POST-GLACIAL VEGETATION CHANGE AT RIDLEY LAKE, NORTH CASCADES, WASHINGTON, USA

A lake sediment record of pollen, charcoal and macrofossils is analyzed to reconstruct vegetation and fire history surrounding Ridley Lake in the North Cascades, Washington. Ridley Lake is situated at 3140 feet, 48.949ºN: 121.030ºW, along the east side of Ross Lake. The Ridley Lake sediments record nearly 15,000 radiocarbon years of vegetation and climate history, with age control through three tephra deposits and four radiocarbon dates. Preliminary results suggest that general trends in climate change interpreted from the pollen agree with climate histories evidneced in other lake records from Western Washington and the Pacific Northwest. Immediately following deglaciation, the landscape is dominated by a mix of shrub alder, Artemisia and other herb species adapted to cold, dry conditions. Small percentages of spruce pollen suggest spruce forests nearby, though not immediately surrounding the lake. The post-glacial vegetation is replaced by mixed forest communities of spruce and alpine fir species. During the mid-Holocene, as conditions warmed during the thermal maximum, forest composition is more diverse, including fir, spruce, alder, birch, Douglas-fir and western hemlock. The unique geographical location of this site between moist, coastal forests and drier interior forests appear to have contributed to the diversity of vegetation in the Late Holocene. The results of this study combined with other regional lake sediment records of pollen, charcoal and macrofossils, provide an intricate picture of the complexity of Holocene vegetation change in a region of high topographic relief and confirms general paleoclimatic trends previously documented in the Pacific Northwest.