Colorado Plateau shrublands

• Category: Desert Biomes.
• Geographic Location: North America.
• Summary: This truly spectacular landscape of rugged red-rock sandstone plateaus and canyons was carved by the Colorado River and its tributaries. Although mainly semi-arid, this region supports a wide variety of plant and animal life.

The Colorado Plateau stretches across vast portions of southeastern Utah and western Colorado, and has extensions into northern Arizona and northwest New Mexico. This region is a semiarid landscape of rocky plateau and canyon with some of the most spectacular scenery in the world. Open woodland of pine and juniper, as well as low shrubland, blanket the landscape. The plateau's long human presence ranges from archaic hunter-gatherers to ancestral Pueblo cliff dwellers, and from the Navajo Nation to Mormon settlers and today's outdoors enthusiasts.

Geography

The sedimentary rock here was carved by the Colorado River and its tributaries to form sheer-walled canyons, buttes, mesas, badlands, and plains. Several large rivers flow through this region, mostly extending from the Rocky Mountains and draining through the Grand Canyon. Isolated mountain ranges are scattered throughout, reaching elevations of 11,500 feet (3,500 meters) where in the past they have supported glaciers. The wide elevation range, from mountaintops to the bottom of the Grand Canyon, creates a broad diversity of biological zones, from alpine meadows too cold to support trees down through montane forests, foothill woodlands, and desert scrub on dry, dusty plains and rocky flats.

Precipitation amounts range from a low of 5 to 8 inches (13 to 20 centimeters) per year in the 8 deserts and arid canyonlands, to almost 20 inches (50 centimeters) per year in the higher pinyon-juniper woodlands. The southern part of the ecoregion differs from the north in having a summer monsoonal precipitation pattern.

Vegetation

By virtue of its isolation, complex geology, specialized landforms, and range of elevation, the Colorado Plateau supports numerous species that are endemic, that is, found nowhere else in the world. More than 300 plant species are known to be endemic to this region. Two-needle pinyon pine and Utah juniper dominate among the tree types. These species occur as short trees and shrubs, either in dense clumps or in more open, rocky savannas. Pinyon-juniper woodlands and shrublands typically are found at elevations ranging from 4,000 to 8,000 feet (1,200 to 2,400 meters) above sea level. Severe climatic events, such as frosts and drought, are thought to limit the distribution of pinyon-juniper woodlands and scrub vegetation to relatively narrow elevation belts on mountainsides. Rocky Mountain juniper may replace Utah juniper at higher elevations.

At lower elevations, often where soils are very thin and drought-prone, dwarfed pine trees and short shrubs are common. Under the tree canopy may be low shrubs, grasses, or bare ground. Other common plants include greenleaf manzanita, big sagebrush, Gambel's oak, bitterbrush, and James's galleta grass.

Additional characteristic types of vegetation are found in the short shrublands, some dominated by blackbrush and/or Mormon tea. This scrub vegetation is abundant throughout the Colorado Plateau on rolling hills and gentle slopes at elevations of 1,800 to 5,000 feet (550 to 1,500 meters). Soils in these areas are shallow, alkaline, and gravelly or sandy, lying over sandstone or limestone bedrock. Other common plants include Torrey's jointfir, spiny hopsage, and (especially where soils are sandy) sand sagebrush. Where the bedrock is mostly limestone, it is common to see black sagebrush or Bigelow sage. Semi-arid grasses such as Indian ricegrass, purple threeawn, blue grama, and muttongrass are common.

In arid environments, water evaporation often leads to salt accumulation on the soil surface, limiting the plant species that can survive there. Throughout the Colorado Plateau, saline basins often include open shrubland composed of one or more saltbush species, such as four-wing saltbush, shadscale, cattle saltbush, or spinescale. Other shrubs may include Wyoming big sagebrush, rubber rabbitbrush, and winterfat.

Wildlife Fauna

Of known animal species that dwell on the Colorado Plateau, there are various species of mammals, reptiles, amphibians, and fish and more than 250 species of birds. A number of once-common native animal species on the Colorado Plateau have become increasingly rare, and some, particularly native freshwater fish, have become extinct.

Due to natural endemism and dynamic land-use history, the Colorado Plateau supports some 170 plant and 70 wildlife species listed under the U.S. Endangered Species Act and more than 200 other species considered by various scientific authorities to be vulnerable. Mammals that may be found in various parts of the Plateau are: coyote, gray fox, ringtail, long-tailed weasel, badger, mountain lion, bobcat, mule deer, bighorn sheep, marmot, antelope, desert woodrat, long-tailed vole, porcupine, and jack rabbit. Among the birds here are Mexican spotted owl, turkey vulture, golden eagle, red-tailed hawk, northern harrier, American kestrel, and blue grouse. Collared lizard, long-nosed leopard lizard, short-horned lizard, and sagebrush lizard number among the reptiles, along with tiger salamander, Great Plains toad, and Great Basin spadefoot toad. Representative fish include cutthroat trout, Apache trout, and mottled sculpin.

Human Impact

People have lived throughout the Colorado Plateau for perhaps 15,000 years, with the earliest known inhabitants being hunter-gatherer societies. Some researchers believe that the introduction of ancient human societies and their big-game hunting led to the extinction of various large animals across the region. These prehistoric animals included mammoths, mastodons, camels, protohorses, a stately deer called the stag-moose, as well as giant ground sloths and beavers the size of today's black bears. These large herbivores were prey for the also-large carnivores: saber-toothed cats, savage short-faced bears, cheetahs, maned lions, and dire wolves. It is still debated how these and scores of other large animal species underwent a mass extinction around 13,000 years ago—roughly the same time this region first became populated by humans. Regardless, these first people continued to exist here in small numbers.

