POSTGLACIAL VEGETATION AND FIRE HISTORY OF THE SOUTHERN MISSION VALLEY, NORTHWESTERN MONTANA

Although much is known about the environmental history of the Yellowstone region, comparatively little is known about the vegetation and fire history of the Rocky Mountains of northwestern Montana. The Mission Range was intensively glaciated in late-Pleistocene time, and has been occupied by humans for at least 10,000 years. These two factors make the region an ideal location for researching how climate and humans may have affected vegetation and fire regimes during the late-glacial period through the Holocene. Pollen and charcoal records from a 7.25-m-long core from East Twin Lake were used to reconstruct the vegetation and fire history of the southern Mission Valley, Montana. The pollen and charcoal data show an abundance of Pinus (P. albicaulis or monticola) Artemisia, Poaceae, and Alnus pollen prior to 14,300 cal yr BP, suggesting an open landscape with shrubs and grass, cold relatively dry conditions, and an absence of fire activity. Increased percentages of Pinus (P. Ponderosa or contorta), Picea, and Abies pollen at 14,300 cal yr BP mark the development of a mixed conifer forest, relatively cool and wet climatic conditions and the first indication of fire activity. A large increase in Pseudotsuga/Larix, in addition to Artemisia, Poacaea, and Alnus pollen between 10,000-4500 cal yr BP coincides with a pronounced increase in charcoal accumulation indicating high levels of fire activity were associated with the advance of Pseudotsuga/Larix forests, especially between 7000-4900 cal yr BP. This period is consistent with other records from the northwestern U.S. that register warmer drier conditions than at present in response to the early-Holocene summer insolation maximum. After 4500 cal yr BP, increases in Pinus, Picea, and Abies pollen indicate establishment of the present-day mixed conifer forest, with an initial decrease then increase in fire activity between 2000-1000 cal yr BP. The East Twin Lake record shows similarities with other paleoenvironmental sites from the Pacific Northwest that record warm and dry conditions during the mid-Holocene and experience dry summer conditions (Whitlock & Bartlein, 1993). Increases in fire activity during the last two millennia suggest humans may have had the greatest influence on fire regimes during this time period.