Fire history, fire regimes, and development of forest structure in the central western Oregon Cascades

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Weisberg, Peter J. 1998. Fire history, fire regimes, and development of forest structure in the central western Oregon Cascades. Corvallis, OR: Oregon State University. 256 p. Ph.D. dissertation.


Fire history and fire regimes were reconstructed for a 450 km2 area in the centralwestern Oregon Cascades, using tree-ring analysis of fire scars and tree origin years at137 sampled clearcuts. I described temporal patterns of fire frequency, severity, and size.and interpreted topographic influences on fire frequency and severity. I then evaluatedthe influences of fire history and topography on the development of forest structure.
Ninety-four fire episodes were reconstructed for the 521-year period from 1475 to1996. The average mean fire interval, Weibull median probability interval, andmaximum fire interval of 4-ha sites were 97 years, 73 years, and 179 years. respectively.Fire regime has changed over time as a result of climate change, changing anthropogenicinfluences, and patterns of fuel accumulation related to stand development. Firefrequency and severity patterns were weakly but significantly associated with spatialvariation in hillslope position, slope aspect, slope steepness, and elevation. Firefrequency was lower for higher elevations, lower slope positions, and more mesic slopeaspects. Fire severity was lower for higher elevations, lower slope positions. more north-
facing slopes, and more gradual slopes. Three fire regime classes were defined andmapped.
Forest stand structures were strongly associated with stand age. fire history andtopography. The number of years since the last high-severity fire was an importantpredictor for nearly all measured aspects of stand structure. Low-severity fires wereimportant for creating variability in tree diameter sizes. reducing tree density andallowing more rapid diameter growth. and creating stand structures with many large snagsand few overstory shade-tolerant trees. However, stands of the same age. and of the samegeneral fire history, often had different structures. Much of this variation was explainedby differences in topography. The strongly positive influence of wet aspects and highelevations on the relative dominance of shade-tolerant tree species has been important forshaping the structure of forest stands. Development of old-growth stand attributes (i.e.,high stand basal area. maximum tree diameter, variability of tree diameters, and densityof large Douglas-fir trees) appears to have been slowest on steeper slopes. wetter aspects,and higher elevations.