When Kyra Clark-Wolf paddles her inflatable across a tree-lined Northern Rockies lake, she’s not snapping out a line in search of trout. The trophies she pulls out of the water look more like dirty dishes than prized wildlife.
She’s actually mucking around on the lake bottom with a core sampler, pulling up tubes of mud whose layers tell the stories of hundreds of years of forest growth alternating with periods of devastating wildfires.
Clark-Wolf is a post-doc in ecology at the University of Colorado, and she likens her lake studies with co-author Philip Higuera to sticking a straw into a glass and pulling up a sample of chocolate milk. If the milk could detail 4,800 years of fire.
The researchers set out to compare silt layers, which feature debris easily pinpointed to known years of fire, against a precise set of historical climate data for northern Montana and Idaho lakes. They wanted to see how forests surrounding the lakes recovered after fire. Healthy layers of pollen and seeds settling on top of charcoal debris would be a good sign of resilience.
“It’s a little counterintuitive to use a lake to study fire,” Clark-Wolf said. “But it turns out that they’re really useful because they’re the low points on landscapes, so they collect lots of materials from the surrounding forest. And those materials get buried and preserved in layers of lake mud that build up over time, and can sit there for thousands of years, without degrading.”
Pollen layers tell the researchers that after fires, greenery returned to the lake environment and started pumping out the yellow gold once again as the forest regenerated. A half-centimeter of bottom mud can reveal the natural history of five to 20 years of the surrounding forest.
And what they found provides both hope and a warning for current climate change, Clark-Wolf said in an interview. Northern Rockies forests appeared to do well after ancient fires, even during warmer periods that can stress forest regrowth.
“Forests sort of have these limits, the upper bounds of the conditions that they can handle,” Clark-Wolf said. “Until you’ve hit those limits, they’re generally able to cope with fire activity and return to a similar condition as before the fire. Within the last 5,000 years, we just really haven’t hit those limits.”
She quickly followed the note of optimism with a decidedly minor key: “That doesn’t mean that we won’t.”
Especially in the Southern Rockies, including at Colorado’s forest-surrounded lakes, temperatures have soared faster with climate change. Those “upper bounds” have been sorely tested with massive wildfires since 2000 like the Hayman fire, East Troublesome, Waldo Canyon, Cameron Peak and others.
“In these warmer, drier areas in the Southern Rockies and the Southwest, forests are in fact having a harder time recovering after fires,” Clark-Wolf said.
In their study, published in the Journal of Ecology, the researchers also reference previous studies of 2020 wildfires in Colorado, including the East Troublesome and Cameron Peak fires. Colleagues have taken mud samples from Odessa Lake and other water bodies in Rocky Mountain National Park, as well as multiple lakes in the Park Range of Colorado, said Higuera, from the Department of Ecosystem and Conservation Sciences in the University of Montana’s College of Forestry and Conservation.
“It’s not that fire in any of these ecosystems is a new thing,” Higuera said. “But two things are happening with higher temperatures now: We’re shortening that time between fires, and we’re making it harder for that vegetation to come back after fires. So both those things are making it more and more likely to have different forest and vegetation composition, when they do come back.”
Higuera and others now want their future research to add in the human factors that come with modern wildfires in more populated places. They have a good picture, from 1,000-year-old mud, of past fire activity around Silver Lake in Montana’s Northern Rockies, and in Colorado’s Southern Rockies.
But the 2020 studies showed them new dangers from human-ignited fires like East Troublesome and Cameron Peak, which could further increase the frequency of major wildfires in Colorado.
“That’s different than in the past; and different than in the Northern Rockies,” he said.