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Gunnison Basin

 Upper Gunnison Basin, Colorado Plateau

 Gunnison Basin

Gunnison

 

 

 

  

The Upper Gunnison Basin is a high-elevation valley situated at the eastern edge of the Colorado Plateau . Rimmed by some of the highest ranges of the southern Rocky Mountains, the only outlet lower than 10,000 feet elevation is to the west where the Gunnison River passes through the Black Canyon. This narrow gorge appears to act as a filter for the movement of lower-elevation species into and out of the basin. Ecologically, the flora and fauna in the Upper Gunnison Basin appear to be depauperate in taxa,and the region has been recognized for its unique biogeographic characteristics.

 
The biotic communities of the basin include sagebrush-steppe, which is particularly prevalent from 7,500 feet to 9,000 feet elevation.

Gunnison Basin
Sagebrush-steppe

 

 Gunnison Basin

Mixed-conifer forests dominated by Douglas-fir, subalpine forests of primarily Engelmann spruce and subalpine fir, subalpine grasslands, and alpine tundra blanket the surrounding ranges. Aspen occurs across a wide range of elevations forming both serial and persistent stands. Forests of lodgepole pine are common in the northern half of the basin. Notable is the lack of pinyon-juniper woodlands in the basin. Though Rocky Mountain juniper occurs on rocky outcrops at lower elevations, pinyon pine is absent. Fossil pollen, macrofossils, and radiocarbon-dated charcoal found within archaeological features indicate pinyon pine was common in the basin from 8,000 to 3,000 years before present. Similarly, ponderosa pine has a limited distribution in the Upper Gunnison Basin, though archeological excavation of ponderosa pine charcoal suggests it to was once more widespread.

 

Humans have occupied the Upper Gunnison Basin for nearly 10,000 years. Based on collective radiocarbon dates and the diversity of archaeological features, prehistoric occupation peaked approximately 6,000 years before present when the climate was warmer and moister and pinyon pine woodlands existed within the basin. The impact of these people on the basin's biota is unknown. More recently, the Uncompahgre Utes hunted in the Upper Gunnison Basin in the summer while spending winters at lower elevations. It is possible that the Spanish visited the Upper Gunnison Basin as early as the 1500s and on through the 1700s, but they left no trace of their presence. 

Gunnison Basin 

 

 Gunnison Basin

On the semi-arid Colorado Plateau elevation is an important factor in determining what assemblages of species or biotic communities occur in a given location. As one moves upward in elevation temperatures generally decrease and precipitation increases. C. Hart Merriam's studies of the biota from the top of the San Francisco Peaks at 12,600 feet to the bottom of Grand Canyon at less than 3000 feet impressed upon the scientific community the importance of elevation and latitude on the location of biotic communities. Subsequent studies have revealed that elevation is only one of a complex of factors that determine the presence or absence of species, populations, and communities. These factors include aspect (i.e., which direction the slope is facing), wildfire history and frequency, and soil type.
 

Evidence from paleoecological studies suggest that elevational ranges for particular species have fluctuated over time in response to major climatic changes. For example, today on the San Francisco Peaks, subalpine forests of Engelmann spruce generally grow from 9500 to about 11,500 feet (see figure above). At the height of the last glaciation (about 20,000 years ago) this species grew as low as 7000 feet, and probably not much higher than 9000 feet, far lower in than its current distribution. Timberline in the region, which today is about 12,000 feet, was apparently much higher during much of the middle Holocene (7500-3000 years ago) and a bit lower than today about 1500 years ago (see figure below). These significant fluctuations are a result of species responding to the dynamic nature of the earth's climate over time.

Gunnison Basin 

 

