Mass Timber Realities: To Glue or Not to Glue
A key and frequently repeated benefit of mass timber is that it possesses less embodied carbon to manufacture (compared to typical steel and concrete) and that it is an amazing vessel for carbon storage, holding onto the biogenic carbon it sequestered through its life as a tree. This is true, but sometimes the claims of carbon storage can be overblown, and business-as-usual industrial tree farming is leaving a lot of potential carbon on the table. Why is that, and what can we do to improve things?
The challenges begin before the timber is even timber. Many trees which are slated to become the bones of buildings are grown in plantations that somewhat mimic a natural forest, but they are often cut every 30-40 years. This is known as rotation age, and it is obviously much shorter than most trees’ natural lifespan. These industrial short rotations are terribly disruptive for the ecology in these plantations, and so this practice cuts short one of our best carbon sequestration tools—if the trees could be retained for another 30 years, they would sequester a great deal more carbon.
By specifying a bio-based material like timber, we have a unique ability to influence the natural environments they come from. In the case of maximizing carbon sequestration, for instance, if we specify 2x12 solid sawn timber, we are sending a demand signal that larger trees are desirable. These days, in lieu of larger members like that, the more common product is engineered wood made of small fragments glued back together—this is the easiest way to make commercial use of younger trees, whereas it’s hard to cut many 2x12’s out of a Douglas Fir tree that has only grown for 30 years. We can also consider specifying larger solid sawn members for beams and columns, rather than just using glulams. We can forgo the “cut and mill only to reconstitute the wood with glue” approach.
It’s important to note that I’m not advocating for cutting down old growth! Most architecture specifications restrict the use of old growth, and the past three decades have seen some regulations put in place to help protect those last stands of giants from being cut (though it does still happen; unfortunately most often on public lands—but that is a whole other story). There is a large difference between old growth wood vs. wood that has been allowed to grow a few extra decades to better optimize for carbon sequestration. Some will say that young trees sequester wood more quickly, which is true in terms of speed, but more mature trees sequester more carbon overall—the best analogy I have heard relates to bank accounts. A child’s new bank account might carry a high yield interest rate but on a small principal amount, whereas another bank account that has a larger principal amount in it at a lower interest rate will still accrue more interest over time.
I’m also not saying engineered wood isn’t a part of the solution—many small trees will still need to be harvested to help with forest thinning projects for wildfire resilience. And engineered wood has offered extremely important pathways for incorporating more parts of a tree into long-lived wood products, rather than just as mulch or fuel. It’s also true that, to coax enough space and light into a stand of trees to allow them to grow to a longer life span, it’s often necessary to do an initial cut of half of the trees to open up the forest floor. This is known as variable retention harvest, and means that smaller trees around a 30 year age will still need to be cut, so engineered wood is a great option for making best use of them. So the complete solution requires diversity in tree age, feeding into a diversity of products intended to be durable and kept from decomposing that carbon back into the atmosphere.
Specifying larger members to encourage longer rotations is just one of many ways we can try to help positively influence the source forests and rural communities that deliver these products. We are already seeing projects willing to use “ugly wood” from forest management efforts that take down fire-prone beetle kill pine, or the use of siding cut from Juniper that was taken from invasive-removal projects in the high desert, not to mention many other projects that have specified wood managed by Tribal sources and others that are practicing longer rotations and wildfire mitigation projects. As we think about working as an industry toward Intentional Sourcing, we can explore the full range of sizes, appearance, and other variations that wood offers. And with each choice, we can find ways to benefit not just our projects, but our supply chain partners and their communities, our forests, and the earth, itself.
