Get reliable rangeland science

By Beth Burritt, Utah State University

• Are We Losing Our Aspen?
• Fire Ecology in Aspen Forests
• Forage: Livestock and Wildlife
• Placing Value on Aspen Ecosystems
• Aspen Management Options
• Further Reading

Before we dive into aspen ecology and issues, it is important that readers understand two key concepts. First, aspen reproduce both asexually via root suckers and sexually from seed germination. Thus, large groups of aspen trees may be genetically identical clones, many of them still attached by underground root networks. Conventional management practices rely heavily on strong suckering responses following burning or cutting. Second, aspen come in two primary forms: seral (meaning they are relatively short-lived and eventually overtopped by competing conifers) and stable (not competing with conifers; long-term growth in pure or nearly pure stands of aspen). Understanding these different aspen "functional types" is crucial to restorative efforts, otherwise well-intended actions may lead to aspen loss. Photo Credit: WanderingtheWorld (www.ChrisFord.com) / Foter / CC BY-NC

Are we Losing our Aspen? Aspen "decline" comes in two flavors: long-term decline related to conifer encroachment and short-term decline (a.k.a., Sudden Aspen Decline or SAD) initiated by drought. For decades we have been hearing about great losses in aspen coverage in the western states due to advancing conifer succession. Some of these changes were caused by past management practices, such as fire suppression and active removal of aspen or management for higher value conifer timber species. More recent evidence, however, suggests that a regionally moist 20th century generally kept fire events limited, allowing conifer expansion and aspen decline. In other locations, aspen actually expanded its range so we must be careful not to oversimplify regional patterns.  This interaction with conifers does not explain losses in stable (nearly pure) aspen forests.  Thus, we must understand at least two aspen "types" which depend on different ecological mechanisms to persist.

SAD is defined as the rapid die-off of both overstory canopy trees and supporting root systems where the end result is the death of complete aspen clones.  Strong evidence has been presented that this type of rapid die-off has occurred in southwest Colorado, but corroborating support is limited elsewhere.  There has been much broader patterns of mature tree mortality across the West and in southern Canada associated with the 2001-2004 drought.  In many instances, combined effects of overstory drought-induced die-off and intense herbivory of young aspen by browsers has led to "de facto SAD" where we witness complete aspen community collapse. Photo by Phil Kemp, US Forest Service, Mancos-Dolores District.
 

Fire Ecology in Aspen Forests. Aspen has conventionally been thought of as "fire dependent," meaning that it requires forest fires to thrive. The quick-sprouting root system of an aspen clone  rapidly regenerates after all types of disturbance (i.e., landslides/avalanches, insects, disease, drought, tree harvest) including burning. Moreover, recent discoveries of high genetic diversity in aspen communities and common occurrences of seedling (sexual reproduction) establishment following fire is leading practitioners to question traditional aspen management. Fire suppression during recent decades is thought to be partially responsible for long-term aspen decline, however several experts have questioned this assertion. Likely, there are several causes for the lack of fire, most notably long periods of climatic moisture that increased the number of conifer trees in some aspen forests over the past century. Stable (nearly pure) aspen is much less conducive to wildfire or prescribed burning; rejuvenation in these forests is dependent on more continuous, low-level, tree mortality and regeneration.

Use of fire for restoration is a viable means of reducing conifers and promoting aspen suckers and seedlings in seral forests. Where disruption of fire cycles due to past fire suppression is evident, using fire to restore aspen is recommended.  Some managers favor a combination of harvest and burning, particularly where the possibility of escaped fires can damage property. Photo by Bob Campbell, US Forest Service.

Aspen as Forage for Livestock and Wildlife. Quaking aspen are widely revered by range and wildlife managers alike for their diverse and nourishing array of understory plants. Young aspen sprouts are particularly favored by deer, elk, cattle, and sheep for their highly nutritious leaves and twigs. During the late summer and early autumn aspen may be the only green, nutritious, component of forests available as forage. Fortunately, aspen leaves contain defense compounds (phenolic glycosides and condensed tannins) that may deter herbivores, at least partly, from devouring young suckers.  However, with high animal concentrations—often a mix of wild and domestic herbivores on the same landscape—defense mechanisms may not sufficiently deter browsing on regenerating aspen suckers. If these consumption patterns persist over many years aspen forests begin to lose their age and structural complexity.  This situation seems particularly serious is stable (nearly pure) aspen forests where uncommon large disturbances offer little hope of broad forest rejuvenation.

Ongoing monitoring activities are attempting to document where livestock and wildlife browsing is threatening future aspen trees. In some cases, there is clear evidence of aspen collapse related to intensive browsing. The presence of large carnivores, such as wolves, grizzly bears, and cougars, may deter aspen browsing by keeping livestock and wildlife on the move. Domestic livestock may be more easily herded to prevent over browsing aspen. Range and wildlife managers are now working collaboratively to curtail aspen recruitment failure while continuing to sustainably utilize forage resources. Cooperative research and monitoring, as well as prudent use of fire, tree harvest, and post-treatment protection, will be required to overcome the widespread lack of aspen recruitment. Photo by Teresa Prendusi

Placing a value on Aspen. We often hear about the diverse values of aspen forests, but what evidence is there to support such assertions? Much of the "value" of aspen lies in its apparently innate beauty; a rich gold or fluttering green among the sea of conifer and sagebrush. Utah recently recognized aspen as its State Tree largely on the basis of esthetic appeal. Digging a little deeper, however, we can tap into, perhaps even monetize, other aspects of aspen worth. For example, aspen is among the most biodiverse vegetation types in the Intermountain West. Additionally, range managers are well aware of the high value forage resident among aspen groves. A diverse array of wildlife utilize aspen for cover, food, habitat, and water. There is some evidence that healthy aspen communities facilitate water conservation. Thriving beaver colonies create migrating water retention ponds, as well as raising water tables and increasing wildlife habitat. Aspen is valuable for a variety of wood products, including flooring, mine supports, particleboard, pet bedding, excelsior (wood fiber packaging and cooling devices), and ski/snowboard manufacturing. Aspen forests make for cherished camping and recreation destinations, popular ski resorts, and general tourism promotion. Finally, aspen may be used as a fire break via selective cutting or light burning around developed areas.What is the value of aspen?  Ask any westerner if they could go without it. License: Creative Commons 2.0 (by-nc-sa)
 

Aspen Management Options. Conventional treatments to stimulate aspen regeneration are widely available (e.g., Shepperd et al. 2008), although we now know that a strong sprouting response is not enough to protect from post-treatment browsers. In order to sustain aspen on the landscape land stewards are adopting "resilience management" strategies. In essence, this entails preserving options. Where fire was historically an important regenerative tool in seral aspen, judicious use of selective harvest practices and prescribed and wildlife fire management may be invoked. A key component of resilience management means understanding ecological function: not all aspen stands are alike in terms of their ecology and earlier human impacts. Treating aspen as a "one-size-fits-all" prospect may cause irreversible damage.  Thus, locally-based knowledge, current science, ongoing monitoring, course correction (where necessary), and institutional support—all key elements of "adaptive management"—provide a recipe for resilience. The complexity of resilience management will require multiple perspectives if they are to be successful.

These approaches will become particularly important with the advent of climate warming in the West. Changing climates are expected to bring two opposing elements into the resilience equation for aspen: 1) warming temperature may cause decreases in suitable aspen habitat as these communities move upslope, and 2) expected increases in fire frequencies and extents may provide opportunities (at least temporarily) for aspen expansion or recolonization. Photo Credit USFS

Basic Rangeland Types

The United States has 15 basic rangeland types that are economically important from the standpoint of forage production and total area.

