CORVALLIS - The low-lying, dense and drippy Oregon Coast Range tends to resist forest fire better than its Cascade Range counterpart, but research now suggests that it may make up with severity what it lacks in frequency.
A group of studies is providing a new and improved image of the fire types, frequency and history of Oregon's ancient forests, and one Oregon State University project gives a view of the Coast Range 500 years back in time.
This research showed major, sweeping fires that occurred about 150 and 450 years ago, and large events that killed fewer trees about 240 and 350 years ago. A pattern emerges of catastrophic burns about every 100 years - and more than 150 years since the last one.
The study was based on the analysis of more than 4,000 tree stumps near Triangle Lake west of Eugene, which with their scars can provide a fairly accurate and detailed history of fire events.
"Our goal is to identify the fire history and effects of climate, landscape and topography that can affect fire behavior," said Peter Impara, a doctoral student in the OSU Department of Geosciences. "Such information is essential for the new moves towards ecosystem management."
Impara said his study, with its limited but fairly precise view backwards in time, coincides with a different research project at the University of Oregon which has used charcoal sediments in lake beds to determine fire events backwards up to 9,000 years.
That study has identified many major fires in the Coast Range, Impara said, but in the distant past has also found periods up to 1,100 years long with only a couple big fire events.
The large Tillamook burn of 1933, Impara said, may provide a glimpse of the factors at work in Coast Range fires. For many years, the wet, cool winds blowing off the ocean produce regular and heavy precipitation, reducing frequent fire but building up high fuel loads.
Then, an unusual drought sequence for an extended period may dry out the coastal forests, and strong, hot, summer winds can blow eastward over the Cascade Range. With an ignition source, the stage is set for a major firestorm that can burn thousands of acres in hours or days.
"What we're seeing in our historical data is that the Coast Range doesn't burn so often, but when it goes, it really goes," Impara said. "And in those big burns, topography is less of a controlling factor. Almost everything burns."
Fire, ecologists know, is a critical component of the forest ecosystem. It can kill trees, rejuvenate stands, change the tree species mix and age classes, affect nutrient flow and soil health, and have profound impacts on wildlife. As such, an understanding of its past occurrence is essential to establishing management systems for healthy ecosystems.
Impara said his study, among other things, will give an estimate of the amount of old growth forest that existed in the Coast Range during the past 500 years. Contrary to some assumptions, these forests always had a blend of age classes and tree types.
His research project also identified two fairly distinct fire zones in the Coast Range. About two-thirds of the way across the range, coming inland from the coast, a warmer and drier "microclimate" tended to produce somewhat less severe but more frequent fire events.