Inventorying U.S. forests to determine complete carbon removal capacity

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David MacFarlane (view larger image)

By 2050, the United States seeks to reduce its carbon emissions by 80 percent, a goal set forth by the Obama administration as part of a comprehensive plan to address climate change. For carbon that does enter the atmosphere, the planting of trees and the prevention of deforestation can help. Carbon, in the gaseous form of carbon dioxide, is taken in by trees and thus removed from the environment. Researchers such as Michigan State University (MSU) AgBioResearch scientist David MacFarlane have been working to measure the amount of carbon in trees, hoping to learn the capacity of these natural resources as carbon-removal mechanisms.

The U.S. Forest Service enlisted six universities across the country — MSU, the University of Maine, the University of Georgia, Virginia Tech University, the University of Montana and Oregon State University — in a five-year project to collect data on various tree species and their carbon content.

“For a long time, scientists have known that trees are related to the global carbon cycle,” MacFarlane said. “So when people started talking about how we could offset emissions from human activities, one of the ways is to grow trees. This has to be converted from the idea phase to learning how much carbon is actually in trees. This project started in 2011, and we’ve been collecting a large amount of data.”

A national forest inventory is taken by the U.S. Forest Service’s Forest Inventory and Analysis (FIA) program on a network of permanent sample plots across the country. The plots help researchers estimate the number of trees, the timber volume of various species, the speed of growth and other valuable information describing U.S. forests. FIA’s early inventories focused on national and regional timber stocks. Now, MacFarlane said, scientists’ viewing forests as living ecosystems that serve many purposes is expanding the scope of FIA.

Trees are composed largely of carbon dioxide and water, with small amounts of nutrients from the soil as well. To get an accurate measurement of the amount of carbon in various types of trees, researchers must cut down trees of different sizes and species. They then cut each tree into manageable pieces, measure the green weight of each piece and, in the lab, remove the water by heating the wood. The final step is to determine the carbon content of the dry mass.

“It turns out that, in general, a chunk of a tree is about half water,” MacFarlane said. “The dry mass is about half carbon. So if you know the mass of a tree, you can use that rule of thumb and say about a quarter of that green mass is carbon. But the only way to truly know the mass of a tree is to weigh it. We don’t want to cut down a whole bunch of trees unnecessarily just to
weigh them, so we have to be smart about it and collect as much data from each tree as possible.”

Mathematical equations have already been developed for determining carbon content. According to MacFarlane, however, there hasn’t been enough data collected to make the equations reliable.

“The universities on the project have monthly conference calls to talk about progress, and this is when we share the new data,” MacFarlane said. “We didn’t have a lot of great data from across the country, so the equations that were developed previously were just sort of cobbled together. They’ve tweaked the equations slightly, and now they have two methods, but the two get different answers. Without a comprehensive dataset, we can’t be very accurate in our estimates.”

In response, the group developed a plan to target species that are prevalent across the country. Each researcher would study similar trees and how they grow in various areas of the United States, reporting any new findings back to the team. In December, MacFarlane traveled to Portland, Maine, to the National Forest Inventory Symposium. The weeklong meeting featured researchers from the Forest Service showcasing findings from their work. MacFarlane and his colleagues presented on testing existing equations with new data, as well as new models for improving estimation. He believes that, as the dataset grows and the equations are refined, the public will take notice.

“It’s nice to plant trees and talk about the good it does, but what people really want to know is, ‘How good is it?’” MacFarlane said. “Public investment in forest carbon offset projects is significant. We can start treating carbon offsets like a commodity. We know how much a bushel of wheat costs. It varies, but we can quantify it pretty easily and assign a dollar value to it. But how much do forests offset carbon emissions? As we collect more data, we’ll continue to learn.”

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