| Abstract |
- Fire history and fire regimes were reconstructed for a 450 km² area in the central
western Oregon Cascades, using tree-ring analysis of fire scars and tree origin years at
137 sampled clearcuts. I described temporal patterns of fire frequency, severity, and size,
and interpreted topographic influences on fire frequency and severity. I then evaluated
the 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 to
1996. The average mean fire interval, Weibull median probability interval, and
maximum 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 anthropogenic
influences, and patterns of fuel accumulation related to stand development. Fire
frequency and severity patterns were weakly but significantly associated with spatial
variation in hillslope position, slope aspect, slope steepness, and elevation. Fire
frequency was lower for higher elevations, lower slope positions, and more mesic slope
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
mapped.
Forest stand structures were strongly associated with stand age, fire history and
topography. The number of years since the last high-severity fire was an important
predictor for nearly all measured aspects of stand structure. Low-severity fires were
important for creating variability in tree diameter sizes, reducing tree density and
allowing more rapid diameter growth, and creating stand structures with many large snags
and few overstory shade-tolerant trees. However, stands of the same age, and of the same
general fire history, often had different structures. Much of this variation was explained
by differences in topography. The strongly positive influence of wet aspects and high
elevations on the relative dominance of shade-tolerant tree species has been important for
shaping 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 density
of large Douglas-fir trees) appears to have been slowest on steeper slopes, wetter aspects,
and higher elevations.
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