Mixed severity fire regimes & forest structure at Crater Lake National Park, Oregon, USA

Forrestel, Alison; Stephens, Scott

The west side forests of Crater Lake National Park (CLNP) are unique in that they represent one of the few places in the Cascade Range where an elevational gradient from mixed conifer to high elevation forests remains intact and has never been logged. The fire history along this gradient is not well documented or understood. Park management documents, including the Fire Management Plan, describe the fire regime of these forests as ranging from mixed severity (mean fire return interval of 40-70 years) in mid-elevation red fir forests to a low frequency, high severity regime (mean fire return interval greater than 100 years) for high elevation mountain hemlock forests (National Park Service 2007). However, both the patch mosaic structure of these forests as well as the fire behavior and fire effects in recent wildland fires in this region suggest that the fire regimes may be more mixed in severity and that fire return intervals may be shorter than previously thought. In addition, there may be potential for fire spread from lower to higher elevation forests that is absent in more fragmented ecosystems. Along the western slope of Mount Mazama, at CLNP, a gradient of forest types are found. At the highest elevations, from 1800 to 2300 meters, forests are dominated by open stands of mostly pure mountain hemlock. As elevation decreases, forests are dominated by a mix of red fir and mountain hemlock. The lowest elevations, ranging from 1500 to 1700 meters, are dominated by mixed conifer forests which are comprised of Douglas-fir, white fir, incense-cedar, western hemlock and western white pine. Lodgepole pine forests are found across the elevational gradient within the study area, primarily on flat cold air drainages with poor soils (Zeigler 1978). Figure 1 shows typical stands of each of the major forest types in the study. CLNP was established in 1902 and most of the forests within the park boundary, including all of the forests in the study area, have never been logged. Prior to this study, there have been few published fire histories that include these forest types at CLNP. There is published fire history information from other forest types within CLNP and from these forest types in other parts of the Cascades or Sierra Nevada ranges (see Table 1). One study, in the Panhandle region of CLNP, reported fire return intervals of nine to forty-two years in ponderosa pine forest (McNeil 1975). Another study, that used a combination of stumps on an area owned by the Winema National Forest adjacent to CLNP and cores from a burned area within park boundaries, reported a fire return interval of thirty-nine years in red fir forests (Chappell 1991). Other studies of fire history in red fir forests south of CLNP report fire return intervals ranging from twenty to close to one hundred years (Pitcher 1987, Chappell 1991, Taylor and Halpern 1991, Taylor 2000, Bekker and Taylor 2001, Taylor and Solem 2001, Scholl and Taylor 2006). The fire history of subalpine mountain hemlock forests is not well documented but these forests are thought to have fire free intervals of hundreds of years (Agee 1993). Bekker and Taylor (2001, 2010) found fire return intervals of 20 years (based on direct analysis of fire scars) and 100 years (based on fire extent maps using the same set of scars) for red fir – mountain hemlock forest in the southern Cascades. They found that moderate and high severity fires occurred in this forest type, but they were not able to detect impacts of fire on forest composition or structure (Bekker and Taylor 2010). In addition to evidence for mixed severity fire regimes from similar systems, recent wildfires at CLNP have been mixed in severity. For example, the 2006 Bybee Fire burned over 1100 ha of lodgepole pine, red fir and mountain hemlock forests within the study area. Burn severity, based on data obtained through the Monitoring Trends in Burn Severity program, was mixed within the Bybee Fire...

Mixed severity fire regimes & forest structure at Crater Lake National Park, Oregon, USA

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