• Alpine Tundra
• Califonia Annual Grassland
• Cold Desert (sagebrush steppe and salt desert)
• Eastern Deciduous Forest
• Hot Desert
• Mountain Browse
• Northern Mixed Prairie
• Oak Woodland
• Palouse Prairie
• Pinyon-Juniper Woodland
• Shortgrass Prairie
• Southern Mixed Prairie
• Southern Pine Forest
• Tallgrass Prairie
• Western Coniferous Forest

Adapted from: Holecheck, J.L., R.D. Pieper, and C.H. Herbel. 1998. Range Management Principles and Practices, Third Edition.

Alpine Tundra 

The highest range type in altitude is the alpine tundra which occupies those mountain areas above timberline that are characterized by short, cool growing seasons and long, cold winters. Low-growing, perennial, herbaceous, shrubby vascular plants dominate the vegetation along with cryptograms such as mosses (Selaginella sp.) and lichens (Cladonia sp.). Permafrost prevents the growth of trees. Alaska hosts the most alpine tundra in the United States, followed by Colorado, Washington, Montana, California, Oregon, Idaho, Utah, New Mexico, Arizona, and Wyoming. The 1,000 mm to 1,500 mm of precipitation occurs mainly as snow, and the land is constantly buffeted by winds and cold temperatures. Even during the growing season, air temperatures are often at or near freezing. These harsh conditions are very stressful to plants, and the flora is limited to 200 to 300 species that commonly occur in alpine tundra. Members of the bluegrass (Poaceae), sedge (Cyperaceae), saxifrage (Saxifragaceae), rose (Rosaceae), mustard (Brassicaceae), buckwheat (Polygonaceae), and pink (Caryophyllaceae) families have wide alpine distribution along with shrub species of the willow (Salicaceae) and heath (Ericaceae) families. Much of the alpine tundra has been designated as wilderness areas because of aesthetics, remoteness, and its importance as a water source in the western United States. Comparatively, livestock grazing ranks low in importance and is generally limited to sheep that are herded to minimize excessive use of rare, flat areas.

California Annual Grasslands 

California annual grasslands are found primarily west of the Sierra Nevada Mountains. The climate of the region is Mediterranean, characterized by mild, wet winters and long, hot, dry summers. Rainfall varies from about 200 mm in the southern foothills to almost 1,000 mm in some areas near the coast. Most of the precipitation comes between October and May, with almost no precipitation during the summer months. Summers are hot and long, with an average frost-free period of 200 to 260 days. Soils vary across the region with only the less fertile areas remaining as rangeland.

The California annual grassland has one of the longest livestock grazing histories of the western range types, dating back to the Spanish settlements of the seventeenth century. Under pristine conditions, the California grasslands were quite beautiful, but they have suffered severe degradation since the arrival of European settlers. The original vegetation of the California annual grasslands was comprised of cool-season bunchgrasses; however, numerous fires and overgrazing from a very early date probably account for the change from perennial to annual grasses. Presently, less than 5 percent of the native perennial grasses remain. The cool-season annuals that replaced the perennial grasses have nearly ideal environmental conditions in California. This is because they have adequate moisture and temperature for growth and reproduction in winter while their seeds remain dormant during the dry summer period. Today, the vegetation is dominated by slender oat (Avena barbata), wild oat (Avena fatua), soft brome (Bromus mollis), ripgut brome (Bromus rigidus), foxtail brome (Bromus rubens), and little barley (Hordeum pusillum).

Because the grasslands of California are dominated by annuals, they have some unique problems. Generally, they are not very responsive to grazing intensity, although heavy grazing does cause erosion problems, some change in species composition, and reduced forage production. Season-long grazing has been superior to specialized systems. These ranges should be grazed lightly during plant germination and establishment in the fall, but grazing intensity can be increased substantially in the early spring. Primary problems in this type of rangeland include inadequate forage quantity in the fall and winter and inadequate forage quality in the summer. Forage quantity fluctuates drastically from year to year, necessitating operators to be able adjust their stocking rates rapidly or have a good reserve of harvested forage.

Cold Desert 

The cold desert type, often referred to as the Great Basin, is comprised of two distinct vegetation communities, the sagebrush grassland and the salt desert. These two rangeland types intermingle with each other over vast portions of the intermountain United States (the area between the Cascade Mountains and the Rocky Mountains). Differences in vegetation and managerial components between the two cold deserts warrant a separate discussion of each.

Sagebrush grassland

Encompassing nearly 39 million hectares, the sagebrush grassland is one of the most extensive of the western range types. It covers parts of Oregon, Idaho, Nevada, Utah, Montana, Washington, and Wyoming, with about 65% of the land mass in federal control and 35% in private ownership. Compared to the salt desert, sagebrush grassland has higher precipitation and less saline soils and typically occurs at the higher elevations of the cold desert. Precipitation ranges between 200 mm and 500 mm. Summers are dry in the northern half of this range type, and most of the precipitation comes in the late fall, winter, and early spring as snow. In the southern part, most of the precipitation also comes in the winter, but slightly more precipitation comes in the summer. At higher elevations the growing season is often no more than 100 days. Soils are primarily volcanic material of the order Aridisols. Depth varies from shallow to deep, and topography is highly variable.

The dominant shrub of the region is big sagebrush (Artemisia tridentata). Important grasses of the sagebrush grassland are bluebunch wheatgrass (Pseudoregneria spicata), bottlebrush squirreltail, (Elymus elymoides), Idaho fescue (Festuca idahoensis), western wheatgrass (Pascopyrum smithii), Indian ricegrass (Oryzopsis hymenoides), needle-and-thread (Hesperostipa comata ), and basin wildrye (Leymus cinereus). Bluebunch wheatgrass dominates the understory in the northern half, with western wheatgrass dominating the understory in the southern half if grazing has not been abusive. Abusive grazing replaces the bunchgrasses with cheatgrass (Bromus tectorum) and increases sagebrush density.

Other important shrubs are rabbitbrush (Chrysothanmnus sp.) and antelope bitterbrush (Purshia tridentata). Sagebrush grasslands generally recover very slowly from overgrazing. Control of invasive and shrubby species can speed recovery.

Salt desert

The salt desert shrubland inhabits the lower areas of the Great Basin, where drainage is often restricted and the water table is high. This region occurs primarily in the states of Utah and Nevada as a mosaic with sagebrush grassland. The salt desert shrubland is one of the least productive of the range types because of climate and soils. Precipitation in the region ranges from 80 mm to 250 mm, the lowest precipitation of all types except the Mojave Desert. The xeric climate is exacerbated by the high salt content of the soil, produced when evaporation causes salts to accumulate at the soil surface. Generally, summers are quite dry with one-half of the precipitation falling as snow in winter and the remainder as rain in the spring or fall. Precipitation is highly variable from year to year, and there is very little vegetation growth in the dry years. Despite the cold winters and hot summers, the growing season is approximately 200 days. Soils are primarily Aridisols with varying degrees of alkalinity and salinity. Vegetation is characterized by a few species of low, spiny, grayish, and widely spaced microphyllous (small-leaved) shrubs in the Chenopodiaceae and Asteraceae families. Shadscale saltbrush (Atriplex confertifolia) dominates the area, with winterfat (Krascheninnikovia lanata) being the second most common shrub. Vegetation canopy cover is usually less than 10%. Much of the salt desert has historically been severely overgrazed, and vegetation recovery is slow but can be accomplished with moderate stocking rates. Traditionally, the range type was used as winter range for sheep which summer in the surrounding mountains. However, control of undesirable plants has not been economically feasible, enabling halogeton (Halogeton glomeratus), a poisonous forb to become dominant in much of the salt desert shrub.

