Late Holocene Environments

Late Holocene insect records from the Colorado Front Range show a progression from warmer-than-modern to cooler-than-modern summers, and back to warm again. At 3000 yr b.p., the calibrated TMAX estimate from Lake Isabelle was 1.8°C above the modern value. An assemblage just a few decades younger (and in fact, overlapping in radiocarbon age) from Longs Peak Inn yielded a calibrated TMAX estimate 0.3°C cooler than modern levels. Mean summer temperatures apparently remained near modern levels until sometime after 2400 yr b.p. A brief warming pulse was inferred from a 900-yr-B.p. assemblage from Mount Ida Bog, then temperatures returned to near-modern levels by 400 yr b.p. Winter temperatures finally warmed to near modern levels at 900 yr b.p., then cooled again by 400 yr b.p. The 900-yr-B.p. warming may correspond to what historians refer to as the "Medieval warm period." The subsequent cooling, or "Little Ice Age," is suggested by the cooling in summer temperatures, but it is more strongly indicated by a cooling of mean January temperatures by perhaps 14°C below modern levels. Additional well-dated late Holocene insect assemblages are needed to clarify the timing and intensity of climatic change during the last few thousand years.

It would be appropriate to compare the paleotemperature estimates derived from the fossil insect data to estimates derived from other proxies, such as tree rings, pollen, and glacier mass balance reconstructions. However, this is not possible at the present time for the following reasons. Tree-ring research in the Colorado Front Range has largely been limited to studies of the past few centuries (Mast et al. 1998; Veblen et al. 2000) and has focused more on reconstruction of drought episodes than on paleotemperatures (Woodhouse 2001). Pollen studies from this region have only provided general outlines of changes in temperature regime; few quantitative temperature estimates have been attempted. One exception to this was Fall's (1997) study of tree line fluctuations on the western slope of the Colorado Rockies. Fall estimated that temperatures near the upper tree line in her study region were 2-5°C cooler than modern temperatures before 11,000 yr b.p. She also estimated that mean July temperatures were 1-2°C warmer than modern levels between 9000 and 4000 yr b.p. She attributes a downslope shift in tree line from 6000-4000 yr b.p. to a decrease in effective moisture, rather than to changing temperatures. She interpreted pollen records from the last 2000 years as being indicative of essentially modern climatic conditions.

The reconstruction of paleotemperatures in the Colorado Rockies, based on past glacial limits, is hindered by several factors. First, this sector of the Rocky Moun tains has apparently been relatively dry throughout at least the Late Pleistocene, so montane glaciers here have never been as extensive as they have been in the Central and Northern Rockies (Elias 1996a). Second, even in locations where past glacial limits have been mapped and dated (Leonard 1989), it is quite difficult to derive paleotemperature estimates from these limits. The reason for this difficulty is that the size of glaciers is controlled by multiple factors, including temperature, precipitation, slope, and aspect. Without sufficient moisture, large glaciers cannot become established, even during intervals of prolonged low temperatures, such as glacial stadials.

There are some important biotic lessons to be gleaned from the fossil record of the Colorado Front Range. First, it is evident that the Pinedale glaciation exerted long-term effects on the shaping of biotic communities. For instance, postglacial communities were limited to the species able to survive the Pinedale glaciation and become reestablished in the alpine zone following deglaciation. This means that the current group of species in the alpine ecosystem may not be the best fit for the environment—they are simply the best fit among those species able to persist regionally through the last glacial cycle. We have no measure of past versus present species diversity in alpine tundra plant communities, because the pollen of the alpine plant species that is preserved in the fossil record can only be identified to the generic or family level, in most cases. However, because alpine tundra now exists in "habitat islands" rather than in continuous belts along the Rockies, postglacial colonization by alpine tundra species would have been made more difficult. This is in contrast to the montane and subalpine vegetation, which exists in more-or-less continuous belts along elevational zones in the Rockies.

Second, there is some fossil and modern evidence that the ecotone between the alpine and subalpine ecosystems is not in equilibrium with the modern climate, but rather is a relict of a prior warming event in which the tree line migrated upslope to its current elevation. Burned patches of forest that occur near tree line have been very slow to recover. New seedling establishment in these areas appears to be much poorer than it would be if these upper forest stands were truly in equilibrium with modern climate.

There is also good evidence that postglacial warming took place 500-1000 years in advance of the ultimate upslope migration of the tree line in the early Holocene. At sites such as La Poudre Pass and Long Lake, the evidence for the establishment of subalpine forest stands near the elevation of the modern tree line begins at about 9000 yr b.p. On the western slope of the Rockies, however, Fall (1997) found pollen evidence for subalpine trees becoming established at modern tree line elevations as early as 10,000 yr b.p. Perhaps differences in precipitation account for the differences in the timing of establishment of trees near their elevational limit between the western and eastern slopes of the Colorado Rockies. In some regions, however, it appears that at the century to millennial timescale, the response of major components of the vegetation in high altitude ecosystems of the Colorado Front Range lags behind major temperature changes.

Finally, the fossil insect record indicates that during the last 14,000 years, regional climates have often changed abruptly, almost in a stepwise fashion between major thermal regimes. The more gradual temperature changes previously inter preted from regional palynological studies now appear to be an artifact of vegetation response lag, specifically the lag in response of trees growing near tree line to changing temperature regimes.

Acknowledgments I thank Elyse Ackerman-Salazar, who prepared the Sky Pond samples for fossil insect identification, and Dr. Mel Reasoner, Brunel University, London, who coordinated the collection of sediment cores from Sky Pond. Kathy Anderson prepared the climate envelopes for beetle species found in the fossil assemblages. Financial support for Front Range paleoecological research has come from Long-Term Ecological Research grants from the National Science Foundation, DEB-9211776 and DEB-9810218.

