Chris Vogel believes climbing a 160-foot-tall ladder in a snowstorm is one of the best cardiovascular workouts in science. “It’s not dangerous,” the University research scientist quipped. “Just cold.” But when an instrument fails on the University of Michigan Biological Station Flux Tower, he doesn’t have any choice but to climb.

Sarah Royce
The University of Michigan Biological Station Flux Tower this summer. (AJ HOGG/Daily)

Built in 1997, the tower is part of Ameriflux, an umbrella organization of nearly 150 research sites stretching across the Americas, from Canada to Argentina. Ameriflux is an effort to understand the way different ecosystems – such as tundra and farmland – respond to changes in carbon dioxide in the atmosphere.

Knowing this, researchers can predict how land-based ecosystems will respond to climate change. In the forest ecosystem, the Flux Tower contributes by measuring the addition and removal of carbon dioxide from the atmosphere.

“It’s a great resource that frames a lot of what we do,” said Ecology and Evolutionary Biology professor Knute Nadelhoffer, who is also the Biological Station director. “It allows us to put biological and ecological measurements into a region and climate context.”

Situated in the middle of a University-owned forest located in the 10,000-acre Biological Station, the Flux Tower, a collaboration with researchers from Ohio State University and Indiana University, is located 20 miles south of the Mackinac Bridge.

The tower will help answer one major question facing the researchers: How can forests help mitigate climate change by removing carbon dioxide from the atmosphere?

All the fossil fuel burned today – the source of the excess carbon dioxide that is causing climate change – was once plant litter. That dead plant material fell to the ground, was buried, and converted over long periods of time into oil or coal.

Nadelhoffer says research at the Flux Tower aims to shed light on how quickly today’s ecosystems are sending that carbon back into the earth rather than into the atmosphere. “What proportion of annual tree growth is getting stored, as the end product of plant litter decomposition, in soil over the longer term?” he asks.

How quickly forests and other ecosystems can do this is a key question that researchers have yet to fully understand. Nadelhoffer likens it to “storing a little for retirement in a 401K – but in this case, you don’t know how much you are contributing.”

“You toss your money – your carbon – on the forest floor and wait to see how much is lost. It’s like negative interest,” he said. Much of the carbon is cycled through plants or animals again and again before it goes into long-term storage in the ground.

To accurately measure the flux, or transfer, of carbon dioxide, the Flux Tower team has installed instruments recording wind speed, wind direction, carbon dioxide concentration, temperature and humidity 10 times per second.

That means they collect 36,000 data points for each measurement every hour. These measurements are converted into hourly data points that tell researchers how much water, heat and carbon dioxide is moving up from or down into the forest.

But you don’t need high-tech instrumentation to get a practical lesson in how the forest affects the atmosphere.

When a new instrument needs to be installed on the tower, Vogel starts on the forest floor, next to a research building about half the size of a semi-trailer. It hums with the sound of pumps pulling air samples from the top of the tower to the ground. About 40 feet above the surface, he finds himself surrounded by the branches of the forest canopy. As he climbs past the tips of the trees, he is still 70 feet below the tower top.

“There is nothing like actually experiencing the sudden increase in wind speed as you climb above the canopy,” Vogel said. “You just don’t get that from a graph of the data.”

The tower data, however, measures only the net transfer of carbon dioxide as measured from the top of the tower. To double-check this measurement, the Flux Tower team also measure the biological processes within the forest.

These ground measurements include tree diameter growth, the amount of leaves on the trees, the amount of carbon dioxide coming from the soil, and other sources and sinks for carbon in the forest.

The comparison of the ground and tower measurements surprised Vogel.

On a yearly basis, the two sets of data only match within about 25 percent. But once the Flux Tower data were added up over a five-year period, the two data sets began to converge. This led to the realization that there seems to be a one-year lag between the tower data and the ground-based data.

One hypothesis of the cause of this “mystery carbon” is that the trees take it from the atmosphere in the fall, and store it as starch in their stems and roots. The carbon uptake doesn’t show up as growth until the following year.

Although the cold weather has slowed research for now, Vogel hopes to catch the mystery carbon once the leaves bud in the spring.


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