Eastern Deciduous Forest 

Although much of the eastern deciduous forest has been heavily modified by farming, logging, and industrialization, it is becoming increasingly important for livestock production. Many of these areas are managed using ecological rather than agronomic principles. The growing season lasts from 120 to 240 days, with snow and frost common in the winter. The 800 millimeter (mm) to 2,000 mm annual precipitation is uniformly distributed throughout the year. Soils of the order Alfisol support deciduous forest trees, including maples (Acer sp.), birches (Betula sp.), oak (Quercus sp.), hickories (Carya sp.), beeches (Fagus sp.), and basswood (Tilia sp.). Grasses such as bluestem (Andropogon sp., fescue (Festuca sp.), timothy (Phleum sp.), bluegrass (Poa sp.), brome (Bromus sp.), ryegrass (Lolium sp.), and orchardgrass (Dactylis sp.) thrive in open canopy areas where trees have been thinned or completely removed. Nitrogen and phosphorus fertilization can further boost forage production in the low fertility soils.

Hot Desert 

The hot desert is one of the largest western rangeland types, yet compared to the other rangeland types, it is relatively unimportant from the standpoint of livestock production. This type is found in southern California, southern Nevada, Arizona, New Mexico, southwestern Texas, and northern Mexico. Elevations range from 925 m to 1,400 m. Precipitation in the desert climate varies from 130 mm to 500 mm and increases with elevation above sea level. Precipitation occurs primarily during winter, peaking in January and during summer with the wettest months being July, August, and September. May and June are extremely dry. Summer rains occur as convection storms because of solar heating. The frost-free period is generally over 200 days during the year, and it is not uncommon for areas to go two to three years without a killing frost. The Mojave Desert, Sonoran Desert, and Chihuahuan Desert occur in this type.

Vegetation of the hot desert type did not evolve with grazing by large herbivores; however, this area was one of the first types in the United States to receive grazing by domestic livestock. Severe overuse and lack of adaptation to grazing probably explain the large-scale decline of black grama and other climax grasses in the past 100 years. Almost all the plants found in the hot desert type are warm-season species. The warm, sunny climate of the Mojave and Sonoran deserts has attracted large numbers of people to these desert areas. Currently, tourism, wildlife, water, and recreation are the most important products of this rangeland type. Livestock grazing is still an important rangeland use in the Chihuahuan desert; however, large tracts of the Sonoran Desert are being converted into housing and other urban uses in Arizona. Management of livestock focuses on water development to improve livestock distribution and carefully adjusting stocking rates to forage availability. Recovery from overgrazing in these arid regions is often slow to non-existent.

Mountain Browse 

The mountain browse range type occurs primarily in the Rocky Mountains and Sierra-Cascade Mountains of the western United States and is most prevalent in Colorado, Utah, Oregon, and Idaho. The climate is intermediate between that favoring grassland and that favoring forest, although at an average of 460 millimeters (mm) to 500 mm precipitation is not high enough to sustain forests. Temperatures range from 35°C in the summer to -34°C in the winter, with a growing season of 100 to 120 days. Soils are mostly Entisols and Inceptisols. The topography of the mountain browse type is variable, and elevations range from 1,200 meters (m) to 2,800 m. The ridges and dry, rocky slopes are dominated by shrubs 1 m to 10 m tall. Important species occurring throughout the type are chokecherry (Prunus virginiana) and several species of buckbrush (Ceanothus sp.). Gambel oak and true mountain mahogany (Cercocarpus montanus) are two of the most important shrubs associated with this type in the Southwest. The occurrence of antelope bitterbrush (Purshia tridentat) and other important forage species make this range type an important winter feeding area for big game.

Northern Mixed Prairie 

The northern mixed prairie encompasses the western half of North and South Dakota, the eastern two-thirds of Montana, the northeastern one-fourth of Wyoming, and the southeastern part of Alberta and southern Saskatchewan in Canada. The climate is characterized by long, severe winters and warm summers. Average frost-free period ranges from about 140 days in the south to less than 100 days in Canada. Two-thirds of the 300 millimeters (mm) to 650 mm of average annual precipitation comes as rains during the summer, with the majority falling in June. Most soils are of the order Mollisols; however, much of the region is still rangeland due to the short growing season, drought, and severe winters. It is the second most important western range type from the standpoint of livestock production. The northern mixed prairie is home to the highest diversity of grasses of all the western range types, including short, mid-, and tall grasses, as well as cool- and warm-season grasses. Consequently, the northern mixed prairie is one of the best of all types for both wildlife and livestock from a nutritional standpoint. The diversity of grasses provides a continuous excellent feed source throughout the year: cool-season bluebunch wheatgrass (Pseudoregneria spicata) and various bluegrasses provide early spring feed; green needlegrass (Nassella viridula) and needle-and-thread (Hesperostipa comata), western wheatgrass (Pascopyrum smithii), and various forbs provide excellent late spring feed. Little bluestem (Schizachyrium scoparium), blue grama (Bouteloua gracilis), and sideoats grama (Bouteloua curtipendula)provide high-quality summer and fall forage, while the shrubs and blue grama provide excellent winter feed. Nevertheless, supplementation is often required for livestock during the winter because of frequent snow cover. The northern mixed prairie hosts the widest variety of shrubs of all the western range types. This is a primary reason why it supports an abundant and diverse wildlife population.

Oak Woodland 

Rangelands dominated by oak species (Quercus sp.) can be divided into three categories: the shinnery oak (Quercus havardii) type found in southeastern New Mexico, west Texas, and south into Chihuahua, Mexico; the Gambel oak (Quercus gambelii) types in the central and southern Rocky Mountains; and the open savannah dominated by tree oaks in California, Oregon, southern Arizona, and central Texas. Oaks are sensitive to winter cold and occur primarily in areas with a maximum temperature near 36°C and a minimum temperature of about -34°C. Oak species occur on rolling uplands and foothills with a variety of well-drained soils. Precipitation in the oak woodland type ranges from 360 mm per year in the chaparral ranges of southern Arizona and New Mexico to around 1,000 mm per year in the California chaparral. In general, the size of the oak trees decreases in response to decreasing precipitation as one moves from the east to the west in the southern Great Plains.

Although most oaks have limited forage value for livestock, they are a valuable food source for many wildlife species. Deer, elk collared peccary, wild turkey, and other upland game birds forage on the acorns. Deer and elk also browse on the leaves of Gambel oak. The new growth of deciduous oaks such as shinnery and Gambel is poisonous to cattle and sometimes sheep, but goats can browse most oak species without harm and can even be used to control some oaks on rangeland. Oaks are sprouters, making control with herbicides and fire difficult. However, widespread control of oaks is not recommended. Rather, management should be focused on maintaining a mosaic of habitats to provide adequate forage for wildlife and livestock while preserving the aesthetic and ecological values of the oak woodland range type.

