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

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

Results

A sample of packrat midden from Red Creek (RC2) that dates to 3320 B.P. Needles of lodgepole pine were recovered from this midden found in the lower basin.

Twenty packrat middens were selected for detailed analyses based on their age and location in the Upper Gunnison Basin. Conventional and accelerator mass spectrometry (AMS) radiocarbon analyses of packrat feces and individual plant remains indicate that the middens range in age from 3320 to 240 B. P. Plant taxa were identified from eight of the 20 middens. Conifer species that no longer occur in the midden area were identified from five of these localities. Most other plant taxa identified from each of the 20 middens do not represent differing environments than those found in the areas today.

Discussion

Previous investigations on the environmental history of plant communities over the last 10,000 years in this Basin have included analyses of stratigraphic sediments (Andrews et al. 1975), pollen (Andrews et al. 1975; Markgraf and Scott 1981; Fall 1985; Short 1986), and macrofossils (Andrews et al. 1975; Carrara et al. 1984; Stiger 1993). The pollen record at high elevations in the Basin indicates that the period 11,000 to 5,000 B.P. was characterized by upward expansion of subalpine forests during a warm, dry period, followed by cooler conditions and retraction of subalpine forests from 4000 to 3000 B.P. (Markgraf and Scott 1981; Fall 1985; Short 1986). Modern communities and environmental conditions appear to have developed by 2000-1500 B.P. In addition, pinyon trees (Pinus edulis) are notably absent throughout the Basin except for a few isolated trees or small stands that have been recolonizing the area in recent years; juniper occurs in numerous areas. Interestingly, based on archaeological and palynological evidence, pinyon trees are present in the basin from about 5000 to 3000 B.P. before becoming extirpated perhaps from climatic change (Stiger 1993). Two other plants, ash (cf. Fraxinus sp.) and ground cherry (cf. Physalis sp.), also have been identified from the archaeological record dated at 7650 B.P., respectively; neither of these taxa currently occur in the Upper Gunnison Basin and are found at lower elevations (< 2280 m) in Colorado.

Information obtained from ancient packrat middens collected in the Upper Gunnison Basin indicate the expansion of major forests at 3320 B. P. to lower elevations in the central Basin. This interpretation is based on the abundance of needles of ponderosa pine and lodgepole pine in the midden at Red Creek and west of Haystack Cave. Both sites have present vegetation dominated by sagebrush and a few scattered trees of juniper and Douglas fir. Although isolated stands of ponderosa pine occur within 1 km of RC 2, the nearest lodgepole pines are found approximately 15 km to the north at an elevation of 3100 m. This latter pine is a fire-adapted species and, while it can grow at all elevations up to 3658 m, the lowest elevation in which it occurs in Colorado is approximately 2300 m. Thus, lodgepole forests probably extended into lower elevations of the Upper Gunnison Basin prior to 3000 B. P. in response to cooler, and perhaps drier, conditions at that time.

The period from approximately 3000 to 1500 B. P. appears to have been relatively stable climatically and with conditions in the Basin similar to those at present. Pollen and plant remains from middens dating to this period show few differences from the plant communities at those localities today (Indian Creek, Ute Gulch, and Lost Canyon). Analysis of the midden at Palisades Site 2, however, indicates that a warm period occurred in the Gunnison Basin by 950 B. P. This change in climate is evinced by the pollen record from this midden which is dominated by sagebrush and weedy species and has a low frequency of pine species.

The warm event at 950 B. P. is followed by another cooling trend. Data presented here indicate movement of bristlecone and limber pine forests into lower elevations at approximately 240 and 560 B. P. in the northern and eastern edges of the Gunnison Basin, respectively. These species prefer cool and dry environments. The expansion of these forests into these areas correlates with global cooling events in the late 14th century and between A. D. 1500-1850 (the Little Ice Age; Grove 1988).

Further interpretations of paleoecological data gathered in this project are limited by lack of a larger sample size of middens of different ages. Preliminary studies completed here indicate that valuable information can be gained from the paleoecological record on local climate change in the Upper Gunnison Basin. As this information increases, a detailed record of community and climate change in this mountain environment will allow correlations with the archaeological record to assess human responses to environmental change in the Basin during the Holocene.

Acknowledgements

This project was funded by grants from the Colorado Historical Society and the City of Gunnison. We thank Curecanti National Recreation Area for providing access to park lands for midden surveys and collection, and to Mr. Sam Beckley for access to private lands. Radiocarbon dates were provided by Beta Analytic, Inc., and by Dr. Thomas Stafford (University of Colorado).

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