Fire history and fire regime were interpreted from tree ring analysis of 4320 stumps at 178 sites in a 25 by 55 km area in the central Oregon Coast Range. A total of 27 fire episodes were identified in a 516 year period, with sizes estimated at 18 to 544 km2 and a mean of 97 km2. The mean fire return interval (MFRI) was 85 years; the natural fire rotation (NFR) for the 516 year period was 271 years.
Fire size estimates were smaller and frequency was lower in the pre-settlement period (1478 - 1845) than in the post-European settlement period (1846 - 1909) with a mean size of 66 vs.192 km2 and NFR of 452 vs.78 years. Fire size and frequency both declined after fire suppression began in 1910 (mean fire size 86 km2, NFR of 335 years). Seventeen of the 27 fire episodes identified were low-severity and affected less than 15% of the study area, occurred between 1585 and 1844, and were concentrated in the eastern 1/3 “Valley Margin” portion of the study area. Less frequent, larger “widespread” fire episodes, two in the 1500s and two in the mid-1800s, each affected greater than 50% of the study area and obliterated most pre-existing stands: only 347 trees examined (8%) were greater than 400 years old, and none exceeded 516 years. Upper hillslope positions experienced more frequent, more severe fires than lower hillslope positions, where 44% of the trees sampled exceeding 400 years of age were found.
Fire episode size varied by as much as 800% for the earliest fires depending on how much erasure was presumed to have occurred. MFRIs may have underestimated true return intervals by half, since many sites recorded only two fires. Old-growth stands were more abundant than shown in previous studies. 48% of the sites sampled contained trees greater than 200 years of age; most of the stands in the Interior/Coast appear to have been greater than 200 years of age in 1850. The species composition, structure, and temporal variability of old growth stands probably differed between the eastern 1/3 and western 2/3 of the study area as a result of the contrast in fire regimes.
Donald L. Henshaw, Frederick J. Swanson, Julia A. Jones, Peter C. Impara
Fire in the central Oregon Coast Range was investigated in this study as a process that creates and reacts to pattern at several spatial and temporal scales. Dendrochronologic data were used to identify fire episodes over the last 500 years (the maximum tree-age record in the study area) and to characterize the fire regime variables of frequency, severity and size for a 1375 km2 study area of the central Oregon Coast Range. The extent of significant fire episodes was identified, mapped, analyzed, and compared to results of similar fire studies in the western Cascades (Teensma 1987; Morrison and Swanson 1990; Hemstrom and Franklin 1982). This study lays the groundwork for an analysis of the interactions among the process of fire and the landscape patterns of climate, landforms, topography, and vegetation. The established fire episodes were compared to a long-term fire history developed from lake core analysis of charcoal in a separate study (Long 1995). An analysis of the fire record was also used to reconstruct age classes in the study area over time as a method of assessing the effects of fire and estimating historical old growth occurrence.