In northwest Montana’s Swan Valley, a pile of about 100 small logs, 10 feet long or so, are neatly stacked, surrounded by berry bushes, a few white wildflowers and towering larch trees. Surrounding the logs are several acres of US Forest Service land, which was cleared last year of dead, downed and dense understory trees to reduce the risk of wildfires. The woodpile that remains is too small to be processed into lumber, plus the sawmill just off the highway that was recently closed. So the wood can be sent to a pulp mill, if the price is right. Or it can sit in the forest for years. Smaller limbs can be burned in a prescribed fire. But Ning Zeng, a climate scientist at the University of Maryland, is also making the pile bigger. He sees another solution: burying the logs and all the planet-warming gases they would otherwise release underground.
This is the idea behind a carbon sequestration technique called wood vaulting. Forests in much of the western US are overgrown, full of tangled trees and brush ready to burn. The Forest Service’s wildfire crisis strategy calls for the removal of excess vegetation on up to 50 million acres of federal, state, tribal and private lands by 2032. Scientists and climate technology companies alike say tree canopy can help store some of the carbon dioxide equivalent, in the form of flammable vegetation, which the Forest Service must deal with in the coming years — an an estimated 2.2 billion metric tons. This is about as much CO2 as cement production worldwide sent out in 2016, and as many as forests worldwide remove out of the atmosphere last year.
“There’s more wood in the forest than there are markets for it,” says research forester Nate Anderson, who studies product supply chains for the Forest Service’s Rocky Mountain Research Station in Missoula. Appreciating the carbon stored in wood vaults could change that.
If done properly, burying the debris can help limit its release greenhouse gases which warms the atmosphere and contributes to climate change. “I see no reason why it can’t actually be very significant and reach millions of tons of carbon dioxide sequestered per year in the US alone,” said Sinéad Crotty, the director of the nonprofit Carbon Containment Lab. Daniel Sanchez, a professor who studies CO2 removal at the University of California, Berkeley, agrees. “Wood vaulting is a new approach that we think is relatively low-cost and relatively scalable,” he said.
Investors, including Bill Gates, poured in millions of dollars in recent years in the beginning of wooden safes. A handful of small-scale sites are underway across the country, including in Maryland, Nevada, Texasand Colorado. The Department of Energy recently awarded $50,000 for two companies, including Zeng’s Carbon Lockdown Project, to build a wood vault in Montana — one of many carbon dioxide removal, or CDR, pilot projects to receive funding. According to the 2023 Intergovernmental Panel on Climate Change Synthesis reportCDR technology — which can include direct air capture technology, land-based carbon sinks, and more – is an “inevitable” component to limiting warming to 1.5 or even 2 degrees Celsius, which experts say is necessary if we want to avoid irreversible effects of climate change.
Timber vaulting is conceptually simple: Dig a hole with an excavator and bury small trees, woody debris and other plant material not large or valuable enough to sell. The vaults look like a layer cake of trees, gaps filled with dirt, and more trees piled on top, finished with a frost of topsoil.
Once companies acquire biomass, not just any hole will do. The intrusion of water, oxygen and even termites can compromise a safe’s durability by encouraging decomposition. Digging vaults in clay or silt soil, away from groundwater, is considered best practice. (Some alternative methods actually completely submerge the wood in water, but this is less common.) Conditions inside the vault must remain stable—ideally forever, without human intervention—for projects to live up to their promises. The same types of sensors already used in landfills can be installed to collect data such as oxygen, moisture and methane levels over time.
Digging holes 15 to 25-plus feet deep can mean disturbing soil, destroying habitat, or removing nutrients from the landscape—all potential drawbacks of wood vaulting—so companies are considering using already degraded locations, such as old industrial sites or mines. (Several also have plans to rehabilitate wood vault sites after trees are buried, to plant the soil with native seeds for pollinator habitat or pasture use.) The ideal location for a wood vault is close to the source of its biomass, reducing emissions from transportation . and logistics. Sufficient labor to transport biomass and build vaults, plus some sort of protection that they won’t just be dug up in a few decades, are other key factors.
