Paleoenvironmental Interpretations of Holocene Insect Fossil Assemblages from Four High-Altitude Sites in the Front Range, Colorado, U.S.A.

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Abstract
A series of 4 Holocene age sites in the Front Range, Colorado, forms an altitudinal transect from subalpine forest to alpine tundra in the modern environment. Insect fossil assemblages from these sites were analyzed for paleoenvironmental changes, including tree-limit shifts. Tree-limit altitudes have been inferred from the forest-tundra insect species ratios of the fossil assemblages, supplemented by plant macrofossil data. A climatic optimum is suggested by maximal forest-tundra ratios between 9000 and .apprx. 7000 B.P. From 7000-4500 B.P. the insect evidence suggests the continued nearby presence of conifers at the sites, as supported by the plant macrofossil record. Faunal evidence indicates a tree-limit decline at 4500 B.P. Declining forest-tundra insect ratios, combined with the conifer macrofossil record, suggest a climatic deterioration from 4500-3100 B.P. followed by a rapid amelioration, from 3000-2000 B.P. A gradual decline in the forest-tundra ratios occurred after 2000 B.P., reaching 1:1 ratios at or before 1000 B.P. The timing of Holocene changes was broadly similar to the chronology of changes from the insect fossil record at La Poudre Pass and a north-south transect of Holocene paleobotanical sites from Alberta [Canada] to southern Colorado.