2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 5
Presentation Time: 2:30 PM


WALSH, Megan K., Geography, University of Oregon, 107 Condon Hall, 1251 University of Oregon, Eugene, OR 97403, WHITLOCK, Cathy, Earth Sciences, Montana State University, P.O. Box 173480, Traphagen Hall #200, Bozeman, MT 59717 and BARTLEIN, Patrick J., Geography, Univ of Oregon, 1251 Univ. Oregon, Eugene, OR 97403-1251, mwalsh2@uoregon.edu

The Willamette Valley, located between the Coast and Cascade ranges of southwestern Washington and northwestern Oregon, has been the setting of major changes in climate, vegetation, and human activity during the Holocene. The fire history of the region during this time period, however, has been poorly studied. An 11,000-year long, decadal-resolution record of fire history from Battle Ground Lake, WA (elev. 154 m), was reconstructed using macroscopic charcoal analysis and compared with pollen and regional paleoclimatological data. The charcoal influx record shows a general increase in fire frequency from ~8 fire episodes/1000 years at the beginning of the Holocene until the mid-Holocene, when the fire frequency peaked at ~14 fire episodes/1000 years around 6500 cal yr BP. The vegetation during this interval was xeric (e.g., Quercus and members of the Poaceae family), due to the warm, dry conditions that prevailed at that time. A decrease in the fire frequency from its peak in the mid-Holocene to ~4 fire episodes/1000 years at present was accompanied by a shift to more mesic vegetation surrounding the site, marked by a decrease in Quercus and an increase in Pseudotsuga, Abies, and Cupressaceae (cedar), as the regional climate became wetter with less summer drought. Although the regional climate likely changed gradually over the past 6500 cal yr, major changes are seen in the charcoal influx record, most notably after 3000 cal yr BP. At this time as fire frequency decreased, the magnitude of the charcoal influx peaks more than doubled, as did the time between fire events. The change in fire activity after 3000 cal yr BP was accompanied by little change in the vegetation, except a major increase in Pteridium-type pollen and a further increase in Cupressaceae. The Battle Ground record illustrates the role that interactions between fire and vegetation, as well as the direct effects of climate, have in shaping a region's environmental history. Although this record provides important information on the fire, vegetation, and human history of the northern Willamette Valley, several additional sites are concurrently being studied to gain an understanding of these changes during the Holocene across the Valley as a whole.