Portland State University. Department of Geography
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geography
1 online resource (viii, 64 pages)
High variability in historical fire patterns characteristic to mixed-severity fire regimes is expected to have contributed to a structurally heterogeneous landscape throughout much of the forested ecosystems of the western United States. After more than a hundred years of fire exclusion in the region, many forests have shifted to a more homogeneous structure, which raises concern regarding these forests' ability to sustain expected increases in fire activity with a warming climate. The shift is not uniform across the west, however, and differences in historical disturbance patterns and changes due to land management are not well characterized in forests across a wide range of environmental settings. While there is broad agreement on the high (low) impact of fire exclusion on the structure and composition of low (high) elevation, dry (wet) forests in the west, much less is known about its effect in mid elevation, moist mixed-conifer forests.
In order to provide reference conditions of wildfire patterns, this study reconstructs the variability in historical fire occurrence and severity and tree establishment in a moist mixed-conifer forest in northeastern Oregon's Blue Mountains. We used a novel multi-proxy approach, combining remote sensing, dendroecology, and mathematical models to comprehensively sample and reconstruct multi-century fire frequency and severity, and forest structural patterns. Based on over 550 tree-core and 100 fire-scar samples collected at 38 plots, we quantified historical fire severity and assessed the spatial heterogeneity of disturbance and establishment patterns across 9,300 ha in two subwatersheds. We identified high variability in historical fire patterns over fine scales between forest stands. Despite speculation that moist mixed-conifer forests experienced primarily infrequent and severe fires, relatively frequent fire (median intervals ranging from 14-42 years) of all severities (low, medium and high) at fine scales was common throughout the study area, with an abrupt decrease in fire activity after 1900. Concurrent post-fire establishment pulses of both shade-intolerant and shade-tolerant species were common, suggesting an initial floristics vegetation succession model. The results of this study highlight the complexity of the mixed-severity fire regime, long-term challenges and untested assumptions of post-fire responses of shade-tolerant species, as well as assumed infrequent fire activity in moist mixed-conifer forests, and thus can help inform discussions around appropriate restoration of fire-excluded, moist mixed-conifer forests of the northern Blue Mountains Ecoregion.
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Platt, Laura Marie, "Variability in Historical Fire Patterns of a Moist Mixed-Conifer Forest in the Northern Blue Mountains of Oregon" (2020). Dissertations and Theses. Paper 5543.
Available for download on Tuesday, July 27, 2021