Document: WIL-3-68-7

Climate variability and forest fires in the Pacific Northwest.

KEETON, W.S.* and J.F.FRANKLIN

University of Washington, Seattle, WA 98195, USA ¹

Abstract:
Understanding the relationships between past climate variability and wildfire activity helps us predict potential changes in fire regimes caused by future climatic changes. We analyzed relationships between historic forest fires and dominant modes of climatic variability affecting the Pacific Northwest, which are Pacific Decadal Oscillation (PDO) and El Nino/Southern Oscillation (ENSO). We used fire data spanning the 20th century for USDA Forest Service Region 6, individual states (Idaho, Oregon, and Washington), and 21 national forests that represent the region's physiographic provinces. We used a reconstructed fire occurrence index for the interior Columbia Basin covering the years 1503-1938. Forest fires showed statistically significant correlations with warm/dry phases of PDO at regional and state scales; relationships were variable at the scale of individual national forests. Warm/dry phases of PDO were especially influential in terms of the occurrence of very large fire events throughout the Pacific Northwest. No direct statistical relationship was found between ENSO and forest fires at the regional scale, although relationships may exist at smaller spatial scales. However, both ENSO and PDO were correlated with summer drought, as estimated by the Palmer Drought Severity Index (PDSI), and PDSI was strongly correlated with fire activity at all scales. We conclude that even moderate (+/- 0.5 C decadal mean) fluctuations in climate over the 20th century have had detectable influences on drought-related wildfire activity. Potential future increases in regional temperature and summer drought intensity are likely to have major impacts on fire regimes.

Keywords: fire, natural disturbance, climate variability, climate change, drought, Pacific Decadal Oscillation, El Nino/Southern Oscillation

This abstract is being presented at: 10:30 AM in session:
Poster Session #12: Disturbance Ecology.