Forest offers clues to fire’s history
A central Idaho ponderosa pine forest is giving scientists a rare glimpse at thousands of years of fire history.
What they’re seeing suggests a hot, fiery future where even the most ambitious fuels-thinning projects will be unable to halt intense fires brought on by global climate change, according to findings published in the latest issue of the journal Nature.
The research also implicates global warming – not overly thick forests – as the leading cause of the massive fires that have been raking the West for nearly 20 years, including the 1988 fires in Yellowstone National Park and the 1991 firestorm in Spokane.
The study analyzed 8,000 years of sediment accumulated in the valley of the South Fork Payette River near Lowman, Idaho. Buried in the sediment are records of massive, prehistoric wildfires. After the fires, rain and snowmelt flowed over a forest floor sealed by the heat and ash, creating a cement-like slurry of charcoal, boulders and bits of trees. The most severe fires caused the thickest blankets of sediment.
Most fire history is based on studying tree rings and typically extends back no further than 500 years, which is the age of the oldest ponderosa pines, said Grant A. Meyer, co-author of the study and associate professor of earth and planetary sciences at the University of New Mexico. “What we know based on tree rings comes from a period of time that was generally cooler and wetter.”
Meyer is referring to a well-documented period from about 1350 to the beginning of the 20th century known as the “Little Ice Age.” Rocky Mountain glaciers grew larger during this period. And the wet, cool conditions allowed grasses to flourish on forest floors, which provided tinder for frequent but cool ground fires.
Foresters often look back on this period as a golden age, when cool fires and old trees seemed to be in harmony, Meyer said. But the time before the Little Ice Age probably provides a more accurate glimpse of the interplay between forests and fire.
Predating the Little Ice Age and tree ring records was a 400-year period of hot, dry temperatures with multiple decades-long droughts, Meyer said. Debris flows dating to the medieval warm spell hold a record of several massive wildfires that burned to the ground the ponderosa pine forest above the Payette River.
“The fires must have been quite severe,” Meyer said.
Evidence from a variety of sources suggests temperatures now are hotter than during the medieval period, the scientists wrote. Most scientists believe the current warming trend is the result of a buildup of greenhouse gases caused by the burning of coal and oil. “The rapidity and magnitude of twentieth-century global climate change is probably greater than has occurred for millennia,” Meyer and his two colleagues wrote in Nature.
Fire had always been allowed to rid the forest floor of excess fuel, yet this did not protect the forest from large-scale destruction by fire during the warming trends, said Jennifer Pierce, a University of New Mexico graduate student and lead author of the study. This flies in the face of the prevailing view that forests can be protected by thinning projects.
Thinning the forests is effective in some areas, but historical evidence now shows that fires will only be hotter and larger on a planet that continues to heat up, Pierce said.
“Management policy needs to incorporate climate change, instead of trying to get back to these Little Ice Age conditions where fires were small,” Pierce said. “That’s just not realistic. Maintenance of those conditions will be difficult in the face of global warming.”
Retired U.S. Forest Service fire ecologist Stephen Arno said climate should not be ignored, but forest thinning remains the best defense against catastrophic wildfire. Recent years have provided “innumerable examples” where thinned forests have slowed the rapid spread of forest crown fires, said Arno, author of the 1999 book “Flames in Our Forest.” He also has a forthcoming book about restoring fire to the West.
“Fuels management can work to mitigate the severity of fires. Even with climate change, you’re still going to need to control fuel buildup,” he said.
Arno lives in Florence, Mont., and has spent the last 30 years trying to restore balance to a ponderosa pine forest on his property. The land now looks like it did before it was logged in the 1880s, with large spaces between the trees, Arno said. Frequent prescribed burns keep the forest floor clear and protect the property from becoming a landscape of ash should lightning or arsonists strike.
“There’s no real possibility of crown fires here,” he said.
Overly thick forests, not climate change, are the main reason fires are growing bigger and hotter, Arno said. After a century of keeping fire out of the forests, “Well, wonder of wonder, these wildfires are getting worse and worse.”
The recent spate of fires has fueled a renewed push to thin forests of the West. Last year, the Bush administration instituted the Healthy Forest Restoration Act, aimed at making it easier for federal land managers to conduct forest thinning projects. Several projects are now being planned for the Inland Northwest.
The forest thinning effort is “draconian” and does nothing to address global warming, which is the biggest threat to western forests, said Cathy Whitlock, a Montana State University earth sciences professor who wrote an accompanying commentary to the Nature article.
“The preponderance of evidence that the system is changing is so great,” Whitlock said. “We should consider this long-term perspective before embracing one-size-fits-all management strategies. … We are in this mindset that all forests need to be thinned and we need to restore ground fires to forests.”
Whitlock said she doesn’t have answers on how to better manage the forests, but she said climate change can no longer be ignored. “Forests are one of the great resources of the West. We need to have a strategy that’s going to sustain them in healthy conditions in perpetuity.”