LATE QUATERNARY RECONSTRUCTIONS OF FIRE AND VEGETATION ACROSS AN ELEVATION GRADIENT IN THE NORTH CASCADE MOUNTAINS, WASHINGTON STATE, USA

The long-term history of fire and vegetation was reconstructed at two lakes in the North Cascade Mountains. Thunder Lake is located at 365 m elevation, and the Panther Potholes (a pair of small lakes) are located at 1100 m elevation, approximately 8 km up the Thunder Creek drainage from Thunder Lake. Lake sediment cores were sampled and processed using similar methods. The Thunder Lake core is 700 cm in length and dates back to 14,000 cal BP. The Panther Potholes core is 840 cm in length and dates back to 10,500 cal BP. Site histories reflect general trends in the late Quaternary vegetation history of the Pacific Northwest. The sparse, post-glacial vegetation assemblage recorded between 14,000-13,000 cal BP at Thunder Lake is commonly described in reconstructions throughout the region. Expansion of dry mixed forests of A. lasiocarpa and P. engelmannii between 13,000 and 9500 cal BP at Thunder Lake is also typical of regional vegetation histories. Many records indicate a warm dry period between 10,000 and 8000 cal BP in the Pacific Northwest, and this is suggested by both sediment records; forests were dominated by Pseudotsuga menziesii at Thunder Lake and a dry, mixed conifer forest at the Panther Potholes. Fire frequency was at its highest during this period at the Panther Potholes. As climate became cooler and moister over the past 7000 years, mesic tree species established at both sites. Tsuga heterophylla and Thuja plicata likely established at Thunder Lake c.7500 cal BP and c. 6000 cal BP, respectively, as suggested by a sharp increases in pollen percentages. These species established much later at the Panther Potholes (c. 5200 cal BP), as indicated by the presence of macrofossils. Recent increases in mesic species over the past 2000 years suggest continued cooling, particularly with the establishment of Chamaecyparis nootkatensis and Tsuga mertensiana at the Panther Potholes. Fire frequency actually increased during the past 3000 to 4000 years at the Panther Potholes, possibly due to changes in lightning ignitions, anthropogenic ignitions and/or an increase in available fuels. Together, the two lake sediment records suggest a strong influence of regional climate throughout the late Quaternary with site-specific differences in forest species composition and succession due to differences in elevation.