Fire history and fire regimes were reconstructed for a 450 square meter area in the central western Cascades using tree-ring analysis of fire scars and tree origin years at 137 sampled clearcuts. Temporal patterns of fire frequency, severity, and size, and interpreted topographic influences on fire frequency are described. Influences of fire history and topography on the development of forest structure are evaluated.
Ninty-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 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. Fre severity was lower for higher elevations, lower slope positions, more north-facing slopes, and more gradual slopes. Three fire regime classes are defined and mapped. Study parameters include several variables describing fire scar and tree origin years, reconstructed fire history, fire frequency, fire severity, fire size, forest stand structure.
Donald L. Henshaw, Frederick J. Swanson, Peter J. Weisberg, Sheryl K. Giglia
Purposes regarding fire history include: (1) Reconstruct fire history and characterize fire regime over the past several centuries; (2) Analyze temporal changes in fire frequency, severity, and size over the time period of record; (3) Describe spatial variability in fire frequency and severity, and identify key environmental influences on fire pattern; and (4) Delineate portions of the study area with qualitatively different fire regimes.
Purposes regarding interactions between fire history, physical environment, stand structure include: (1) Quantitatively examine the influence of fire history on forest stand structure, especially as stand structure relates to the development of old-growth characteristics; and (2) Describe how fire history influences on stand structure vary for different environments, where topography is used as a proxy for features of the physical environment that relate to tree growth and population dynamics. Forest structure is limited to the diameter structure of trees and snags, at the scale of 4-ha sites.