Seminar Abstract

Susan Prichard

November 4, 2003

Holocene fire and vegetation dynamics in a montane forest, North Cascades Range, Washington, USA

The Pacific Northwest is dominated by prominent mountain ranges, but few paleoecological investigations have focused on high elevation sites in this region. We reconstructed the Holocene fire and vegetation history of a montane site in Thunder Creek Watershed, North Cascades National Park based on lake sediment charcoal, macrofossil and pollen records. This study provides some of the first paleoecological data for the North Cascades Range and provides a comparison of vegetation change to other mountain sites.

During the early Holocene (>10,500 to c. 7700 cal year BP) forests were likely open grown and dominated by lodgepole pine. During the mid Holocene (c. 7700 to c. 5200 cal yr BP) lodgepole pine became uncommon and Douglas-fir and western white pine were dominant. Shade tolerant species including western red cedar and western hemlock first appeared at the beginning of the late Holocene (5200 cal yr BP to present). Alaska yellow cedar appeared most recently at 2000 years BP.

Fire frequency does not significantly change over millennial time scales, but fire return intervals are highly variable. Of the 62 detected fires, 39 have a quantifiable response in the macrofossil record. Overall, peaks in charcoal accumulation rates are associated with an initial peak followed by a drop in macrofossil accumulation rates. Climate is a major driver of vegetation change over millennial time scales, and fire and other disturbances are important agents of change at decadal to century time scales.