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

Graduation date: 1999Fire history and fire regimes were reconstructed for a 450 km² area in the central\ud western Oregon Cascades, using tree-ring analysis of fire scars and tree origin years at\ud 137 sampled clearcuts. I described temporal patterns of fire frequency, severity, and size,\ud and interpreted topographic influences on fire frequency and severity. I then evaluated\ud the influences of fire history and topography on the development of forest structure.\ud Ninety-four fire episodes were reconstructed for the 521-year period from 1475 to\ud 1996. The average mean fire interval, Weibull median probability interval, and\ud maximum fire interval of 4-ha sites were 97 years, 73 years, and 179 years, respectively.\ud Fire regime has changed over time as a result of climate change, changing anthropogenic\ud influences, and patterns of fuel accumulation related to stand development. Fire\ud frequency and severity patterns were weakly but significantly associated with spatial\ud variation in hillslope position, slope aspect, slope steepness, and elevation. Fire\ud frequency was lower for higher elevations, lower slope positions, and more mesic slope\ud aspects. Fire severity was lower for higher elevations, lower slope positions, more north-facing slopes, and more gradual slopes. Three fire regime classes were defined and\ud mapped.\ud Forest stand structures were strongly associated with stand age, fire history and\ud topography. The number of years since the last high-severity fire was an important\ud predictor for nearly all measured aspects of stand structure. Low-severity fires were\ud important for creating variability in tree diameter sizes, reducing tree density and\ud allowing more rapid diameter growth, and creating stand structures with many large snags\ud and few overstory shade-tolerant trees. However, stands of the same age, and of the same\ud general fire history, often had different structures. Much of this variation was explained\ud by differences in topography. The strongly positive influence of wet aspects and high\ud elevations on the relative dominance of shade-tolerant tree species has been important for\ud shaping the structure of forest stands. Development of old-growth stand attributes (i.e.,\ud high stand basal area, maximum tree diameter, variability of tree diameters, and density\ud of large Douglas-fir trees) appears to have been slowest on steeper slopes, wetter aspects,\ud and higher elevations