Graduate Thesis Or Dissertation
 

Spatial and temporal patterns of fire in the forests of the central Oregon Coast Range

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  • 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 <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 >50% of the study area and obliterated most pre-existing stands: only 347 trees examined (8%) were >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 >200 years of age; most of the stands in the Interior/Coast appear to have been >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.
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