How Do You Measure The Amount Of Carbon In A Tree?
The latest national climate assessment says forests play a key role in keeping our air clean.
According to the report, America’s forests stored the equivalent of 11 percent of the country’s carbon dioxide emissions over a 25 year period.
That’s because when trees breathe they suck up carbon dioxide, release oxygen, and store that leftover carbon in their trunks.
But how scientists determine the amount of carbon stored in a tree is a question open for debate.
When Bob Marra goes into the woods, he takes a tool with him. It’s a hammer -- his magic sonic hammer.
“It’s called a sonic hammer. But I call it the ‘magic’ sonic hammer, just because it looks kind of cool,” Marra said.
Marra is a biologist with the Connecticut Agricultural Experiment Station. While the hammer isn’t magical, it did do something pretty cool: help us look inside a tree.
To do that, Marra hammered nails into the trunk of a sugar maple in northwest Connecticut, girdling the tree with sensors. Then, he circled and tapped on each nail. Each tap was recorded by a computer.
Marra’s recording sound waves. Measuring how fast sound travels from the nail he hits, to all the other nails around the tree.
It’s called “sonic tomography.” Think of it like a CAT scan for trees. A way to peer inside a trunk without drilling to see if a tree is rotting -- or solid wood.
“The denser the wood, the faster the sound waves,” Marra said.
Dense wood is really good at storing carbon. But if a tree is less dense inside, that could indicate decay. And also, that the tree might not be as good at storing carbon as we think.
Using a grant from the National Science Foundation, Marra tested his tomography idea --- scanning around 70 trees in northwest Connecticut.
He found dozens were rotting inside, even ones that on the outside, looked good.
“What’s going on inside of these trees, is kind of hidden to us, for the most part,” Marra said. “Trees that, otherwise, look to be perfectly fine and you would have no reason to think otherwise, can have internal decay taking place.”
Marra said that’s an important consideration -- especially when it comes to carbon storage or “sequestration.”
“If we’re going to look to forests as a way to sequester carbon, we should develop much more accurate estimates of how much carbon is actually sequestered.”
That’s because there are whole markets based on this.
Take, for example, California. Its aggressive pollution regulations have fostered an expansive cap-and-trade program. California polluters can offset emissions by buying up carbon credits. And landowners across America can profit by “proving” their forest is really good at storing atmospheric carbon.
Rajinder Sahota is with the California Air Resources Board, which oversees the program. She explained the process.
“What you do, is you have a measurement at the beginning of that time period that says, ‘here’s how much is in my forest,’” Sahota said.
Then, through audits, landowners prove their land, over time, can store carbon in a way that’s better than business as usual.
“Here’s how my forest looks relative to what is the common amount of stored carbon,” Sahota said. “And here’s how much, if I undertake some activities, I can increase that carbon storage in my forest.”
But measuring all that? Well, here’s where it gets tricky.
“You look at any tree. Especially a hardwood tree. You look at its shape. That’s really complex,” said Christopher Woodall, a researcher with the U.S Forest Service.
Woodall’s equations are used by California to calculate stored carbon.
“You estimate the volume. And then you got to figure out the biomass within that volume,” Woodall said. “And then, turn that into an estimate of carbon.”
To do that, foresters don’t go out and look at every tree. Instead, they sample. Measuring a variety of trees and plugging those numbers into a complex model.
But forestry science is evolving. Woodall has since published work saying the equations need to be improved. In part, because new technologies are making biomass estimates more efficient and precise.
“I think we’re not too far away from not necessarily sampling trees in the U.S., but actually having a true census. Eventually, with a combination of satellites … and with drones and laser scanning, we’re headed to the point where we might be able to know something about every tree in the U.S.,” Woodall said.
He said that could happen soon or in 50 years. But for now, scientists are taking baby steps, trying to assess the role of forests in climate change. Because, as Woodall said, it’s too important to ignore.