U.S. Joint Fire Science Program


Date of this Version


Document Type



Fire Science Brief, Issue 98, March 2010


US government work.


The Willamette Valley in northwestern Oregon and southwestern Washington is a fertile agricultural region that supports a variety of farming activities. It is also a densely populated region with extensive urban and suburban development, including residences in the wildland/urban interface. Over the millennia, the valley floor has been shaped by numerous forces, including flooding of the Columbia and Willamette rivers, naturally and human-ignited fires, Euro-American forest clearance, and most recently, agriculture. Over the past 18,000 years the regional climate has undergone major changes. From the big chill of the last great Ice Age, to a period of warming that reached a maximum in the early and middle Holocene (9,000–4,000 years ago), to the colder conditions of the late Holocene (4,000 years ago to present), including the Little Ice Age (1450–1850 AD). As a result, the vegetation of the valley has changed from a subalpine parkland, to a closed conifer-dominated forest, to the open, oak savanna and prairie habitat that greeted early settlers. Today, however, only remnant patches of the pre-settlement vegetation remain, including oak savanna. The indigenous people inhabiting the Willamette Valley are also assumed to have changed during this time, from hunter/gatherers at the end of the Ice Age to more sedentary groups in the late Holocene who used fi re to maintain supplies of nuts, berries, and tubers. Many researchers believe that humans used fi re to manipulate their environment with increasing frequency during the Holocene, but the extent of their burning and the ecological consequences are a matter of debate. Were past vegetation changes caused by deliberate burning, climate fluctuations, or a combination of both? Studies of fire and vegetation history using the pollen and charcoal found in lake sediments from sites in the valley indicate the answer is not simple.