in species, climate, and production over time. Therefore, although we frequently refer to the sagebrush steppe as an “ecosystem,” it is composed of many different ecosystems. This is certainly true within the scale that we usually evaluate and manage sagebrush steppe. Thus, when we manage a sagebrush-grass area we need to understand both the inherent characteristics of that particular site and also the characteristics of that particular site at the present time.
In this class, we are going to learn that the sagebrush steppe of western United States is much different today than it was when the pioneers trekked across it. We are going to learn that the ecosystem itself is functioning much different today than it did prior to European settlement. And we are going to learn that humans have played a major role in these changes. We are also going to learn that the future of the sagebrush steppe of western United States is dependent upon us – not only our understanding of the workings of the ecosystem, but the appreciation that we have for it as a society.
Distribution
The distribution or range of the sagebrush steppe ecosystem depends on climate and has fluctuated over the past 30 millennia. In general, big sagebrush favors areas that receive the majority of the precipitation during winter months and the least during summer months. Annual precipitation amounts range from about 6 in. to about 22 in. Soils are quite variable and include the moderate to deep loess’s, alluvial, lacustrine, and sedimentary deposits, and volcanic deposits of basalt, andesite, rhyolite, and tuff. Topography ranges from nearly flat to gently rolling to relatively steep.
The sagebrush steppe ecosystem extends over much of Utah, Nevada, southern Idaho, eastern Oregon, western Montana, Wyoming, and Colorado, as well as smaller disjunct areas of eastern Montana, southern British Columbia, southern Alberta, southwestern North Dakota, western South Dakota, Washington, California, Arizona, and New Mexico. Estimates of total acreage vary from some 95 million acres to 270 million acres. Even if the lower estimate is accepted as reasonably accurate, sagebrush-grass vegetation is still one of the largest, if not the largest, range ecosystem in the western United States. However, it is estimated that the area occupied has been reduced by 40-50 percent since pre-European settlement and less than 10 percent remains in an unaltered condition today. Millions of acres have been converted to agriculture, cities, roads, transmission lines, energy developments, exotic plants, and woodlands. More will be said about the management of the sagebrush steppe in the next class.
Sagebrush − Soil Relations
Soil type considerations arevery important in the distribution of sagebrush taxa. Althoughthere are many exceptions, the general distribution of sagebrush isrelated to soil moisture, temperature, depth, and parent material. Some observations regarding soil relations inthe Great Basin are summarized in the followingparagraphs.
In general, Basin Big Sagebrushtends to occupythe deep soils with minimal development in the low to moderateprecipitation zone, whereas Wyoming Big Sagebrushoccupies soils ofmoderate depth. As moisture conditions and temperatures improve, and elevation increases,Mountain Big Sagebrushdominates until itgives way to Sub-alpine Sagebrushon deep soils at high elevations.The position of tall Three-tip Sagebrushalong themoisture gradient overlaps the upper portion of Wyoming big sagebrushand the lower portion of mountain big sagebrush. Apparently,it is not associated with a particular kind of soil.
On shallow soils, Dwarf Sagebrush species replace the tallspecies. Black Sagebrushoccupies the lower position along themoisture gradient and is restricted to limestone-derived soils inthe drier areas. Where Low Sagebrushis associated with Black sagebrush,the former consistently occurs in the cooler and higher moisturesituations. Low sagebrushalso occurs on shallow, non-calcareoussoils with strongly developed clay-pans in southwesternIdaho and eastern Oregon. These soils are supersaturated duringthe spring, but during the summer the plants are under considerablemoisture stress.Alkali sagebrushoccurs in habitats similar to those thatsupport low sagebrushon shallow soils with clay-pans. Sometimesboth species are found in the same stand. Fringed sagebrushalsooccurs on similar habitats, but on the more shallow and rockyportions.
Sagebrush Steppe Plant Communities
Native sagebrush-grass vegetation is dominated by woody species of Artemisia with an understory of perennial grasses and forbs. Vegetal cover is usually not continuous and considerable bare ground is often exposed. Sagebrush generally occurs at elevations from 5,000 to 7,000 ft., but some species grow at elevations as low as 800 ft. and others as high as 11,500 ft. On some soils and under some moisture conditions the sagebrush over-story is co-dominant with other species of woody plants.
