Carbon coppice
Or: Why willow?
If you’ve talked to me in the last three months, you’ve heard me talk about willow. Specifically, willow coppice. I provide a mildly manic monologue, with gesticulation, upon request. But for now, some context on why I’ve been talking about it. The basic concept is this: Sure, trees hold carbon, but I think trees can be a negative emissions technology.
It’s not a new idea. But it’s still worth breaking down. Starting with the last part first: “Trees can be a negative emissions technology.”
Yes, trees take in carbon dioxide and release oxygen. A lot of the carbon becomes the tree itself, specifically: cellulose, hemicellulose, and lignin -- organic polymers that make up wood. So, as long as the tree stays alive, that carbon is safely out of the atmosphere and inside the tree where it doesn’t contribute to climate change (yay). It’s no small amount, either -- when a tree is alive, ~30% percent of it, by weight, is carbon; it’s about 50% by weight when it’s dry. (Do your own calculations with this amazing guide (pg 4.) A single tree 7 feet in circumference and 75ft tall has ~1 ton carbon (3.67 tons of C02e) in it. Ergo: forests sequester a lot of carbon.
Now: “Trees can be a negative emissions technology.”
Trees die. When they die, about 90% of the carbon in them goes back into the atmosphere via decomposition. It takes a while--say 5 to 100 years--but the vast majority of the carbon they sequester gets released back into air as bacteria, fungi, and bugs eat the tree. The remaining ~10% goes into the soil. That means that when a tree is growing, it’s a net sequester of CO2, and when it’s rotting, it’s a net emitter. The same goes as you scale up from tree to forest. Young forests sequester a lot of carbon, while old forests are approximately neutral, as the growth of new trees is equal to the decomposition from old trees. That’s no dig on old forests. They’re some of our most valuable carbon sinks, and they produce huge amounts of oxygen and other positive externalities like clean water and animal habitat. But it’s important to understand that while growing plants sequester carbon, forests don’t continue to sequester lots of carbon (in net terms) after reaching maturity.
Furthermore, trees burn. Without fire suppression, approximately 4% of the land on earth would burn each year. Nowadays, it’s less--due to fire suppression efforts. Nonetheless, when trees burn, they release the vast majority of their carbon back into the atmosphere. The wildfires in Australia in early 2020 released approximately 900M tons of C02 (2x Australia’s year emissions) in less than a month. So, while forests can store carbon, it’s not guaranteed they will forever.
How to get around this? Let trees grow, and then periodically convert them into something that sequesters carbon and persists better than a tree: biochar.
And finally: “Trees can be a negative emissions technology.”
With this all in mind, it’s becoming clear that if we want to take A LOT of carbon out of the atmosphere, we’re going to have to plant new trees, in addition to keeping our old forests. Please don’t get me wrong - we need our old forests, and should not cut them down, let alone cut them down and make them into biochar.
So, trees --> biochar! Right? Yes! Well, with some caveats. Most trees don’t grow that fast. The faster the plant grows, the more CO2 it’s capable of sequestering, so if we’re prioritizing carbon sequestration, we need to prioritize speed. Grass grows fast, right? What about grass? Or corn? Great question, and one we’re going to spend a lot more time on (but not right now).
But clearly, it’s not just about speed. It’s also about size - grass grows fast for a year, but then slows down. So plant redwoods! They’re enormous. But, as you might guess, they grow slowly. So what to do?
Grow willow. Why willow? Because it...
has a similar biochemical profile to hardwood, allowing it to be easily processed into biochar.
grows quickly (~10 tons/acre/year on average).
doesn’t have to be harvested by hand, reducing the implied GHG of harvesting the crop.
once planted, can be harvested every three years without replanting.
maximizes the natural growth curve of the plant, maximizing carbon drawdown.
Until now, all trees have been thought of as carbon sinks. Which is valid, but by combining the process of pyrolysis with a fast growing tree that we treat as a crop, rather than as a forest, we turn the sink into a mechanism for negative emissions. Trees can be a negative emissions technology.
The best tree for this: willow coppice.
Interviews and more:
Can we use biochar to reduce methane emissions from cows?
I chat below with Heather Nobert of the University of Nebraska on research she’s doing to test biochar as a feed additive for cows to help reduce methane emissions. Promising stuff! 🐄
How do biochar and forest fires fit together?
I chat below with Chris Carstens, CTO and Cofounder of Carbo Culture on his work building cutting edge pyrolysis tech, forest fire management in the Western US, and he shares some biochar-related wisdom, including that dinosaurs ate biochar.
Find yourself convinced by the writing above, and want to know more about willow coppice?
Check out the start of a locally planted willow coppice at Wally Farms - pictures below, and more videos and whatnot in the Instagram feed.
Can we make biochar and help farmers heat their greenhouses at the same time?
Erik Schellenberg, owner of Black Creek Farm & Nursery here in the Hudson Valley, thinks so. And I tend to agree. Check out the video of him here, and click through to see my twitter thread on the same topic below.
Finally, how do we get biochar in the hands of local farmers and gardeners, and start a distributed, backyard drawdown?
I spoke with Silvia Sandri, head of the Local Carbon Network project at All Power Labs. Neat proof of concept thus far, and would be really excited to see this scale up. Interview below:
https://localcarbon.net/ is a great place to get information on LCN
Also https://medium.com/local-carbon-network