Looking for fire in mud at the bottom of a lake sounds like a prank scavenger hunt, but it really can reveal ancient evidence.
Below layers of charcoal left over from the 1910 Great Burn, Phil Higuera and his colleagues at the University of Montana have found traces of forest fires dating back 2,500 years. With an $860,000, four-year grant from the National Science Foundation, they have expanded their lake mud collections through the Rocky Mountains from Colorado to Montana.
“You can go to a disk of sediment that’s 2,000 years old, pull out a pine needle and it looks the same as one you pull off the forest floor today,” Higuera said. The project targets small, deep, high-mountain lakes that collect sediment over the years but don’t release it easily. The mud forms a time capsule that contains pollen, ash and isotopes left over from plant material that show what the forest was like decade by decade.
Using a raft and long polycarbonate tubes, the researchers pull 3-inch cores of mud 6 to 10 feet long from the lake floor. Those cores can be segmented to pinpoint the “Little Ice Age” between the 1700s and 1900s, the “Medieval Climate Anomaly” between 800 and 1,100 years ago, and specific historic events like the Great Burn. Chemical markers from those events help calibrate each mud core so it can be compared to others from different lakes.
“This lets us utilize the past as a set of experiments to see how ecosystems respond to climate change and environmental change in general,” Higuera said. After major forest fires are detected, the plant matter in subsequent years shows how the local forest responded. The tempo of heavy and light fire years gets revealed, along with a picture of what effects different climates had on the landscape.
Similar results can be recovered by studying ancient trees. A team of researchers from Portland State University recently published their findings from 1,767 fire-scarred trees in 97 South American sites across a variety of landscapes. Some of the trees dated as old as 990 A.D. and showed changes that correspond with the Southern Annular Mode (SAM) climate cycle. The cycle tracks changes in westerly wind patterns that bring warm, dry weather in the Southern Hemisphere.
"We found that wildfire activity over the centuries has been increasingly favored by the warm phases of SAM going back to 1665," lead study author Andres Holz said. Wildfires in those warm, dry periods grew in frequency in the 20th century, and the trend appears to accelerate in the 21st century. Holz’s study was published in the National Academy of Science’s PNAS journal.
While the lake mud project got started in Colorado, Higuera said expanding its scope to Montana has greatly increased the vegetative picture. Lakes in western Montana catch a mix of dry and moist forest types that’s missing from the more arid Colorado mountain ranges.
While Higuera’s expertise is in fire history, this technique falls under the more esoteric name of paleoecology.
“It’s studying processes that take place over thousands or hundreds of years,” Higuera said. “We have to understand what’s going on with Lolo Peak fire last week and understand the past in ways that make it relevant to the present and future.”