Introduction

Ecosystems are the products of regional biotic history, shaped by environmental changes that have occurred over thousands of years. Accordingly, ecological changes take place at many timescales, but perhaps none is more significant than the truly long-term scale of centuries and millennia, for it is at these timescales that ecosystems form, break apart, and reform in new configurations. This is certainly true in the alpine regions, where glaciations have dominated the landscape for perhaps 90% of the last 2.5 million years (Elias 1996a). In the alpine tundra zone, the periods between ice ages have been relatively brief (10,000-15,000 years), whereas glaciations have been long (90,000-100,000 years). Glacial ice has been the dominant force in shaping alpine landscapes. Glacial climate has been the filter through which the alpine biota has had to pass repeatedly in the Pleistocene.

This chapter discusses climatic events during the last 25,000 years (figure 18.1). At the beginning of this interval, temperatures cooled throughout most of the Northern Hemisphere, culminating in the last glacial maximum (LGM), about 20,000-18,000 yr b.p. (radiocarbon years before present). The Laurentide and Cordilleran ice sheets advanced southward, covering most of Canada and the northern tier of the United States. Glaciers also crept down from mountaintops to fill high valleys in the Rocky Mountains. In the Southern Rockies, the alpine tundra zone crept downslope into what is now the subalpine, beyond the reach of the relatively small montane glaciers. By about 14,000 yr b.p., the glacier margins began to recede, leading eventually to the postglacial environments of the Holocene. It is now becoming apparent that the climate changes that drove these events were sur-

Geologic Epoch

Physical environment

MCR temperature reconstructions

Vegetation history

Chronology (,4C yr BP)

Holocene

Glaciers expand during Little Ice Age

Glaciers retreat to high mountain cirques; postglacial alpine soils begin to develop

TMAX oscillating with warm intervals at 3500 and 900 yr BP; cold intervals at 3000 and 200 yr BP

Treeline descends after 4000 yr BR then remains + stable after 2000 yr BR

TMAX > modern from ,0,000 to 5300 yr BP

5000

Treeline rises to modern levels and beyond, from 9000-4000 yr BP

Late Glacial Interstadial

Late Pleistocene

Glacial margins retreating

Beetles Indicate rapid warming by 13,200 yr BP

No upslope movement of forests

Mountain glaciers cover regions above 2450 m; widespread permafrost in high elevations

Very cold, dry climate on Eastern Slope; TMAX depressed by 10-11oC

Treeline depressed by 300-700m TMAX depressed by 2-5oC

20,000

Figure 18.1 Summary of geologic epochs in the Late Quaternary, associated paleoenviron-mental changes at high elevations in Colorado, and radiocarbon chronology. TMAX stands for the mean temperature of the warmest month of the year. Glaciological data are from Madole and Shroba (1979). MCR temperature reconstructions are from this chapter. Vegetation history data is from Fall (1997).

prisingly rapid and intense. This chapter examines the evidence for these climatic changes and the biotic response to them in the alpine zone of Colorado.

To reconstruct the environmental changes of this period, we must rely on proxy data, that is, the fossil record of plants and animals, combined with geologic evidence, such as the age and location of glacial moraines in mountain valleys. As of this writing, the principal biological proxy data that have been studied in the Rocky Mountains are fossil pollen and insects. This chapter focuses mainly on the fossil insect record because it has supplied quantitative estimates of past climates. For the most part, pollen analysis in this region has provided only qualitative climate reconstructions, although there are exceptions (i.e., Fall 1997).

372 Century to Millennial Timescale Methods

The remains of beetles are very valuable as proxy data, that is, as indirect evidence for past environmental conditions. Beetles are the largest order of insects. They have been the main insect group studied from Quaternary sediments, and, in fact, they are the most diverse group of organisms on Earth, with more than one million species known to science (Crowson 1981). In addition, their exoskeletons, reinforced with chitin, are extremely robust and are commonly preserved in large numbers in lake sediments, peats, and other types of deposits. In most cases, beetles have quite specialized habitats that apparently have not changed appreciably during the Quaternary (Elias 1994). This characteristic makes them excellent environmental indicators. The exoskeletons of beetles and some other insects are covered with exquisite microsculpture, enabling paleontologists to identify fossil exoskele-tons to the species level in at least half of all preserved specimens, even though insect exoskeletons are most often broken up into the individual plates in fossil specimens.

Beetles are very quick to colonize a region when suitable habitats become available. They often respond more quickly than plants, which, until recently, were relied on almost exclusively as indicators of environmental change on land. Like plant macrofossils, insect fossils are generally deposited in the catchment basin in which the specimens lived. Thus they provide a record of local conditions, in contrast to pollen, which can be carried many miles on winds and often gives a more regional "signal."

Studies of insect fossils in two-million-year-old deposits from the high arctic have failed to show any significant evidence of either species evolution or extinction. Beetle species have apparently remained constant for as many as several million generations (Elias 1994)

Insect fossils are generally extracted from organic-rich lake or pond sediments or peats. Ancient stream flotsam, deposited in fluvial sediments and later exposed along stream banks, is often a rich source of insect fossils.

Insect fossil data are usually presented as minimum numbers of individuals for each species identified. Paleoclimatic reconstructions are generally made on the basis of the climatic conditions in the region where the species in a given assemblage can be found living together today, that is, the climate of the region where their modern distributions overlap. This method has recently been refined, by focusing on the climatic conditions associated with beetle species' modern ranges (the "climate envelope" of the species), rather than on the geographic overlap of their modern distributions. This is called the Mutual Climatic Range (MCR) technique.

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