Historical Fire-Climate Relationships in Contrasting Interior Pacific Northwest Forest Types

Describing the climate influences on historical wildland fire will aid managers in planning for future change. This study uses existing historical climate reconstructions and a new fire history from the southern Blue Mountains in eastern Oregon, USA, to: 1) characterize historical fire-climate relationships, and 2) determine if climatic influences on fire differed among dry sites dominated by ponderosa pine (Pinus ponderosa Dougl. ex Laws) and more productive sites with significant older fire intolerant grand fir (Abies grandis [Dougl.] Lindl.) structure. Similar to results from other fire history studies across the American West, this research documents an increased incidence of burning in the southern Blue Mountains prior to 1900 associated with more arid conditions as measured with Palmer Drought Severity Index (PDSI). Positive values of the El Niño-Southern Oscillation (ENSO) were associated with fire years when multiple sites burned within the 688 000 ha study area. Although ponderosa pine and grand fir study sites were significantly different with respect to site productivity as well as historical and contemporary species composition, there were only marginal differences in historical mean fire return intervals between these forest types. There was also no significant difference in reconstructed mean temperature, precipitation, drought index, or ENSO during fire years between forest types prior to 1900 CE. However, the years prior to fire in ponderosa pine sites were moister than average, suggesting that antecedent climate was more important in the production of herbaceous fuel in ponderosa pine sites.