Another concern with timber vaulting is that it may encourage more logging than necessary. But so far the industry has focused on burying the remnants of wildfire risk reduction treatments, as well as trees that have already burned or been danger trees removed from urban environments. Guidelines of Stripe Inc. s Frontier Fundone of the leading funds buying carbon dioxide removal credits from start-ups, recommended the remains of wildfire risk reduction projects as a sustainable source.
The science of how long these vaults can keep CO2 out of the atmosphere is still being worked out. “We want to be as clear as possible when we promise anything around durability,” Crotty said. If done properly, Sanchez thinks vaults might be able to store CO2 for hundreds to thousands of years; companies share numbers ranging from 100 plus on 1,000 plus years.
While it’s hard to say exactly how long wood vaults can store carbon, past discoveries hold clues. A bulldozer at Carbon Lockdown Project’s Canada site found a red cedar tree buried deep in the ground; Zeng still has it in his office today. He says further analysis (yet to be published in a peer-reviewed journal) confirmed that it was 3,000 years old but had lost only 5 percent of its carbon.
Another way to analyze wood vaulting’s potential is by comparing how much carbon it can store with other more established techniques. Wood vault’s carbon yield is high compared to that of biochar, a charcoal-like soil amendment derived from partially burned organic matter. Biochar can retain about 30 percent of its biomass’s original carbon, while wood vault is thought to store more than 90 percent, according to Sanchez.
Wood vaulting is also quite cheap compared to other methods. Calculations from one of Zeng’s test sites found it costs $105 per metric ton of CO2, mostly in transportation costs. For comparison, the average cost of biochar in California is $400 per metric ton of CO2, and direct carbon capture technology can cost anywhere between $600 to $1,000 per metric ton of CO2. “This is the transformative aspect of this idea compared to many other biomass utilization strategies,” Zeng said. “It’s going to come down to economics.”
Several companies are currently trying timber vaulting, mostly on private land. Zeng has the Carbon sequestration projectwhich began with a research site outside of Montreal, in 2013. He is now working on a property in Maryland for trees removed from urban environments that would otherwise have been mulch.
Then there is Mash reforestation, a company that aims to replant forests after burning them for carbon credits. CEO Grant Canary said he was particularly interested in burying the already burned trees that posed a danger to workers and the small trees they planted. The company plans to store between 5,000 and 20,000 metric tons of CO2 equivalent derived from trees on hundreds of acres of recently burned private land in central Montana. Mast Reforestation is working with Zeng’s Carbon Lockdown Project on the site, and Canary said construction could be underway as soon as late 2024 or early 2025. As a winner of the Department of Energy’s award, the project promises more than 17,000 carbon dioxide removal credits to the federal government by the end of 2028.
Kodama Systems is also in the permitting phase for a wood vault to store approximately 1,000 metric tons of CO2 equivalent in western Nevada. Material could be in the ground as soon as later this year. Dry pastures in this area are considered the best for timber vaulting; researchers at the Carbon Containment Lab also say the Four Corners region of Arizona, New Mexico, Colorado and Utah has high potential for wood vault development. Carbon Containment Lab scientists work with companies, including Kodama Systems, to collect their own independent data.
So what is needed to transform wood vault projects from the handful of pilot sites that currently exist to a carbon dioxide removal technology at scale? More science to verify longevity and storage claims, more money to start additional projects, and more buyers of carbon dioxide removal credits. While companies and scientists have set their sights on one day working with the Forest Service, government land management agencies are notoriously slow to try anything new, meaning a public-private partnership on wood vaults is still a long way off.
For Zeng, standing in the middle of the woods, it was enlightening to see everything from mill infrastructure to potential log vault sites in Colorado, Montana and beyond, melding theory with on-the-ground practice. “I’m really encouraged on this trip,” he said. “The dots are connected.” Although wood vaulting is not a one-handed solution to overgrown forests, wildfire risk and a rapidly warming climate, it can be a simple solution to sequester some carbon and also reduce wildfire risk.
