Abstract
Although mixed-severity fires are among the most widespread disturbances influencing western North American forests, they remain the least understood. A major question is the degree to which mixed-severity fire regimes are simply an ecological intermediate between low- and high-severity fire regimes, versus a unique disturbance regime with distinct properties. The Klamath-Siskiyou Mountains of southwestern Oregon and northwestern California provide an excellent laboratory for studies of mixed-severity fire effects, as structurally diverse vegetation types in the region foster, and partly arise from, fires of variable severity. In addition, many mixed-severity fires have occurred in the region in the last several decades, including the nationally significant 200,000-ha Biscuit Fire. Since 2002, we have engaged in studies of early ecosystem response to 15 of these fires, ranging from determinants of fire effects to responses of vegetation, wildlife, and biogeochemistry. We present here some of our important early findings regarding mixed-severity fire, thereby updating the state of the science on mixed-severity fire regimes and highlighting questions and hypotheses to be tested in future studies on mixed-severity fire regimes. Our studies in the Klamath-Siskiyou Ecoregion suggest that forests with mixed-severity fire regimes are characterized primarily by their intimately mixed patches of vegetation of varied age, resulting from complex variations in both fire frequency and severity and species responses to this variation. Based on our findings, we hypothesize that the proximity of living and dead forest after mixed-severity fire, and the close mingling of early- and late-seral communities, results in unique vegetation and wildlife responses compared to predominantly low- or high-severity fires. These factors also appear to contribute to high resilience of plant and wildlife species to mixed-severity fire in the Klamath-Siskiyou Ecoregion. More informed management of ecosystems with mixed-severity regimes requires understanding of their wide variability in space and time, and the particular ecological responses that this variability elicits.
Highlights
Fire regimes—the characteristic pattern and effects of wildland fire in a given area—are major drivers of the ecology of many vegetation types and are widely applied as a guiding framework for management of fire-prone ecosystems globally (Bond and van Wilgen 1996, Schoennagel et al 2004, Noss et al 2006)
M-S fires are defined as a combination of low- to high-severity fire effects within the perimeter of a single fire, their ecology may not be a simple intermediate between the two; rather, M-S fires and fire regimes are hypothesized to give rise to unique patch dynamics and ecosystem responses
The range in fire effects—fire severity, patch size, and legacy generation—appears to be a major driver of ecosystem dynamics in these systems, as we have described here
Summary
Fire regimes—the characteristic pattern and effects of wildland fire in a given area—are major drivers of the ecology of many vegetation types and are widely applied as a guiding framework for management of fire-prone ecosystems globally (Bond and van Wilgen 1996, Schoennagel et al 2004, Noss et al 2006). The interaction between patch-size and seed source dynamics was remarkably similar between small M-S fires and the exceptionally large Biscuit Fire; most high-severity burn area was ,400 m from edge, and conifer regeneration was generally abundant within this distance (Shatford et al 2007, Donato et al 2009a).
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