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View of the forest from the top of the tower. Photo Credit: David Foster, Harvard Forest. Creative Commons attribution CC BY-ND

A tower in the forest that tells the tale of carbon

In the middle of a 3,000-acre forest, there is a silver, metal tower. Industrial in style and size, it looms above the tree canopy. Like a cell tower, it could be called an eyesore. It is clearly a man-made object in the middle of an otherwise pristine-looking forest.
In the middle of a 3,000-acre forest, there is a silver, metal tower. Industrial in style and size, it looms above the tree canopy. Like a cell tower, it could be called an eyesore. It is clearly a man-made object in the middle of an otherwise pristine-looking forest.

But this tower holds the secrets to the carbon flux of the forest, taking measurements every hour for the last thirty-tree years. This tower is called an eddy flux tower, and it is where scientists take measurements of long-term ecosystem health. These measurements contribute to the baseline understanding of how forests are affected by climate change.

Carbon is a trendy topic but there’s conflicting information – we’re emitting too much but trees are storing a lot of it, too. This conversation may seem like a new topic, but scientists have been studying the carbon cycle – and the role humans play in it— for a long time. 

The oldest eddy flux tower is located in the Harvard Forest, a long-term ecological research site in central Massachusetts. The towers rely on a method called eddy covariance, a type of meteorological method that measures the exchange of gas and energy between ecosystems and atmospheres. The eddy covariance method was discovered in 1951 and has since become a key element to studying the carbon cycle.

Every hour, it measures the gas exchange between the forest ecosystem, the respiration from the leaves and trees, and the air. The tower stands above the tree canopy and has sensors from the ground level to the top to capture the full profile of the forest. When the data from the sensors are aggregated together, scientists can examine how much carbon is emitted in the atmosphere and how much carbon the forest is storing. 

Oldest eddy flux tower in the Harvard Forest. Photo Credit: David Foster, Harvard Forest. Creative Commons attribution CC BY-ND

The tower at the Harvard Forest is just one of many long-term climate recorders. “You name a place, there’s a tower there,” says Ian Smith, a PhD student at Boston University who relies on eddy flux to study forest vegetation. More towers mean more measurements, which allows scientists like Smith to compare the carbon fluxes in one area to another. That way, they can see how different landscapes are being affected by ever-rising greenhouse gas emissions. 

Audrey Plotkin Barker is a forest ecologist using both on-the-ground measurements of trees and eddy flux data to study long-term change in forest ecosystems. “I love walking up the steps of the tower, because you get to walk through the canopy and get a photon view of the forest. You see the photosynthesis in a different way.”

Most recently, Barker Plotkin’s data formed the basis for a study in Ecological Monographs that found that New England trees are accumulating more carbon than ever before. 

The study was a synthesis of eddy flux data alongside inventory methods. Barker Plotkin and her co-authors also found that one part of the forest was continuing to store carbon—much more than they ever thought was possible —a rate that has doubled since 1992.

“When the tower began in 1990 people really thought forests would be maxing out on ability to accumulate biomass. That’s not what we have seen. The trees are growing strongly.”

When trees grow, they’re storing carbon – most of it is in their trunk, but there is carbon in their leaves, roots, and branches, too. Some scientists believe that one tree can store forty-eight pounds of carbon and forests with older trees store more than younger forests.

One of the several sensors on the eddy flux tower. Photo Credit: Julian Hadley, Harvard Forest. Creative Commons attribution CC BY-ND

A different story emerged in another part of the forest that is dominated by hemlock trees. Hemlocks are suffering from hemlock woolly adelgid, an invasive pest that sucks the sap from a tree slowly kills it by disrupting its normal growth. The massive die-off of hemlock trees turned this part of the forest from a carbon sink to a carbon source. 

Barker Plotkin also says that the towers and the associated research have influenced our understanding of the many ways humans impact the carbon cycle. Humans impact the carbon cycle through the usual culprits—driving, flying, and eating meat to name a few. But Plotkin Barker emphasizes that since the hemlock woolly adelgid is an invasive brought in by human trade, humans indirectly affect the carbon cycle too—and potentially in more ways we haven’t discovered yet.

Ian Smith, who was not involved in the study, cautions thinking of an area purely as a carbon sink. “Every ecosystem has sources and sinks. More carbon dioxide in the atmosphere means humans are likely emitting more carbon. That can increase the strength of a sink, which isn’t always good.” 

Letting trees grow could help slow the effects of climate change, but the authors caution that prioritizing forest health is essential for this process to be successful. Says Barker Plotkin, “It’s a strong case to keep forests, forests.”