Tree rings don’t lie.
It’s an adage Grant Harley uses in his research in dendrochronology, the science of dating events and environmental variations using the growth rings in trees and aged wood.
The subfield dendroarchaeology applies the science to historic constructions. It’s a lot like detective work, said Harley, an assistant professor of geography and geological sciences at the University of Idaho.
His team’s latest case: the Colville Indian Agency Cabin in Chewelah.
Run in around the 1870s by John A. Simms, who used the cabin as a residence, the agency oversaw federal funding support for the Colville, Spokane, Coeur D’Alene and Kalispel tribes, said Susan Richart, treasurer for the Stevens County Historical Society. After the agency relocated to Fort Spokane in the early 1880s, the cabin was purchased in 1906 by a doctor, Samuel McPherson.
The cabin remained with the McPherson family until 2010, when it was gifted to the Stevens County Historical Society.
Upon digitizing records kept at Washington State University that belonged to Simms, Richart said she found the manuscripts did not indicate when the cabin was built. Now, the society has tasked Harley and his team of graduate students to determine the age of the cabin and two other nearby structures known as the McCrae cabins.
“Because we’re a historical society, the facts are important to us,” said Richart, who has largely coordinated the dendrochronology project. “When we realized that Dr. Harley and his bunch could determine it, that was good for them. In the scheme of things, it’s not really that big of a drive from the University of Idaho to Chewelah.”
Harley, who joined the University of Idaho in 2017, said he offers the expertise of the Idaho Tree-Ring Lab to date historic structures free of charge. He said the work makes for valuable research projects for him and his graduate students.
“The most important thing is the people in Chewelah support the cabin and recognize its importance,” Richart said. “A lot of them grew up with it being there and they knew the McPhersons. They’ve been extremely supportive.”
Harley said his team spent three days in May at the cabin to acquire usable cores from the walls and infrastructure. He said researchers extracted roughly “pencil-sized” cores from the logs using a 5-millimeter drill bit.
The drilling does not affect the cabin’s structural integrity, Harley said. Richart said the historical society, which is using a state grant to renovate the cabin, plans to refill the holes.
“The attic was the toughest part because it was really hot and there’s bat guano everywhere,” Harley said. “You’re trying to get yourself in a position where you can stay for like 10 to 15 minutes really still because you have this DeWalt hand drill with this bit in it, and you’re just very, very slowly inching it into the tree, because if you go to fast, it’ll break the core or it’ll start to burn.
“It’s just a very slow, tedious process,” he added, “but what you’re left with is this really nice, long sequence of rings that you can date.”
Harley said the next step is to sand down the sample cores to scan and evaluate the rings, using a microscope to measure them “to the thousandth of a millimeter.” He expects to have a chronology established by this fall.
Field observations, he said, suggest the cabin walls were built from Douglas fir with an average ring count that dates back 250 to 300 years. The attic rafters and ceiling joists, he said, were built from some sort of pine tree.
“Whenever it was constructed, to be determined, they cut down or harvested old-growth or virgin timbers, which is exciting,” he said. “It’s terms people use in forestry: first-growth forests, or forests that hadn’t been harvested or logged or cut down.”
In order to determine the wood’s age, Harley said he and his team first had to find a core from a living tree with rings that go back far enough to use as a reference.
He said the U.S. Forest Service issued a permit for them to extract suitable cores from older trees in the Colville National Forest. Using the dendrochronological principle “longevity under adversity,” Harley said researchers sought trees on dry slopes for two reasons: It’s less likely they were logged in the past due to the difficulty to reach them, and science has found trees more starved of resources tend to live longer.
“It’s literally like matching two different bar codes,” Harley said. “We compare the matching of the pattern both visually and with some statistics. Once we get a convincing match, then we’re able to date those historic timbers.”
While the science doesn’t necessarily date when exactly the Colville Indian Agency Cabin was built, historical evidence allows researchers to draw inferences, Harley said.
“Using a couple different lines of evidence in terms of historical journals, it was really common that for people that were building structures in that day, working with greenwood was preferred amongst craftsmen as opposed to working with seasoned wood or old wood,” he said. “It’s really rare that people would cut down a series of trees, let them sit on the landscape for X number of years and then build a structure.”
Beyond dating the wood, dendrochronology can also show other aspects of the time period, such as climate history.
Harley said researchers, for example, have correlated ring width in Douglas firs and ponderosa pines in the Eastern Washington area with instrumental period weather data to evaluate rainfall levels. The wider the ring, the wetter the year.
“Projects like this, we love to do them. We do them for free. We’re a public university, so we take our time and volunteer our time to core these structures,” Harley said. “We’re just in it for the science.”