Historical variability of fire regimes must be understood within the context of climatic and human drivers of disturbance occurring at multiple temporal scales. We describe the relationship between fire occurrence and interannual to decadal climatic variability (Palmer Drought Severity Index [PDSI], El Niño/Southern Oscillation [ENSO], and the Pacific Decadal Oscillation [PDO]) and explain how land use changes in the 20th century affected these relationships. We used 1701 fire‐scarred trees collected in five study sites in central and eastern Washington State (USA) to investigate current year, lagged, and low frequency relationships between composite fire histories and PDSI, PDO, and ENSO (using the Southern Oscillation Index [SOI] as a measure of ENSO variability) using superposed epoch analysis and cross‐spectral analysis. Fires tended to occur during dry summers and during the positive phase of the PDO. Cross‐spectral analysis indicates that percentage of trees scarred by fire and the PDO are spectrally coherent at 47 years, the approximate cycle of the PDO. Similarly, percentage scarred and ENSO are spectrally coherent at six years, the approximate cycle of ENSO. However, other results suggest that ENSO was only a weak driver of fire occurrence in the past three centuries. While drought and fire appear to be tightly linked between 1700 and 1900, the relationship between drought and fire occurrence was disrupted during the 20th century as a result of land use changes. We suggest that long‐term fire planning using the PDO may be possible in the Pacific Northwest, potentially allowing decadal‐scale management of fire regimes, prescribed fire, and vegetation dynamics.

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Snag numbers and decay class were measured on a chronosequence of 26 wildfires (ages 1-81 years) on the east slope of the Cascade Range in Washington. Snag longevity and resultant snag densities varied spatially across burns in relation to micro-topographic position. Longevity of snags < 41cm dbh was greater for thin-barked Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa) and lodgepole pine (Pinus contorta) than thick-barked Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa). With larger diameter snags, however, Douglas-fir persisted longer than Engelmann spruce. The time period required for recruitment of soft snags > 23 cm dbh was estimated to exceed snag longevity for ponderosa pine, Englemann spruce, lodgepole pine, and subapline fir, causing an “on-site gap” in soft snags for these species. Snags of Douglas-fir ≥ 41 cm dbh stood for a sufficient time (40% standing after 80 years) to potentially overlap the recruitment of soft snags ≥ 23 cm dbh from the replacement stand. Providing continuity in soft snags following stand-replacement events would require a landscape-scale perspective, incorporating adjacents stands of different ages or disturbance histories. Results suggest that standards and guidelines for snags on public forest lands need to be sufficiently flexible to accomodate both disturbance and stand development phases and differences in snag longevity among species and topographic positions.

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