Palouse Prairie 

The Palouse prairie occurs primarily in eastern Washington, north central and northeastern Oregon, and western Idaho. However, only remnants remain after the majority was converted into farmland. Soils are primarily loessal dunes in the order Mollisol. They have excellent textural, structural, and chemical properties for agriculture. Rainfall in the area ranges from 30 centimeters (cm) to 64 cm annually, with approximately 65% to 70% falling during the winter months. Peak precipitation occurs in December and January. The months of July and August have the lowest precipitation, although these months are wetter than in the California annual grassland type. The growing season lasts from 140 to 160 days, extending from May 10 to October 10. Winters in the Palouse country are relatively mild, and summer temperatures are seldom over 35°C.

The Palouse prairie is one of the most productive and beautiful grasslands in the world. The major characteristics distinguishing the Palouse prairie from other North American grasslands is that the climax vegetation is dominated by bluebunch wheatgrass (Pseudoroegneria spicata) or bluebunch wheatgrass and Idaho fescue (Festuca idahoensis). Grasses are almost entirely cool-season bunchgrasses because of the dry summers. The Palouse prairie supports a wide variety of forbs, most of them belonging to the Composite (Compositae) family.

The Palouse prairie grasses evolved with little grazing pressure from large herbivores. The American bison did not use this type because barriers such as the Rocky Mountains and Snake River Canyon restricted access. Both bluebunch wheatgrass and Idaho fescue have very low grazing resistance. For this reason, the Palouse prairie responds rapidly to overuse. The Palouse prairie was the first of the northern ranges to support a livestock industry. Unfortunately, overgrazing has resulted in an increase of downy brome (Bromus tectorum) and Sandberg bluegrass (Poa sandbergii) which can replace perennial bunchgrasses.

Pinyon-Juniper Woodland 

One of the most widely distributed rangeland types in the western United States, the pinyon-juniper woodland (also spelled pinon-juniper), occurs from the state of Washington to 220 kilometers (km) north of Mexico City. Annual precipitation varies from 300 millimeters (mm) to 450 mm, with local areas receiving up to 500 mm. The frost-free period is variable and ranges from 91 to 205 days. Soils are poorly developed and are primarily of the orders Entisol and Aridisol. The climate is characterized by low precipitation, hot summers, high wind, low relative humidity, high evaporation rates, and much clear weather and intense sunlight, making it relatively harsh for tree growth. Nevertheless, a combination of overgrazing, absence of fires, dissemination of seeds by mammals and birds, and possibly climatic change has sparked tree encroachment onto grasslands and prompted original stands to become more dense. Overgrazing of understory species has reduced protective soil cover and resulted in severe soil erosion over much of this range type. As the tree overstory increases, perennial grasses and forbs decrease because of shading and increased competition from the trees. The pinyon-juniper woodland is one of the most depleted rangeland types occurring in the United States, particularly in the northern extent of the region, where very few areas presently support a good grass understory. Recovery from overgrazing is considered to be slow to nonexistent in most areas without control of the trees. Presently, wood from this type may have more economic value as a biofuel and carbon sink than the forage it provides for domestic animals.

Shortgrass Prairie 

The shortgrass prairie extends from northern New Mexico into northern Wyoming, encompassing much of eastern Colorado. The low precipitation of the region (300 millimeters to 500 mm) has prevented cultivation, despite the fertile soils that are primarily Mollisols. Therefore, much of the region remains as rangeland. It ranks third in importance for livestock production among the western rangeland types. The climate is characterized by cool winters and warm summers, with the majority of the yearly precipitation coming from light rains falling throughout the summer. This climate favors warm-season grasses such as blue grama and buffalograss, which have shallow but extensive root systems. The region evolved under intense bison grazing, prompting the most abundant grass species to develop physiological and morphological adaptations to heavy grazing pressure. Livestock are generally grazed year-round on the shortgrass prairie with little hay provision; however, a protein supplement can substantially reduce livestock weight losses.

Winterfat is an important shrub that is palatable for livestock and wildlife and important for pronghorn antelope that thrive in this rangeland type. Scarlet globemallow is an important forb heavily used by cattle, sheep, and pronghorn. Throughout history, people have repeatedly attempted to cultivate portions of the shortgrass prairie, despite the fact that it will not sustain cultivation without irrigation and is subject to wind erosion.

Southern Mixed Prairie

The most important of the western rangeland types for livestock production is the southern mixed prairie, which extends from eastern New Mexico to eastern Texas and from southern Oklahoma to northern Mexico. The precipitation ranges from 300 millimeters (mm) to 700 mm across the region with a frost-free period generally at least 180 days in length. This vast area hosts a wide range of soils and climatic conditions that result in variable production and vegetation communities. There are four basic subtypes of the southern mixed prairie. These are: true mixed prairie, desert prairie, high plains bluestem, and oak savannah.

Soils of the region belong to the order mollisols, entisols, and aridisols. The southern mixed prairie has a long history of grazing, beginning with the buffalo and then domestic livestock brought in by the Spaniards in the 17th century. As a result, most grasses are relatively grazing resistant. Important grasses of the southern mixed prairie are: blue grama (Bouteloua gracilis), buffalograss (Buchloe dactyloides), little bluestem (Schizachyrium scoparium), various threeawn species (Aristida sp.) silver bluestem (Bothriochloa saccharoides), vine mesquite (Panicum obtusum), and sideoats grama (Bouteloua curtipendula). Texas wintergrass (Stipa leucotricha), is the only cool season grass of significance in the southern mixed grass prairie and as such provides an important winter feed source. Several woody invasive plants, such as mesquite (Prosopis sp.), create challenges for land managers and livestock producers over most of the southern mixed prairie, particularly when overgrazing occurs. Multi-species grazing is commonly used in this range type to increase animal production and improve range condition. The Merrill four-pasture, 30-herd grazing system appears to be more beneficial to soils, vegetation, and livestock and wildlife performance and health than continuous grazing.

Southern Pine Forest 

The largest and most important rangeland type in the United States is the southern pine forest. With an annual precipitation of 1,250 millimeters (mm) and a frost-free period of 200 to 365 days, it is also the most important of all rangeland types for livestock production. The warm temperatures and large amount of precipitation ensure high vegetation production limited only by the acidic soils of the region. These highly leached soils are primarily of the order Ultisol.

Pine trees such as longleaf pine (Pinus palustris), shortleaf pine (Pinus echinata), and loblolly pine (Pinus teada) occupy much of the area and are grown for lumber. Most grazing occurs on open or cutover forests where grasses in the genera Andropogon, Panicum, Aristida, Paspalum, Sporobolus, and Cynodon provide abundant forage. Protein and phosphorus deficiencies in the forage require that livestock be supplemented for at least part of the year. Livestock production is second to timber production as the most important land use in the Southeast.

Tallgrass Prairie

The tallgrass prairie is located primarily in the central United States. Climate and disturbance have worked together to keep the area a grassland. Precipitation ranges from 500 millimeters(mm) in the north to 1,000 mm annually in the south, with most of it falling as rain in the summer growing season. Periodic summer drought combined with relatively frequent fires, every three to four years, prevented the successful establishment of trees. The subhumid, temperate environment coupled with the deep fertile soils (Mollisols) of the region make it ideally suited for cultivation. Consequently, much of the tallgrass prairie now grows wheat and corn. Very little rangeland still exists in this region and is confined to the Flint Hills of eastern Kansas and the Osage Hills of Oklahoma because of their thin, rocky soils unsuitable for cultivation.

