HUMAN-CLIMATE-LANDSCAPE INTERACTIONS IN THE WILLAMETTE VALLEY, OREGON AND WASHINGTON, DURING THE LAST 1200 YEARS

Documenting the long-term fire and vegetation history of low-elevation ecosystems, particularly grasslands, is critical for understanding past human-climate-landscape interactions. However, most paleoecological studies in the Pacific Northwest have been carried out in high-elevation forests and are irrelevant for addressing such issues. In this research, we reconstruct the fire and vegetation history of the Willamette Valley, OR and WA, for last 1200 years from five low-elevation sites located along a north-south transect. To do so, pollen and high-resolution macroscopic charcoal records obtained from lake sediments were compared with independent records of climate variability and human activity. Identification of different charcoal particle types (i.e., woody, herbaceous) aided in our characterization past fire events through an understanding of the fuel source. Overall, the results of this research strongly suggest a combination of natural and human influences on fire regimes, but they vary greatly both spatially and temporally. At some sites, shifts in fire activity compared closely with centennial-time scale variations in climate. At other sites (located in former prairie and oak savanna), anthropogenic burning was likely the dominant influence on the fire and vegetation patterns. Perhaps most interesting is that contrary to popular belief, the records show that fires were generally infrequent in the 200-300 years prior to Euro-American settlement of the valley. This may have been the result of Little Ice Age cooling, or could indicate an earlier drop in Native American populations due to disease than originally thought. This research contributes to our understanding of long-term vegetation dynamics and the role of fire, both natural- and human-ignited, in shaping ecosystems. It also provides an historical context for evaluating recent shifts in plant communities in the Willamette Valley as a result of Euro-American settlement, and provides critical baseline information needed to aid management and restoration decisions.