This Fire Regime Synthesis details characteristics of presettlement (i.e., before Euro-American settlement), historical (i.e., any past record), and contemporary (i.e., 1980s to present) fuels and fire regimes in ponderosa pine and montane mixed-conifer communities in the East Cascade Mountains in the northwestern United States. Stand structure and species composition varies within and among these communities due to variation in site characteristics, climate, disturbance history, and successional stage. Although historical landscapes were successionally diverse, they were characterized by a large proportion of low-density, open-canopy forest dominated by large, old, fire- and drought-tolerant trees (mostly ponderosa pine), with small areas of higher-density forest, and small openings without trees. These stand structures are compatible with a presettlement fire regime dominated by frequent, low- and mixed-severity fires, with high-severity fire effects mostly limited to relatively small (<100-200 ha) patches. This fire regime maintained low fuel biomass, dominated by flammable, often continuous surface fuels, with few ladder fuels, and relatively widely spaced crown fuels. Our analyses found presettlement mean fire interval (MFI) averaged 12 years with a maximum MFI of 24 years in the ponderosa pine forest zone, and MFI averaged 20 years with a maximum MFI of 100 years in the montane mixed-conifer forest zone. Historically, lightning-caused fires could ignite as early as May and as late as October, although fire-scar analyses across all forest types suggest a dominance of late growing season and dormant season fires, with some variability among studies. Lightning ignitions were most common, although American Indians also ignited fires, within and outside lightning season. Evidence suggests that most presettlement fires were small (<400 ha), and larger fires were infrequent. Large fires and widespread fire years were most frequent during relatively warm, dry summers at both site and regional scales. Wetter than average antecedent conditions may be related to large fires on the driest ponderosa pine sites.

Stand structure, species composition, fuels, and fire regime characteristics in contemporary stands differ from presettlement stands to the degree that some consider open-canopy forests dominated by large fire- and drought-tolerant trees to be endangered. Dense forest conditions, once-rare, have become common, whereas open-canopy conditions and large, old trees, once common, are rare. Fire exclusion began during Euro-American settlement, first by reducing ignitions, then by reducing fine fuel biomass and connectivity with livestock grazing, and eventually, by engaging in active fire suppression. Fire exclusion led to increased fuel biomass and presence of ladder fuels. Subsequently, contemporary fire regimes are characterized by increasing frequency of large, high-severity fires, due to both increased fuel biomass and the effects of anthropogenic climate change. Despite contemporary increases in fire frequency and severity, there is an overall fire deficit compared to presettlement fire frequency and area burned by low-severity fire.

Hazardous fuel reduction to reduce fire spread and severity is important for protecting infrastructure in the Wildland Urban Interface (WUI) and the few remaining stands of old and large-diameter dry forests in the East Cascades and elsewhere in the West. Landscapes managed to mimic presettlement fire regimes, forest structure, composition, and fuels are likely to be most resilient to future fires and other disturbances. Wildfire and prescribed fire are important tools for achieving these conditions. The effects of contemporary climate and projected climate changes on fire behavior and fire effects must be considered when managing with fire.