The tallgrass prairie evolved under grazing by wild ungulates, primarily, the American bison. It is one of the most grazing-resistant range types, due to the high amount and favorable timing of precipitation. The tallgrass prairie is particularly well-suited for grazing in the summer and fall, but grasses become coarse and have little nutritional value to offer livestock during the winter months. However, extreme overgrazing can shift a tallgrass prairie from tallgrass prairie to mid-grasses to short grasses. A rotational grazing plan is recommended for the tallgrass prairie to optimize livestock performance and range condition.

Tallgrass prairie should be burned every two to three years in the spring to prevent excessive mulch accumulation. The most notable grass species of the tallgrass prairie are little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), yellow indiangrass (Sorghastrum nutans), and switchgrass (Panicum virgatum). Important forbs of the tallgrass prairie are leadplant (Amorpha canescens) and scurfpea (Psoralea sp.). Buckbrush (Symphoricarpos orbiculatus) is the primary shrub on the tallgrass prairie, providing valuable food and cover for wildlife.

Western Coniferous Forest 

Two main plant communities, ponderosa pine (Pinus ponderosa) and Douglas fir (Pseudotsuga menziesii)-aspen (Populus tremuloides), dominate the 33 million hectares that comprise the western coniferous forest rangeland type. Early adoption of good grazing management and high precipitation have prevented this forest from suffering the ecological damage seen in the pinyon-juniper vegetation type. The western coniferous forest supports many different land uses including recreation, watershed, wildlife, and scenic value in addition to livestock production.

As the largest of the true forest types in the western United States, ponderosa pine is found from inland California east to Nebraska and from southern Alberta south into northern Mexico. It is found at elevations between the pinyon-juniper and the Douglas fir zone ranging from 2,000 meters (m) to 2,500 m in the Southwest to as low as 1,100 meters in the northern areas. The ponderosa pine forest is also the most xeric forest type in the western United States receiving between 450 millimeters (mm) and 650 mm of precipitation, with most falling as snow in the north and as rain in the south. Frost can occur in any month of the year, and the growing season encompasses 105 to 140 days. The primary soils are Entisols with Inceptisols occupying the benchlands and ridges and Mollisols common in areas with more level topography.

Immediately above the ponderosa pine zone from 2,500 m to 3,100 m elevation is the Douglas fir-aspen forest. This forest type occurs primarily in Colorado, Idaho, Wyoming, Montana, Oregon, and Washington. The bulk of the Douglas fir-aspen forest type occurs in seral stages dominated by lodgepole pine (Pinus contorta) quaking aspen (Populus tremuloides) and western larch (Larix occidentalis). Soils in the region are primarily Alfisols, Entisols, and Inceptisols, and the topography is very steep. The growing season lasts from 100 to 125 days, and the zone receives 640 mm to 900 mm of precipitation falling mostly as snow.

Learn about Rangeland Plants

• Origin of Range Plants
• Life Span of Range Plants
• Season of Growth of Range Plants
• Cool-Season and Warm-Season Forages

Origin of Range Plants

The "origin" of a range plant is the area where it evolved. It is important to know the origin of a plant because it can affect the way that a plant responds to the environment.

Native plants are those that originated and evolved in North America.

Introduced plants were brought to North America from another continent. Several of these plants were intentionally introduced to rangeland because they have good forage value. Other plants were accidentally introduced to North America, usually as contaminants in crop seeds. Weedy introduced plants were accidentally introduced or brought in for their ornamental value but then "escaped" into rangelands.

Life Span of Range Plants

Annual plants live only one growing season. There are two types of annuals:

• Winter annuals germinate in the fall and form a small rosette of leaves that persist through the winter. The following growing season, the plant continues to grow, flowers, produces seeds sometime in the summer, and then dies.

• Summer annuals germinate in the spring and complete all growth, including seed production, by the end of the growing season and then die.

Biennial plants live two growing seasons. Normally these plants form a basal cluster of leaves the first year, called a rosette, and send up a seed stalk (bolt) the second year.

Perennial plants live from one year to the next, producing leaves and stems for more than two years from the same crown. Most range plants are perennial.

Season of Growth of Range Plants

Cool-season plants make their principal growth during cool weather. At lower elevations, these plants grow in the spring and set seed in late spring or early summer. New growth can appear in the fall if moisture is adequate. Plants that grow at high elevations are usually cool-season plants because of the cool temperatures throughout the growing season.

Warm-season plants make most of their growth during late spring and summer. Seed develops in midsummer and early fall.

Cool-Season and Warm-Season Forages

Learn about grass growth:

• Basics of Grass Growth
• Components of Grass Growth
• Vegetative Growth of Grasses
• Reproductive Growth of Grasses
• Carbohydrate Reserves in Grasses

Basics of Grass Growth

The root crown usually has several to dozens of buds (growing points), and each bud can produce new tillers and roots. New tillers are anatomically and physiologically connected to older tillers. Therefore, several connected tillers may all live and share water, carbohydrates, and nutrients. If one tiller dies, an adjacent tiller with established roots often continues to live and/or the plant can produce a new tiller from a bud located on either the root crown or the base of a tiller.

Perennial bunchgrasses perpetuate from one year to the next by developing buds on the root crown each fall, storing enough energy in those buds for them to survive the winter dormant period, and having enough additional stored energy in the bud for it to produce the initial growth on the tiller's first two to three leaves the next spring. If the plant cannot form buds at the base of a tiller before it enters summer or fall dormancy, or if the buds that do develop lack enough energy reserves to keep the bud alive during the long winter dormant period, the tiller and eventually the plant will die. Only after a tiller produces the first few millimeters of growth on the first two to three leaves is there enough photosynthetically active leaf material for the plant to produce a surplus of energy (carbohydrates) that can be used to produce many leaves (forage) and store energy for growth the next year.

Components of Grass Growth

Plant growth has two components: the creation of new cells and the enlargement of those cells. All new cells in a plant are produced by meristematic tissue. Once a bud has initiated growth of a new tiller, there are only four microscopic growing points on that tiller than can produce new cells for plant growth. One growing point is at the base of the leaf blade. All of the cells that form the leaf blade, which usually is the bulk of the most palatable forage, come from the microscopic growing point at the base of the leaf blade. The second growing point is at the base of the leaf sheath, where the sheath attaches to a node. This growing point can only produce cells used to create the leaf sheath. The growing points at the base of the leaf blade and the base of the sheath, collectively, are called the intercalary meristem. At the very tip of a tiller is the terminal growing point or apical meristem. The apical meristem is the growing point that produces the seedhead of a grass plant. The fourth growing point is the axillary bud at each node. If proper growing conditions occur, the plant may activate this bud to produce an entirely new tiller. Some tillers stay vegetative, while others become reproductive and produce seedheads. Whether a tiller becomes reproductive depends on the environment and hormones produced in the plant.

When a tiller becomes reproductive, the tiller elongates, and the growing points located on the leaves and at the nodes and apical meristem (terminal growth point) become elevated above the ground surface. Elevated growing points are at risk of being removed by a grazing animal. Whether one or more of these growing points are grazed off has implications for how the plant responds to defoliation. When the terminal growth point is removed, the plant cannot produce any additional leaves or a seedhead. New leaves can only be produced if the plant activates either a crown or axillary bud to produce a new tiller. Complete tiller replacement is a slow process and may use substantial amounts of stored energy. Removal of the terminal growth point may have negative or positive outcomes depending upon management goals, how much leaf area remains after grazing, and the ability of the plant to regrow after defoliation and store energy for growth the following growing season.

