By Ray Ring
High Country News
It is comforting to believe that catastrophic fires are something unnatural we have caused, because that implies we can stop them, with our forest thinning and other strategies. But that’s another assumption that may not pan out, says Jennifer Pierce.
Pierce, a third-generation geologist in Carhartts and turquoise earrings, scrapes the dirt with a shovel in the mountains just 50 miles from the National Interagency Fire Center in Boise, Idaho. Around her is an entire landscape of multiple catastrophes. The place looks like it’s been bombed. A series of fires beginning in the late 1980s has burned about 1.5 million acres of federal, state and private land. The fires swept away vast expanses of vegetation on highly unstable, very steep slopes — and that caused many dozens of massive mudslides.
"Don’t call it a mudslide, please," Pierce says, as she crouches on one slide that settled beside the South Fork of the Payette River. "It’s a debris flow."
Pierce is working on a Ph.D. in geomorphology, the study of how landscapes are formed. In the layers of mud, she’s finding a record of fires that have burned through here in the past. Her work, published just a few weeks ago, is part of the cutting-edge fire science that may force land managers to rethink the age-old dogma that says catastrophic wildfires are the fault of humans.
Beginning only about 10 years ago, tree-ring researchers made the connection between fire and climate change, when they showed that widespread fires in the Southwest United States have erupted consistently over the past 300 to 400 years, on roughly the same schedule as the planet’s El Niño-La Niña weather cycle. Paleo-ecologists at the University of Oregon have looked even further back in time; examining lake sediments that contain ancient charcoal fragments, they are discovering that widespread fires have occurred during the warmer, drier periods in the Northwest and Northern Rockies, stretching back thousands of years.
The tree rings and lake sediments show a lot of fires during past warming spells, but are not so clear on how severe the fires were, says Pierce.
That falls into the realm of geomorphology. A catastrophic fire that denudes an entire slope and causes massive erosion leaves a telltale deposit of charcoal fragments and sediment at the bottom of the slope, where the debris flow helps build a fan-shaped deposit called an alluvial fan.
The first researcher to use alluvial fans to link catastrophic fires and climate was Pierce’s mentor, Grant Meyer, a geomorphology professor at the University of New Mexico. His 1995 paper, focusing on Yellowstone National Park, showed that the lodgepole pine forests there suffered catastrophic fires in sync with long-term climate shifts, with big flare-ups during what’s called the Medieval Warm Period — roughly 900 to 1200 A.D. During that period, lakes receded, treelines rose in elevation, and parts of the West were gripped by prolonged drought.