Late Holocene Environmental Change in the Upper Gunnison Basin, Colorado (page 1 of 2)

Author: Steve Emslie, Dept of Biological Sciences, University of North Carolina, Wilmington.

Abstract

View of lower elevation communities in the Upper Gunnison Basin. The Gunnison River with its riparian habitats is visible in the foreground.

Paleoecological and archaeological research in the Upper Gunnison Basin, southwestern Colorado, is providing an environmental history of this intermountain region extending over the past 40,000 years. This region has long been recognized for its unusual ecological characteristics including an absence of plant and animal taxa that should occur here, but do not. For example, pinyon (Pinus edulis), ash (Fraxinus sp.), and ground cherry (Physalis sp.) are rare or absent from the Basin. Vertebrates that occupy the same elevational ranges and habitats as those within the Basin, but are absent here, include Woodhouse’s toad (Bufo woodhousei), Short-horned lizard (Phrynosoma douglassii), Collared lizard (Crotaphytus collaris), Western rattlesnake (Crotalus viridis), Sagebrush vole (Lemmiscus curtatus), and Thirteen-lined ground squirrel (Spermophilus tridecemlineatus) (Hammerson 1986; Fitzgerald et al. 1994). Fossil and archaeological evidence indicates that many of these species existed in the Basin during the late Pleistocene to middle Holocene (Emslie 1986; Stiger 1993). The development of these ecological anomalies, including local extirpations and isolation, probably has resulted from the unique topography and climate that characterizes the Basin. Pleistocene and Holocene climate change may have been important in altering the composition of Basin communities to produce these modern assemblages.

The Study Area

Map of the Upper Gunnison Basin as delineated by mountain passes and the continental divide.

The Upper Gunnison Basin encompasses an 11,000 km² area in southwestern Colorado within the southern Rocky Mountains and on the eastern edge of the Colorado Plateau. It has an elevational range of 2200-4300 m and no outlet lower than 2650 m except through the narrow gorge of the Black Canyon of the Gunnison to the west. No other large montane basin in Colorado is enclosed by similar geographic barriers and it is this feature that we believe has caused the development of the unusual climate, communities, and biogeographic patterns now present in the Basin. Click here for a map of current vegetation zones and paleoecological sites used in this study.

Partly due to its topography, the Upper Gunnison Basin experiences unusual climate compared to other regions in Colorado. The winters often are extremely cold as cool air settles into the Basin (the record minimum temperature is -42.7° C in January), and the average annual temperature is 3.1° C. In addition, the area is drier than other regions at this elevation and has an annual average precipitation of 27 cm. For comparison, the state annual average temperature is 7° C and precipitation is 38 cm for montane shrublands at 1675 to 2600 m elevation (Fitzgerald et al. 1994). It is possible that these factors (temperature, moisture, topography), or an interaction of these factors, have been responsible for the composition of Basin plant communities that are dominated by sagebrush (Artemisia sp.) at elevations from 2800 to 3100 m, a range where pinyon-juniper forests prevail in areas outside of the Basin. At elevations between 3100 to 3600 m the Basin has a variety of forests that include juniper (Juniperus utahensis), aspen (Populus tremuloides), ponderosa (Pinus ponderosa), lodgepole (P. contorta), bristlecone (P. aristata), and limber pines (P. flexilis), blue and Engelmann spruce (Picea pungens and P. engelmannii), and Douglas fir (Pseudotsuga menziesii); alpine vegetation occurs above 4200 m. In addition, historic records indicate that the interior Basin habitats have remained relatively unchanged and undisturbed since the area was first explored and surveyed by the U. S. Government in 1853 (Beckwith 1854).

A small cave in the Upper Gunnison Basin where ancient packrat middens have been recovered.

A total of 45 preserved packrat middens were located and sampled throughout the Upper Gunnison Basin during field surveys in August and September 1996. Excavation of cave deposits at two sites, Haystack and Cement Creek Cave, have supplemented the midden record with a rich collection of late Pleistocene and Holocene vertebrate remains. These collections indicate that Basin environments in the late Pleistocene consisted of alpine tundra and subalpine forests at elevations of 2500-3000 m.

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