A reproductive tiller may remain vegetative if the terminal growing point (apical meristem) is removed by grazing. Vegetative growth, therefore, is favored by some grazing, which reduces the number of seedheads produced and may stimulate the formation of new tillers. Vegetative tillers usually are less stemmy and more nutritious than reproductive tillers. Forage grasses in irrigated pastures often are maintained in a vegetative state by periodic removal of the terminal growth point. Following removal of the terminal growing point, existing leaves may continue to grow and produce forage if defoliation did not remove the growing point at the base of the leaf blade. In addition to continued leaf growth, the plant also may activate a bud to produce a new (replacement) tiller which can create additional forage from both existing leaves and new tillers. The addition of irrigation water permits the replacement tiller to grow toward completion of its full growth cycle. When soil moisture is available the entire growing season, this process can be repeated many times and a large amount of forage produced. Because some of the leaf blade always remains after grazing, growth continues unimpeded, which results in ample storage of energy (carbohydrates) in the buds used to initiate growth of tillers the following year. Removal of the terminal and other growing points late in the growth phase, however, can harm plants when they lack the resources (water and nutrients) to activate the bud, produce a new tiller, and complete the growth cycle. In this scenario, the buds do not develop or they do not store enough energy to initiate growth the following year.

Vegetative Growth of Grasses

An apical meristem (expanded portion of Figure 1) is responsible for leaf formation. The intercalary meristems at the base of leaf blades and sheaths are responsible for leaf expansion (insert in Figure 2). Each leaf is rolled into a tube-like form in its lower portion and unfurls as the blade extends. Subsequent leaves follow the same pattern.

As new leaves push up from the center of the rolled tube portion of the first leaf, the growth is similar to the extension of a telescope. In Figure 1, leaf 1 is the oldest; leaf 8, the youngest, is emerging. In this example, the growing point (apical meristem) is at or near the soil surface , which provides protection from defoliation by large grazing animals. Defoliation, therefore, removes leaf tissue but, in most cases, will not harm the growing point at the base of the leaf blade. It is this growing point that produces the bulk of the forage.

Grass growth, for either cool- or warm-season species, begins in spring when the soil warms. As the first grass leaf emerges its length or height increases due to new cells being produced at the base of each leaf blade. This growth area (intercalary meristem) is at the base of the leaf blade and adjacent to the growing point for the leaf sheath (insert in Figure 2), which has not yet developed.

Leaves have a definite life span, as do tillers. The first spring leaf normally dies in the summer. Leaves are most photo-synthetically active when they reach full expansion. As they age, their capacity for photosynthesis declines. The excess carbohydrates produced through photosynthesis helps produce additional leaves, seed, roots and buds for new tillers the following year. Thus, photosynthate produced by the plant is used efficiently in growth and maintenance. Once a leaf can no longer produce enough carbohydrates through photosynthesis for its own needs, it dies.

Reproductive Growth of Grasses

As the growing season progresses, grasses begin to allocate their resources to seed production rather than leaf production. Similar to animals and humans, hormones within the plant signal when it is time to start producing reproductive tissue. Hormones signal the apical meristem, which is the growing point of grasses, to produce a seedhead.  The tiller begins to elongate, and as it grows, it elevates the growing points (with the exception of basal buds) above the surface of the ground.  Plants differ in how early and how quickly they initiate tiller elongation and seedhead production.  Differences in this timing affect the plants ability to withstand grazing by herbivores. If a herbivore grazes an elongated stem and removes the elevated growing points, seed production is stopped for that tiller. Consequently, if seed production is a management goal, grazing should cease when tillers begin to elongate. The period of tiller elongation to mature seed is often called the "critical period." Allowing reproductive tillers to mature and produce seeds can increase the density of plants in the stand. Many bunchgrass species are dependent on seed production for reproduction; in these plant communities annual grazing during the critical period can decrease stand density since plant mortality is not matched by new plant recruitment. However, annual seed production is not essential for stand maintenance because few plants typically die each year, and some grasses reproduce by vegetative means from underground rhizomes or above-ground stolons.  In rhizomatous species, grazing during seed production may stimulate future tiller production from the basal buds if there is adequate soil moisture to allow for continued plant growth.

Carbohydrate Reserves in Grasses

The energy (carbohydrates) used to produce the first one to three leaves of a grass plant each spring does not occur from photosynthesis (the production of carbohydrates from CO2 and water that become the leaves, roots, and seeds of the plant), but rather from carbohydrates stored by the plant the previous growing season. Carbohydrates produced the previous growing season that the plant does not use to produce its leaves, stems, roots, or seeds that year are called soluble carbohydrates and can be stored in the plants buds, the root crown, roots, and/or tiller bases. The role of the stored energy is to keep the plant organs alive during the dormant period and to provide the energy needed to produce the first few leaves that emerge the following spring. Most, if not all, of the energy used by the first few emerging leaves comes from energy stored in the bud. Once the first leaves extend, the plant has enough leaf area to successfully conduct photosynthesis to meet the energy needs of the plant and produce the carbohydrates needed for additional leaf production and growth. These carbohydrates also provide enough energy for the roots to grow and the plant to produce seed. When defoliation removes the growing points on the plant’s tillers, the plant must use carbohydrates stored in its buds, root crown, and/or roots to initiate new growth. If the plant does not have the time or resources to fully regrow its leaves and also produce enough soluble carbohydrates to keep its buds alive during the coming dormant season and produce the first few leaves the following spring, the plant can die. The primary role of the stored carbohydrates is to keep plant buds alive during dormant periods and initiate new growth in the spring, not initiate regrowth after grazing. After grazing, regrowth from growing points at the base of the leaf blade allows plants to rapidly regrow without having to use stored energy the plant needs the following spring.

See the fact sheet Carbohydrate Reserve Theory: What You Learned May Be Wrong

There are no clear boundaries that define vegetation types across rangeland landscapes. However, grasses are generally a major component of rangeland vegetation so becoming familiar with grass plant growth is essential.  Rangeland plant species are often grouped into vegetation types to provide a framework for managers to assess the ecological status and trend of plant communities.

Vegetation change and succession on rangelands was historically measured or modeled with the Range Condition and Range Succession Models. These two models are structurally similar but differ in their terminology. Both models viewed plant succession, both before and after disturbance, as a linear process (Figure 2). Following a disturbance, early seral species typically inhabit an ecological site. With time, the plant community progresses through a series of changes toward a potential natural community, or climax community. All plant communities are subject to many stresses that influence the composition and structure of the vegetation. Under the Range Succession and Condition models, little or no stress resulted in vegetation change toward the potential natural community (PNC). Increasing stress above some community specific level would prevent succession toward the PNC and could return the community to some prior composition, lower on the successional ladder.

To describe the stage of succession at any given time, both models compared the current plant composition to the PNC or climax community described. The percent of overlap between the current community and the PNC determined the stage of plant succession and condition of the site. The four categories used were early seral or poor condition; mid-seral or fair condition; late seral or good condition; and the potential natural community or excellent condition. Seral stage descriptors formed the basis of the Range Succession model and condition descriptors the Range Condition model.

Table 1. Vegetation composition by annual biomass production for three Wyoming sagebrush ecological sites (Loamy 8-10 inch) and their range condition and succession scores.

