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Postglacial History of Lost Trail Pass Bog, Bitterroot Mountains, Montana

Peter J. Mehringer, Jr., Stephen F. Arno and Kenneth L. Petersen
Arctic and Alpine Research
Vol. 9, No. 4 (Nov., 1977), pp. 345-368
DOI: 10.2307/1550528
Stable URL: http://www.jstor.org/stable/1550528
Page Count: 24
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Postglacial History of Lost Trail Pass Bog, Bitterroot Mountains, Montana
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Abstract

Studies of sediment, chronology, fossil pollen and charcoal from cores from Lost Trail Pass Bog (2152 m) provide the first postglacial bog, forest, and fire history for the Bitterroot Mountains. The 6.27 m of sediment, dated by 16 radiocarbon dates and two volcanic ashes, represent lake, fen, and bog deposition spanning the last 12,000 yr. Lycopodium spores were introduced as tracers into the 81 constant-volume samples to estimate pollen and charcoal influx. Because of considerable variation between samples, pollen and charcoal estimates were averaged by pollen zones. Glacial ice withdrew leaving a lake by 12,000 yr ago and sagebrush steppe dominated the landscape for the next 400 to 500 yr. If lodgepole and whitebark pine are the diploxylon and haploxylon pine pollen in the record, then by 11,500 yr ago whitebark pine forests replaced the steppe and persisted for the next 3000 to 4000 yr under climatic conditions that were probably cooler than present. Two falls of Glacier Peak volcanic ash, separated by less than 25 yr, occurred about 11,250 BP. By 7000 yr ago, under warmer but not necessarily drier climatic conditions, Douglas-fir and lodgepole pine replaced whitebark pine and charcoal influx increased. The fall of Mazama volcanic ash was dated at about 6700 yr ago. By 5000 yr ago aquatic, fen, and bog microfossils became important. With the return to cooler climates, about 4000 yr ago, Douglas-fir was no longer common in the pine forest. Little vegetational change is indicated after 4000 years ago. However, more charcoal was deposited during the last 2000 yr than during the previous 9500 yr.

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