HISTORICAL FIRE FREQUENCY ON CONTRASTING SLOPE FACETS ALONG THE MCKENZIE RIVER, WESTERN OREGON CASCADES

Fire has been a critical influence on forest dynamics over much of western North America, and knowledge of historical fire regimes provides a valuable reference point for guiding ecosystem management. Spatial and temporal variations in historical (1550-1997) fire frequency were quantified for a 145-km(2) area straddling the McKenzie River in the central western Cascades of Oregon. Fire-sear and tree-origin years estimated from Cut Stumps in 63 clear-cut sites were used to approximately date fires. The estimated natural fire rotation was 162 years for the presettlement period (1550-1849). Natural fire rotation varied considerably over the period of record, ranging from 40 years for the Euro-American settlement period (1850-1924) to 504 years for the recent (1925-1997) fire-suppression period. Such temporal variation in fire regime may be attributed to a combination of climatic and anthropogenic influences. The study area also experienced 2 periods of widespread fire recorded at a majority of sample sites; one in the mid-1500s and the other front 1850 to 1925. Historical fire frequency has also varied spatially over at least 2 distinct scales. At the slope-facet scale, fire frequency was greatest on the generally south-facing slope to the north of the McKenzie River. At the finer scale of individual 4-ha sites, local topographic influences varied according to a site's broad geographical context north or south of the river. Results suggest that meaningful fire-regime characterization may require information derived from multiple spatial scales.