The data in Table 1 show that a potential plant community can have several very different seral stages with the same classification for range condition class or seral stage. Site 1 is a shrub-dominated site with relatively few perennial grasses. Site 2 is predominately a bunchgrass site with few shrubs. Species and particularly life-forms often respond to fire and other disturbances quite differently. The response of both seral stages to the same management action or the same catastrophic disturbance probably will not be the same, despite both having the same classifications for range condition and successional status. One of the serious flaws of the range succession and condition class models is that the same descriptive classification — for example, good/late — can represent two different phases of plant succession and therefore two very different response potentials to management and disturbance. If the descriptive classifier is the only data product retained by the land manager, the manager will not know the response potential of the ecological site following disturbance.

Figures 3a-3c are pictures of the same sagebrush ecological site in three different community phases. Figure 3a is the potential natural community. The community phases in Figures 3b and 3c are both late seral/good condition communities but with very different plant communities. The shrub-dominated site (Figure 3b) will have substantial bare ground after a disturbance and would be very susceptible to invasion by non-native annual grasses or other weeds. The other two sites (Figures 3a and 3c) have sufficient perennial bunchgrasses to remain bunchgrass sites after a disturbance and resist the invasion of annual grasses. The need to better understand the management opportunities and hazards associated with the different community phases for any ecological site’s reference community led to the development of state and transition models.

Written by Brad Schultz, Extension Educator, Winnemucca, Nevada

State and transition models were developed to help land managers make better decisions when managing vegetation for a suite of potential land uses — for example, livestock forage and wildlife habitat — and/or ecosystem services such as erosion control, water infiltration, and wildfire risk reduction. They provide a framework (Figure 8) for describing, understanding, predicting, and controlling ecosystem dynamics; the integrated goal of science and management. State and transition models help understand complex systems, with multiple interactive drivers for changes — competition, precipitation, and response to management actions — that operate at different intensities, frequencies, and durations across different scales of time and space.

Figure 9 is a state and transition model for a Loamy 8-10 inch ecological site from central Nevada. The reference community (PNC) is a Wyoming sagebrush-bunchgrass community. Indian ricegrass (Achnatherum hymenoides) and Thurber’s needlegrass (Achnatherum thurberianum) are the predominant bunchgrasses, representing 30% to 50% of the total annual biomass production. Bottlebrush squirreltail (Elymus elymoides) and Sandberg bluegrass (Poa sandbergii) are minor but constant perennial bunchgrasses in the community. Cheatgrass (Bromus tectorum), an invasive annual grass can establish on sites that are disturbed or mismanaged. Wyoming big sagebrush (Artemisia tridentata wyomingensis, a non-sprouting shrub, is the most common shrub, and at a mid- to late-seral stage has an annual biomass production of 25% to 35% of the total community production.

Building on what has been learned, the green box in Figure 9 is a resilient sagebrush-bunchgrass state, with seven community phases within the resilient state. Depending on the specific management goals for the site there could be more or fewer phases. The arrows connecting the green and yellow boxes are pathways between community vegetative phases within the resilient state of the Wyoming Big Sagebrush Ecological Site. The green boxes represent very resilient vegetative phases that present little or no ecological risk to the integrity of the state. That is, any community phase they change toward will not result in transition to a different vegetative state. If a disturbance or management action removes the desired sagebrush, the perennial bunchgrasses are dense enough to reoccupy the site and exclude the annual invasive grasses, the sprouting shrubs, or few juniper trees.

The framework needed to develop a state and transition model for vegetation management and change.

Figure 8. The framework needed to develop a state and transition model for vegetation management and change. Courtesy of Brad Shultz

Management actions and/or normal successional pathways across decades could result in phase changes from communities described in the green boxes to communities in the yellow boxes. These may or may not be desired community changes, depending on management goals. Regardless, the changes represent increased risk to the integrity or resilience of the ecological site. These added biological risks are from increased sprouting shrubs, sagebrush cover above 15% to 20%, associated declines in the density of bunchgrasses, and the establishment of juniper trees. An increase in the woody species above some site-specific level is an ecological risk because large woody plants out-compete the smaller bunchgrasses for soil moisture, and the bunchgrasses eventually decline. At some point in the successional process, there will be insufficient bunchgrasses to completely occupy the site immediately after a catastrophic disturbance, and the site will likely transition to a cheatgrass community.

The yellow boxes represent community phases that may meet management goals but are communities with an increased risk of transition across a threshold to an undesired alternative state. The yellow lines that approach the threshold represent the reversible portion of the transition process. A change in management at this stage of plant succession may return the community to a phase with more resilience and less ecological risk. Once the transition crosses the threshold, none of the alternative states can move back into a desired phase of the ecological site without extensive and expensive inputs.

The black arrows between the community phases are pathways between each phase. The specific management actions and/or ecological processes that interact to change the vegetation from one phase to another can be documented and used to direct management toward the desired community phase.

Figure 9. State and transition model for a Loamy 8-10 ecological site in Central Nevada. Courtesy of Brad Shultz

Written by Brad Schultz, Extension Educator, Winnemucca, Nevada

State and transition models have four primary components: states, transitions, thresholds, and triggers. The reference vegetative state refers to a recognizable and repeatable plant community (ecological site) that typically occupies a specific type of soil. The abundance of the specific plant species largely depends upon the topography, the specific characteristics of the soil, climatic conditions and its variability, the composition of the previous plant community when it was last disturbed, the type and intensity of the last disturbance, time elapsed since the last disturbance, historic and current land uses, and herbivory (wild and domestic). Management that removes important species or life-forms from a vegetative state and/or introduces non-native plants that alter the state's ecological processes and functions typically results in a different vegetative state. This is illustrated in Figures 4a-4c, with photos from several Wyoming sagebrush (Artemisia tridentata wyomingensis) communities in Nevada.

Figure 4a shows a Wyoming sagebrush site with an intact understory of perennial bunchgrasses and forbs. Removal of the shrubs from a fire or other catastrophic disturbance would result in a community of mostly bunchgrasses: the earliest phase of secondary succession on most sagebrush-grass ecological sites. The bunchgrasses provide sagebrush-bunchgrass states with good ecological resilience to disturbance. Figure 4b is a Wyoming sagebrush community without the perennial bunchgrasses in the understory. Following removal of the shrubs, bunchgrasses cannot reoccupy this site without significant external inputs (e.g., seeding, weed control). This community is unlikely to return to the previous sagebrush-bunchgrass state because cheatgrass (Bromus tectorum), an introduced annual grass, may rapidly occupy the site (Figure 4c). The sagebrush community in Figure 4b is resistant to change back to the sagebrush-bunchgrass state, regardless of the management actions applied. Furthermore, it lacks ecological resilience (i.e., change back to a sagebrush-bunchgrass state) if disturbed. The sudden loss of the sagebrush component in this community would leave the ground barren and prone to invasion by undesired weeds which results in the common transition to a cheatgrass community (Figure 4c). No simple change in management can reestablish the perennial bunchgrasses missing from either community (Figures 4b and 4c); therefore, they have become alternative vegetative states.

Loamy WY sagebrush

Figures 4a-4c. Three vegetative states in a loamy 8-10 inch (precipitation zone) Wyoming sagebrush ecological site in Nevada. The community in Figure 4a has an extensive understory of perennial bunchgrasses that maintain site resilience. Figure 4b shows a community without the bunchgrasses or other perennial herbaceous species. Figure 4c shows a sagebrush-bunchgrass community that lost the bunchgrasses, burned, and transitioned to a cheatgrass community. Courtesy of Brad Shultz.

