ORISE Postdoctoral Scholar USFS Pacific Northwest Research Station Corvallis, Oregon, United States
Abstract: Douglas-fir dominated forests of the west slope of the Cascades are among the moistest and most productive forests in North America. Many of these forests receive more than 2,000mm of precipitation a year, lightning ignitions are relatively rare, and older stands in this region accumulate more biomass than almost any other terrestrial system. The standard model of stand development attributes the phenomenal complexity of older western Cascades forests to stand replacing fire that resets succession at coarse spatial scales followed by a long fire-free period with stand development driven primarily by competition and fine scale disease and windthrow disturbance. An extraordinary period of fire activity in northwestern Oregon in the summer 2020—when three-quarters of the total area burned in the region in the last 70 years burned in just 72 hours—seemed to confirm the importance of infrequent stand replacing fire in productive Douglas-fir forests. This study uses tree ring evidence to determine the range of variability in historical fire frequency and fire extent across the west slope of the Cascades in western Oregon and southwestern Washington. We found that westside Douglas-fir forests experienced frequent non-stand replacing fire circa 1500-1900 CE. These new fire histories provide little evidence that large stand-replacing fires were the dominant disturbance dynamic in west slope Cascades forests. Future management will require recognition of significant variability in the natural fire regime of the western Cascades.