San Francisco Peaks, Arizona 
Introduction

Gunnison Basin

Rising more than a mile above the surrounding pine forestsand grasslands of northern Arizona, the San Francisco Peaks are a prominent feature of the southern Colorado Plateau. Thought by some geologists to be the remains of a large strato-volcano similar to Mt. St. Helens in Washington State, the Peaks rise in dramatic isolation to over 12,000 feet. They are often visible from more than one hundred miles away.The Peaks and much of the surrounding Coconino National Forest are managed by the United States Forest Service. Traditional land uses of the area have included grazing of livestock, logging, and mining of cinders and pumice. Today, recreation and tourism, including downhill skiing at the Arizona Snowbowl, have become the most prominent land-uses of the Peaks. The mountain's caldera, known as the Inner Basin, contains an aquifer that supplies much of the municipal water for the city of Flagstaff, the largest city on the Colorado Plateau. Water is piped southward to the city from a series of wells tapping the basin's aquifer, which is recharged by seasonal snowmelt.Widespread timber harvest supplied lumber and fuel for the pioneers, and extensive clearing circled each settlement. Cattle were introduced in the 1870s and ranching continues today. Cattle subsist on range by summer, and the impacts of grazing are widespread. Riparian habitats of cottonwoods and willows have been greatly altered by the irrigation of hay meadows and cattle usage.At the headwaters of the Colorado River drainage, the Upper Gunnison Basin is the only major watershed not tapped by metropolitan water concerns to the east.

Today, over 85% of the Upper Gunnison Basin is federal land managed by the Park Service, Bureau of Land Management, and Forest Service. Seven Forest Service wilderness areas and a Bureau of Land Management primitive area afford protection to diverse high-elevation landscapes within the basin. Curecanti National Recreation Area preserves a fringe of lower-elevation terrain surrounding Blue Mesa Lake. Grazing continues across much of the basin, though range condition has improved since the 1970s, in part through fire management and reduced cattle utilization. Irrigation of stream-side hay fields and cattle usage continue to degrade riparian vegetation at lower elevations. Timber harvest is minimal, and the impact of logging during the mining era has largely been erased. Presently, recreation and tourism is a prevalent land use in the Upper Gunnison Basin.

Biotic Communities of the Colorado Plateau
The Community Concept

In the late 1880s, C. Hart Merriam surveyed the plant life of the San Francisco Peaks near Flagstaff, Arizona, for the federal Biological Survey. Merriam observed that from the top of the Peaks to the bottom of Grand Canyon, vegetative communities tended to occur along an elevation range. He sought to understand what factors influenced the distribution of the region’s vegetation, and began to categorize distinctive vegetational belts or "life zones."  

Ecologists and biogeographers still categorize ecological communities based on dominant vegetation, but modern classification systems are more complex than Merriam's. In this website we follow the "biotic communities"  concepts of David E. Brown, which include both plant and animal life. (See reference below; for a list of biotic communities of the Colorado Plateau, see the left-side menu). For example, Abert's squirrel can be readily linked to its habitat of ponderosa pine forest while pinyon jays are primarily adapted to the region's expansive pinyon-juniper woodlands.

Modern ecology holds that all ecological communities are composed of individual species whose ecological tolerances happen to overlap, forming what appears to be a specific zone of vegetation. In other words, individual species have over time evolved their own ecological tolerances to different factors such as precipitation, soil types, shade, fires, temperature, etc. Though gambel oak and ponderosa pine commonly live in the same biotic community, if environmental conditions were to change each of these species would respond individualistically, and perhaps would end up in new, different communities. Paleoecological studies on the Colorado Plateau indicate that limber pine may have been one of the region’s lower elevation pines in the past, living among juniper and pinyon-pine in a niche that is today occupied by ponderosa pine. Conditions have since changed, and limber pine is now predominantly seen growing in the subalpine conifer forest at high elevations. The pine has found a new niche, and is no longer a part of modern pinyon-juniper woodland or a ponderosa pine forest biotic communities.

For more information, see: C Hart Merriam and the Life Zones Concept
Map of the Biotic Communities of the Greater Grand Canyon Region.

Resource: Brown, D. E. 1994. Biotic communities of the Southwestern United States and northwestern Mexico. University of Utah Press, Salt Lake City, 342 pp. Research: Paleobotany and Paleoclimate of the Southern Colorado Plateau. The biota of the Colorado Plateau during the middle (50,000-27,500 B.P.) and late (27,500-14,000 B.P.) Wisconsin time periods was dramatically different from that seen today. Differences were primarily a result of major climate changes associated with the last major glacial period. This site examines the environment of the southern plateau during this time. Adapted by R. Scott Anderson from his journal article. 

Elevational Range

Research: Late Holocene Environmental Change in the Upper Gunnison Basin, Colorado. The Upper Gunnison Basin is a high elevation (7,500 to 14,000 feet) region on the edge of the Colorado Plateau in southwestern Colorado. Its unusual ecological characteristics include an absence of plant and animal taxa that should occur here. Fossil and archaeological evidence indicates that many of the missing species existed in the Basin during the late Pleistocene to middle Holocene.

Author: J. D. Carpenter


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