The native sagebrush steppe ecosystem was dominated by an over-story of woody sagebrush (Artemesia) and an understory of perennial grasses [primarily wheatgrasses, fescues (Festuca), bluegrasses (Poa), and wildryes] and perennial forbs. Other important shrubs include rabbitbrush (Chrysothamnus), bitterbrush (Purshia), horsebrush (Tetradymia), chokecherry (Prunus), serviceberry (Amelanchier), hopsage (Grayia), Mormon tea (Ephedra), wild currant (Ribes), mountain mahogany (Cercocarpus), and snowberry (Symphoricarpos). Forbs were present in a much greater variety than grasses, but their distribution was much less uniform. However, one species, arrowleaf balsamroot (Balsamorhiza sagittata), was very widespread and often abundant. Other common forbs present in varying quantities were yarrow (Achilles), pussytoes (Antennaria), locoweed (Astragalus), segolily (Calochortus), hawksbeard (Crepis), larkspur (Delphinium), daisy (Erigeron), buckwheat (Eriogonum), biscuitroot(Lomatium), lupines (Lupinus), foxglove (Penstemon), phlox (Phlox), groundsels (Senecio), violet (Viola), mulesears (Wyethia), and deathcamas (Zigadenus). These forbs are highly variable in characteristics, ranging from mat-formers such as Phloxhoodii and Antennaria to tall, coarse plants such as Balsamorhiza and Lupinusleucophyllus. Root systems vary from stout, deep taproots in the latter two species to spreading, rhizomatous systems in others.
Today we find an understory with fewer perennial grasses and forbs and increasingly dominated by non-native annual grasses and forbs. This is especially true for the Great Basin area, where cheatgrass (Bromus tectorum) oftendominates the understory as a monoculture, and to a lesser extent for the intermountain and Wyoming Basin areas.
We typically classify sagebrush steppe by the dominant species of big sagebrush e.g. basin big sagebrush, Wyoming big sagebrush, or mountain big sagebrush present. However, although big sagebrush species have the widest distribution, there are smaller acreages dominated by the “low” sagebrushes. Also, there are usually multiple sagebrush species occurring together in the same area.
Common Sagebrush Communities of Idaho
Big sagebrush(Artemisia tridentata)is an erect, aromatic, evergreen species, which lacks winter buds. It has numerous ecotypes that range in size from 1 ft. to 5 ft. tall. Growth forms are of two types: multi-stemmed and single trunks and are correlated with sub-specific taxonomy. This species produces two types of branches, vegetative and flowering. The bark on older vegetative branches is stringy and black or dark brown in color. Bark on younger vegetative and inflorescences is heavily covered with trichomes that give the branches a silvery green to gray color. The leaves are also silvery green or gray.
There are three widely recognized subspecies of big sagebrush. Subspecies can be separated on morphological, chemical, and ecological characteristics. Wyoming big sagebrush occupies the drier sites that range from 8 to 13 in. of precipitation.Next is basin big sagebrush with mountain big sagebrush is the wettest areas with 12 to 60 in. of precipitation. Some authorities have divided mountain big sagebrush into three forms, varieties, or subspecies depending on the authority being quoted. One is called subalpine big sagebrush, A. t. ssp. spiciformis, or A. t. ssp. vaseyana f. spiciformis. The second is referred to as “X” big sagebrush, or xeric big sagebrush or A. t. ssp. xericensis. Lastly, a variety of mountain big sagebrush is called A. t. ssp. vaseyana var. pauciflora.
Big sagebrush grows in a variety of soils throughout its range: arid plains, valleys, foothills, to mountain slopes, from 1,500 to over 10,000 ft. elevation. It can be found growing on all 12 soil textural classes, but it is most often found on loams or sandy loams. Two soil characteristics are common: they are well-drained and contain low concentrations of salts.
Depending on subspecies, as many as 40 species of grasses and forbs can grow with big sagebrush. Big sagebrush is not fire tolerant and can be killed by a number of pathogenic fungi, insects, and environmental conditions such as winter kill or winter induced drought. Seed dispersal is highly variable among stands and somewhat dependent on elevation; some starting in mid-fall, others not until early winter. Most seeds germinate immediately after snow melt or after late-winter or early spring storms. The seeds possess no special adaptations for wind dispersal, but are wind dispersed to a maximum distance of about 100 ft. from the mother plant under most environmental conditions. Hence seedlings placement is heavily depended on wind direction from the seed source. Big sagebrush forms a weak soil seed bank. A single mature big sagebrush plant can produced upward of 500,000 seeds, although, excessive browsing can reduce inflorescences production by a factor of 20. The seeds must lie on the soil surface or be buried not more than 0.25 in. for successful emergence.
