Fire history in the ponderosa pine/Douglas-fir forests on the east slope of the Washington Cascades

We collected 490 and 233 fire scars on two ponderosa pine (Pinus ponderosa) /Douglas-fir (Pseudotsuga menziesii) dominated landscapes on the east slope of the Washington Cascades that contained a record of 3901 and 2309 cross- dated fire events. During the pre-settlement period (1700/1750-1860), the W eibull median fire-free interval (WMFFI) and the mean fire-free interval (M FFI) were 6.6-7 years at both sites. The MFFI during the settlement period (1860-1910) varied within 3 years of the pre-settlement value, but increase d to 38 and 43 years for a truncated fire suppression period between 1910 a nd 1996. Increased variation in MFFI among aspect polygons suggests fire re gimes have become more complex since Euro-settlement. In the pre-settlement period, an area equal to approximately 50-60% of the study areas burned ev ery 6-7 years, an amount of fire disturbance apparently in balance with lan dscape and stand vegetation structure. Overlapping fires have created a com plex mosaic of different fire histories on these forested landscapes. Mappe d fire events from the 1700-1910 showed 134 and 157 separate fire history p olygons (FHP) at the two sites. Fire disturbance rates and patterns are sug gested as ecologically defensible reference points for landscape heterogene ity to reduce the potential for catastrophic fires and to establish vegetat ion disturbance management guidelines. (C) 2000 Elsevier Science B.V. All r ights reserved.