1200 years of fire and vegetation history in the Willamette Valley, Oregon and Washington, reconstructed using high-resolution macroscopic charcoal and pollen analysis

Abstract

High-resolution macroscopic charcoal and pollen analyses were used to reconstruct the fire and vegetation history of the Willamette Valley for the last 1200 years. Presented in this paper are three new paleoecological reconstructions from Lake Oswego, Porter Lake, and Warner Lake, Oregon, and portions of previous reconstructions from Battle Ground Lake, Washington, and Beaver Lake, Oregon. The reconstructions show that prior to Euro-American settlement vegetation and fire regimes were influenced by a combination of natural and anthropogenic factors. Battle Ground Lake shows a stronger influence from climate, while Lake Oswego, Beaver Lake, Porter Lake, and Warner Lake were more controlled by human activity. However, human-set fires were also modulated by regional climate variability during the Medieval Climate Anomaly and the Little Ice Age. Fire reconstructions from Battle Ground Lake, Lake Oswego, Beaver Lake, and Porter Lake imply that fires were infrequent in the Willamette Valley 200–300 years prior to Euro-American settlement. The decline of Native American populations due to introduced disease may have led to this reduction in fire activity. The prehistoric record from Warner Lake, however, indicates that fires in the foothills of the Cascade Range were more frequent than on the valley floor, at least until ca. AD 1800. The historic portions of the reconstructions indicate that Euro-American land clearance for agriculture and logging produced the most dramatic shifts in vegetation and fire regimes. All five records indicate that few fires in the Willamette Valley have occurred since ca. AD 1930, and fires today are predominantly grass fires.