Many vegetative states are composed of two or more vegetative phases (community phase) of the potential natural community (PNC). This is particularly true if both herbaceous and woody life-forms inhabit the PNC. A community phase is one of several plant communities that may occur within a state. Two phases of the same community (ecological site) typically reflect different proportions of the species that occur in the PNC. Whether the specific plant community on a site is a phase of the reference community of a vegetative state or a different vegetative state depends upon how the community responds to potential management actions. Community phases that typically respond to common management actions (grazing management, brush control, etc.) by changing to a different desired phase in an acceptable time frame (from a management perspective) are part of the suite of community phases within one vegetative state. When a typical management action cannot change a plant community to a more desired community, and the desired vegetation change requires intensive and expensive external inputs, that community has transitioned to an alternative vegetative state. Figure 5 shows two phases of a black sagebrush-perennial bunchgrass state in west-central Utah. The phase to the right of the fence has been ungrazed for over 60 years and is mostly black sagebrush with widely scattered native bunchgrasses. The vegetative phase on the left is predominately bunchgrasses with widely scattered shrubs. The bunchgrass phase has received dormant-season grazing for 60 years. A simple change in the season and intensity of grazing could shift the shrub-grass phase to more grasses or the grass-shrub phase to more shrubs. The shrub-grass phase to the right of the fence (Figure 5) is slowly losing its bunchgrass component due to competition from the shrubs. Without a change in management to reduce competition from the shrubs, the shrub-grass community phase will become a shrub state with limited management options.

Figure 5. A black sagebrush–bunchgrass vegetation state with two vegetative phases in west central Utah. Courtesy of Brad Shultz.

A vegetative transition is a directional change in a plant community from one state to an alternative state. Most transitions are toward undesirable or less desirable states. Transitions to more desired states are possible but typically require extensive external inputs to the community. Transitions do not reflect annual differences in a community or the expected seral changes within a state (i.e., the community phase changes previously discussed). Many ecological processes, management actions, and disturbances may occur individually, collectively, or sequentially. The cumulative effect of these processes and events may cause the vegetation to change from one state to another or to recycle among the different phases of a state. The ecological processes that interact to cause transitions typically occur across long periods. For example, the change associated with the loss of bunchgrasses on a sagebrush-bunchgrass site often takes decades (Figures 4a and 4b), but the ecological effect often becomes clear-cut only after a sudden intense disturbance, such as fire (Figure 4c).

A threshold is the conceptual boundary between two or more vegetative states (Figure 6). Since vegetation cannot rapidly move from a less to more desired state without substantial external inputs to the system, thresholds once crossed are irreversible boundaries. Once a threshold is crossed, the ecological constraints created by interactions among many environmental variables interact to prevent the vegetation from returning to the previous desired state. Prior to crossing a threshold, desired vegetation change usually can occur with a change in management or the application of a single treatment to the vegetation, provided the action mimics a missing or altered natural process. These actions deflect the transition (i.e., its reversibility) away from the threshold and toward a more desired phase of the desired state. After a threshold is crossed, reestablishment of the previous state is very costly and requires the use of several and/or complex, risky, difficult, and expensive management actions.

Figure 6. The top diagram is a conceptual model of a threshold between two states. The bottom diagram shows the concept for a sagebrush-bunchgrass site in Nevada. Courtesy of Brad Shultz.

Thresholds are crossed because the ecological processes that interact to maintain the plant-soil matrix in a desired vegetation state are degraded, removed, or altered enough to change the function of the site. Changes in vegetation composition and structure interact to affect vegetation and ecological function (e.g., biomass production, water-holding capacity, erosion potential, susceptibility to invasive species, etc.). Examples of different functional thresholds being approached and crossed are shown in Figure 7.

Triggers are events that cause transitions to occur. These events may be caused by natural or human activities and are extreme or unusual events that may occur suddenly or over long periods. Sudden events may be fire or a flood. Examples of long-term events include poor grazing management that removes desired plants, the exclusion of fire from a fire-dependent ecosystem, or the increase of woody vegetation to the point it eliminates desired grasses and forbs. Triggers are often sudden, culminating events (e.g., fire) that interact with either the removal of or a change in the intensity, frequency, duration, or scope of long-term ecological processes that form transitions and thresholds. The collective interaction of the change in ecological processes and the triggering event result in the non-reversible transition to a less desired state.

The top diagram is a conceptual model of a threshold between two states. The bottom diagram shows the concept for a sagebrush-bunchgrass site in Nevada.

Figures 7a-7c. Three examples of different thresholds being approached and crossed. Figure 7a represents a functional threshold (incision and erosion) that if crossed results in complete loss of the meadow riparian area. Figure 7b represents a biological threshold being approached and eventually crossed, due to declining bunchgrass composition. The competition process has been altered; thus, a biological threshold has been crossed. Figure 7c represents crossing both a biological (species and life-form composition) and physical (soil loss from erosion). Courtesy of Brad Shultz.

Plant communities on rangelands typically are composed of a mixture of grasses, forbs, and shrubs. Some rangelands, such as many ponderosa pine forests or pinyon-juniper woodlands, have an overstory of trees and an understory of grasses, forbs, and/or shrubs. All plant communities, regardless of their location, change across time — a process called plant succession. The changes may be in species composition, life-forms (grasses, forbs, shrubs, trees) and life cycles (annual, biennial, perennial); the abundance of each species; the relative proportion of the life-forms present; the size, density, shape, or arrangement of plants (structure); or combinations of these features.

Vegetation change involves both the direction and the rate of change. A plant community can move toward more desired or productive communities or toward less desired communities. The former can be considered as management opportunities and the latter as hazards that should be avoided. Understanding plant succession is important because the composition of plants within plant communities has three important influences:

1. How landscapes function — for example, water cycle, nutrient cycle, soil formation,
2. The type and amount of products or resources and services society can develop and produce, and
3. Identification of management opportunities and hazards.

Figure 1 (below) shows some of the ecological changes that result from plant succession across several to many decades on a sagebrush-bunchgrass rangeland.

Periodic fire is the typical disturbance on shrub-grass rangelands that maintains the balance between the perennial grasses and forbs (perennial herbaceous) and the woody shrubs and trees. This is particularly true for shrubs that do not sprout after a disturbance. Fire removes the non-sprouting shrubs and permits an abundance of other, more fire-adapted species. The lack of fire eventually results in shrub-dominated communities and a corresponding decline of the other species. The shift from a predominately perennial herbaceous life-form to woody species changes both structural and functional components of the ecosystem. The key management question is: Can the change to shrub dominance be reversed, completely or partially, to meet management goals and objectives for desired resources?

Among the important concepts in plant succession on rangelands are the ecological site, community resilience, and community resistance. An ecological site is a unique, identifiable, and repeatable patch of vegetation on a landscape. It has specific (homogeneous) biological, physical, and chemical characteristics and the potential to produce a distinctive kind and amount of vegetation. An ecological site is a product of all of the environmental factors that influence the development of soils and vegetation, including disturbance regimes. Vegetation on an ecological site is not static and can be one of several seral communities, with the same species present but in different amounts. If the biological, physical, or chemical conditions that create an ecological site change, the site can degrade to a different bio-physical state. Resilience refers to the ability of a plant community to return to prior composition and structure after a disturbance. Resistant communities, or one of their seral stages, occupy the site after it has been disturbed. Resistance refers to a plant community’s ability to avoid being changed from its current state.