References

Antevs, E. 1948. Climatic changes and pre-white man. University of Utah Bulletin 38: 168-191.

Bartlein, P. J., B. Lipsitz, and R. S. Thompson. 1994. Modern climate data for paleoenviron-mental interpretations. American Quaternary Association Thirteenth Biennial Meeting, Program and Abstracts, 197.

Benedict, J. B. 1979. Getting away from it all: A study of man, mountains and the two-drought altithermal. Southwestern Lore 45: 1-12.

Berger, A. L. 1978. Long-term variations in caloric insolation resulting from the earth's orbital elements. Quaternary Research 9: 139-167.

Carrara, P. E., W. N. Mode, M. Rubin, and S. W. Robinson. 1984. Deglaciation and postglacial timberline in the San Juan Mountains, Colorado. Quaternary Research 21: 42-55.

Coope, G. R. 1977. Fossil Coleopteran assemblages as sensitive indicators of climatic changes during the Devensian (Last) cold stage. Philosophical Transactions of the Royal Society of London, Series B 280: 313-340.

Coope, G. R., and G. Lemdahl. 1995. Regional differences in the Lateglacial climate of northern Europe based on coleopteran analysis. Journal of Quaternary Science 10: 391-395.

Crowson, R. A. 1981. The Biology of the Coleoptera. Academic Press, New York.

Elias, S. A. 1983. Paleoenvironmental interpretations of Holocene insect fossil assemblages from the La Poudre Pass site, northern Colorado Front Range. Palaeogeography, Palaeoclimatology, Palaeoecology 41: 87-102.

Elias, S. A. 1986. Fossil insect evidence for Late Pleistocene paleoenvironments of the Lamb Spring site, Colorado. Geoarchaeology 1: 381-386.

Elias, S. A. 1991. Insects and climate change: Fossil evidence from the Rocky Mountains. BioScience 41: 552-559.

Elias, S. A. 1994. Quaternary Insects and Their Environments. Smithsonian Institution Press, Washington D.C.

Elias, S. A. 1996a. Ice-Age Environments of National Parks in the Rocky Mountains. Smithsonian Institution Press, Washington, D.C.

Elias, S. A. 1996b. Late Pleistocene and Holocene seasonal temperatures reconstructed from fossil beetle assemblages in the Rocky Mountains. Quaternary Research 46: 311-318.

Elias, S. A. 2000. Late Pleistocene climates of Beringia, based on fossil beetle analysis. Quaternary Research 53: 229-235.

Elias, S. A., K. H. Anderson, and J. T. Andrews. 1996. Late Wisconsin climate in northeast ern USA and southeastern Canada, reconstructed from fossil beetle assemblages. Journal of Quaternary Science 11: 417-421.

Fall, P. L. 1985. Holocene dynamics of the subalpine forest in central Colorado. American Association of Stratigraphic Palynologists Contribution Series 16: 31-46.

Fall, P. L. 1997. Timberline fluctuations and late Quaternary paleoclimates in the Southern Rocky Mountains, Colorado. Geological Society of America Bulletin 109: 1306-1320.

Legg, T. E., and R. G. Baker. 1980. Palynology of Pinedale sediments, Devlins Park, Boulder County, Colorado. Arctic and Alpine Research 12: 319-333.

Leonard, E. M. 1989. Climatic change in the Colorado Rocky Mountains—Estimates based on modern climate at Late Pleistocene equilibrium lines. Arctic and Alpine Research 21: 245-255.

Madole, R. F., and R. R. Shroba. 1979. Till sequence and soil development in the North St. Vrain drainage basin, east slope, Front Range, Colorado. Pages 124-178 in F. G. Ethridge, editor. Guidebook for Postmeeting Field Trips Held in Conjunction with the 32nd Annual Meeting of the Rocky Mountain Section of the Geological Society of America, May 26-27, 1979, Colorado State University. Geological Society of America, Boulder, Colorado.

Mast, J. N., T. T. Veblen, and Y. B. Linhart. 1998. Disturbance and climatic influences on age structure of ponderosa pine at the pine/grassland ecotone, Colorado Front Range. Journal of Biogeography 25: 743-755.

Nelson, A. R., A. C. Millington, J. T. Andrews, and H. Nichols. 1979. Radiocarbon-dated upper Pleistocene glacial sequence, Fraser Valley, Colorado Front Range. Geology 7: 410-414.

Pierce, K. L., and J. D. Good. 1992. Field guide to the Quaternary geology of Jackson Hole, Wyoming. U.S. Geological Survey Open File Report 92-504, 54 pp.

Reasoner, M. A., G. Osborn, and N. W. Ruter. 1994. Age of the Crowfoot advance in the Canadian Rocky Mountains: A glacial event coeval with the Younger Dryas oscillation. Geology 22: 439-442.

Short, S. K. 1985. Palynology of Holocene sediments, Colorado Front Range: Vegetation and treeline changes in the subalpine forest. American Association of Stratigraphic Paly-nologists Contribution Series 16: 7-30.

Short, S. K., and S. A. Elias. 1987. New pollen and beetle analysis at the Mary Jane site, Colorado: Evidence for Late-Glacial tundra conditions. Geological Society of America Bulletin 98: 540-548.

Veblen, T. T., T. Kitzberger, and J. Donnegan. 2000. Climatic and human influences on fire regimes in ponderosa pine forests in the Colorado Front Range. Ecological Applications 10: 1178-1195.

Woodhouse, C. A. 2001. A tree-ring reconstruction of streamflow for the Colorado Front Range. Journal of the American Water Resources Association 37: 561-569.

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