Basin big sagebrush(A. t. ssp. tridentata) is normally an erect shrub 3 to 6 ft. tall with numerous spreading branches giving it a “ragged-top” appearance, it can reach a height of 15 ft. and trunk diameter of 10 in. when growing in deep well drained, sandy soils of sheltered areas. Vegetative branches are numerous. The whole plant is light gray-green in appearance because of the appressed pubescence. The location of flowering stalks is variable – some occurring above the vegetative branches and some occurring within the vegetative branches. The young bark is light brown or silvery but often becoming black with age and shredding. Basin big sagebrush is easily killed by fire. It exhibits a preference for deep, well-drained soils in valley bottoms and lower foothill areas at elevations of 5,000 to 7,000 ft. This preference makes it a good indicator of arable land. Much of the area once occupied by basin big sagebrush is now under cultivation.
Basin big sagebrush is the least palatable of the big sagebrush species for both livestock and wildlife. The shrub may supply some shelter, especially in severe winters, and it may serve as emergency winter feed because of its height, but the plants are not normally sought out by any type of large herbivore.
Most basin big sagebrush sites that have not been heavily grazed or disturbed support a diverse productive herbaceous understory. However, if the understory is weakened by heavy grazing and the sagebrush canopy becomes closed, these sites become unproductive until management is implemented to reduce the canopy cover and reduce grazing pressure.
Wyoming big sagebrush(A. t. ssp. Wyomingensis)is seldom over 3 ft. tall, branches from the base have a twisted and irregular growth form. Flower stalks generally do not extend above the vegetative branches giving the appearance of rounded top. It tends to have a rounded “cushion-like” appearance and is the dominant big sagebrush species of the Great Basin and Wyoming Basin. Wyoming big sagebrush has grayer and somewhat less fan-shaped leaves than black sagebrush and the seed stalks are less persistent. It tends to produce more lateral roots in the upper soil horizons that other big sagebrush taxa.
Wyoming big sagebrush is common on dry, warmer soils at elevations of 5,000 to 7,000 ft. When Wyoming big sagebrush and black sagebrush occur in intermixed stands, Wyoming big sagebrush occupies deeper, much dryier soil with lower fertility than the soils of other big sagebrush taxa. Therefore, understory herbaceous vegetation is less diverse and productive than under either basin big sagebrush or mountain big sagebrush. On the lower part of the Great Basin, where soils are warmer, exotic annual herbaceous vegetation (primarily cheat grass) becomes the dominant understory with excessive livestock grazing and/or disturbance.
Wyoming big sagebrush is palatable to both livestock and big game, especially in winter, and in areas where livestock or big game winter in stands of Wyoming big sagebrush, the plants are heavily browsed. In most instances this plant supplies enough browse, on relatively poor soils, that it should be encouraged to grow rather than discouraged. However, Wyoming big sagebrush is very susceptible to fire.
Mountain big sagebrush(A. t. ssp. vaseyana) is a uniformally growing, flat-topped shrub up to 3.5 ft. tall with a trunk up to 4 in. in diameter – and some plants may be more than 100 years old. Branches spread at the base and frequently layer. Vegetative branches are usually less than 3.5 ft. high, numerous and 12 to 20 in. long. The flowering stems are erect and extend above the vegetative branches.
Mountain big sagebrush is the predominant sagebrush type in the central Rocky Mountains, forming extensive stands of uniform size at elevations of 6,000 to 10,000 ft. It also occurs on well-watered swales and pockets of deep soil at lower elevations. Soils are deep, loose, and moist, with numerous angular rocks.
Mountain big sagebrush communities are the most productive of all the big sagebrush sites. Herbaceous understory is diverse with numerous perennial forbs and high grass production. These communities have more potential for increasing canopy cover than any other sagebrush species. Because it occupies deep, well-watered soils, grass production can be increased by reducing sagebrush canopy cover to below 25%. Therefore, depending upon the management objective, it may be desirable to reduce the canopy cover of mountain big sagebrush periodically.
Like basin big sagebrush, mountain big sagebrush is of little forage value to either livestock or big game except under the most severe winter conditions. However, it often occurs intermixed with stands of bitterbrush which is a highly sought and nutritious browse species by big game. When mountain big sagebrush occurs in common with bitterbrush it provides cover and diversifies the diet of big game. In locations where snow accumulation is heavy it may become unreachable for big game during winter. In much of southern Idaho mountain big sagebrush communities are important late brood rearing habitat for sage grouse.
Threetip sagebrush (A. tripartita) is a round, evergreen shrub up to 3.5 ft. tall. It may have a simple, trunk-like stem or several, lesser stems arising from the base. The bark on young branches is canescent (wooly) but becomes shredded and grayish, light brown to dark brown or black, on older stems. This species can layer and resprout after fire or herbicide treatment. Leaves on vegetative branches are also canescent, and typically deeply divided into three linear or narrowly linear lanceolate lobes, which in turn may be three-cleft.