The perception is that today's large fires are an ecological catastrophe because they burn vast areas with high intensities and severities. However, little is known of the ecological impacts of large fires on both historical and contemporary landscapes. The present paper presents a review of the current knowledge of the effects of large fires in the United States by important ecosystems written by regional experts. The ecosystems are (1) ponderosa pine-Douglas-fir, (2) sagebrush-grasslands, (3) piñon-juniper, (4) chaparral, (5) mixed-conifer, and (6) spruce-fir. This review found that large fires were common on most historical western US landscapes and they will continue to be common today with exceptions. Sagebrush ecosystems are currently experiencing larger, more severe, and more frequent large fires compared to historical conditions due to exotic cheatgrass invasions. Historical large fires in south-west ponderosa pine forest created a mixed severity mosaic dominated by non-lethal surface fires while today's large fires are mostly high severity crown fires. While large fires play an important role in landscape ecology for most regions, their importance is much less in the dry piñon-juniper forests and sagebrush-grasslands. Fire management must address the role of large fires in maintaining the health of many US fire-dominated ecosystems.
We conducted a detailed investigation of fire frequencies, patterns of fire spread, and the effects of fire on tree succession in the western larch – lodgepole pine (Larixoccidentalis – Pinuscontorta var. latifolia) forests west of the Continental Divide in Glacier National Park, Montana. Master fire chronologies for 1650 to the present were constructed based on tree fire scars and fire-initiated age-classes. Two kinds of primeval fire regimes were identified: (i) a mixed-severity regime ranging from nonlethal underburns to stand-replacing fires at mean intervals of 25–75 years and (ii) a regime of infrequent stand-replacing fires at mean intervals of 140–340 years. The former regime is characteristic of the North Fork Flathead valley and appears to be linked to a relatively dry climate and gentler topography compared with the McDonald Creek – Apgar Mountains and Middle Fork Flathead areas, where the latter fire regime predominates. Fire frequency in the entire North Fork study area was 20 fire years per century prior to 1935 and 2 per century after 1935. In the other two study areas it was 3–5 per century both before and after 1935. We suggest that fire suppression has altered the primeval fire regime in the North Fork, but not in the central and southern areas.
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