Farming as a primary subsistence strategy began in the Colorado Plateau shrublands about 2,000 years ago. While most of the region is marginal for agriculture, indigenous communities continued to use mountains and intermountain valleys for hunting and gathering wild food plants. Among the best-known early societies was that of the ancestral Pueblo, who built cliff dwellings among overhanging sandstone throughout the region's canyons. Mesa Verde National Park protects some excellent examples; these dwellings provided both safety and comfort while maintaining access to nearby water sources.

Only in the past 150 to 200 years has human existence on the plateau impacted the environment in a way that threatens this ecoregion. Colonization and settlement by peoples of European decent began in the 1850s, largely by followers of the Mormon Church who had established a new base at Salt Lake City, Utah. At that time, many Indigenous communities, including the Navajo and Hopi nations, were resettled within tribal reservations. This period through the early twentieth century saw the rapid increase in pressure on lands and waters throughout the Colorado Plateau.

Grazing by domestic cattle and sheep reached their peak of intensity around the turn of the twentieth century, causing severe soil compaction and erosion, as well as trampling damage to vegetation. Timber harvesting and mining also occurred throughout surrounding mountain ranges, leading to larger-scale mining activities, especially for coal and uranium. The water resources of the Colorado River and its tributaries were harnessed through dams (large and small) and water diversions for intensive agriculture. Lake Powell here, named for 19th-century explorer John Wesley Powell, is among the largest of Western reservoirs. It resulted from the construction of the Glen Canyon Dam along the Colorado River at the Utah-Arizona border.

Much of what one sees today throughout the Colorado Plateau is the legacy of the past 150 years of intensified land use. On the whole, this region appears to be largely undeveloped, but past overgrazing, infrastructure development, and other land uses have caused biodiversity losses, decreased soil productivity, and the introduction of invasive plant and animal species. Among the many invasive plant species, annual grasses have become ubiquitous in many semi-desert shrublands. In many cases, the introduction of these fine fuels has enabled wildfires to occur at unprecedented frequencies and intensities. Despite this, around 54 percent of the shrublands remain intact even though only about 11 percent is protected.

Along rivers, invasive tree and shrub species have in many cases replaced the native cottonwood-willow vegetation. Overlaying these legacies are the effects of more recent development. In recent decades, an influx of new residents to small towns has put pressure on local infrastructure. This population growth has been accompanied by a surge in recreational tourism, including hikers, bikers, and climbers who find their ways to once-isolated corners of the region.

Today, tribal lands encompass more than one-third of the region. Nearly half of the regional landscape is under public ownership, with the U.S. Bureau of Land Management being the largest federal land manager, along with national forests and parks. Several world-renowned parks characterize the Colorado Plateau, including the Grand Canyon, Arches, Canyonlands, and Mesa Verde National Parks.

Global warming effects on the Colorado Plateau shrublands have been severe. A 2011 study found that even within the national parks, which are the most protected parts of the region, temperature increases over the previous twenty years had led to the decline of some types of shrubs and the increase of others. The researchers suggested that the impact of these changes would likely be even greater on the areas that were not managed by the parks service.

In addition, the region generally has suffered drought on a roughly twenty-year natural cycle (tied to the Pacific Decadal Oscillation, a sea-surface temperature cycle). However, the Western United States has been sweltering under a protracted megadrought since 2000. As of 2024, forecasters believe the warmer weather and lack of rain will continue, potentially damaging much of the plants, grasses, and shrubs that make up the environment. In turn, this will have a negative impact on the region’s wildlife, particularly bird species.

Bibliography

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Blakey, Ron and Wayne Ranney. Ancient Landscapes of the Colorado Plateau. Grand Canyon Association, 2008.

Calvo, Christopher. “Climate Change on the Southern Colorado Plateau.” National Park Service, Southern Colorado Plateau Network I&M Program, 12 Jan. 2023, www.nps.gov/articles/southern-colorado-climate-change.htm. Accessed 14 Nov. 2024.

Duniway, Michael C., et al. "Generalizing Ecological Site Concepts of the Colorado Plateau for Landscape-Level Applications." Rangelands, vol. 38, no. 6, 2016, pp. 342-349. doi.org/10.1016/j.rala.2016.10.010. Accessed 14 Nov. 2024.

"Localized Drought Impacts on Northern Colorado Plateau Landbirds."National Park Service, 14 Mar. 2022, www.nps.gov/articles/000/ncpn‗birds-and-drought.htm. Accessed 1 Aug. 2022.

Munson, Seth M., et al. "On the Brink of Change: Plant Responses to Climate on the Colorado Plateau." Ecosphere, vol. 2, no. 6, 2011, pp. 1–15, doi:10.1890/ES11-00059.1. Accessed 29 June 2018.

Primm, S. "Colorado Plateau Shrublands." World Wildlife Foundation, www.worldwildlife.org/ecoregions/na1304. Accessed 29 June 2018.

Schwinning, Susan, Jayne Belnap, David R. Bowling, and James R. Ehleringer. “Sensitivity of the Colorado Plateau to Change: Climate, Ecosystems, and Society.” Ecology and Society, vol. 13, no. 2, 2008.

Waring, Gwendolyn. L. A Natural History of the Intermountain West: Its Ecological and Evolutionary Story. University of Utah Press, 2011.

Wikramanayake, Eric. “Eastern Himalayan Alpine Shrub and Meadows.” One Earth, www.oneearth.org/ecoregions/eastern-himalayan-alpine-shrub-and-meadows/. Accessed 14 November 2024.

Wilkinson, Charles F. Fire on the Plateau: Conflict and Endurance in the American Southwest. Island Press, 1999.