Breath of the Forest

view from bottom up of television tower
Peter Bakwin, NOAA

By placing their instruments high in this television tower, researchers with the Chequamegon Ecosystem-Atmosphere Study can measure CO2 levels "above the commotion" of forest photosynthesis and decomposition.

Atmospheric carbon dioxide levels are lower above the Atlantic Ocean than scientists would expect, based on the amount of CO2 released by power plants, vehicles, and industry in the United States. Since the prevailing winds blow from west to east across North America, our continent seemingly acts as a "carbon sink"—a place where CO2, a greenhouse gas increasing worldwide, is scrubbed from the atmosphere. Scientists believe that forests effect this cleansing, as trees use CO2 to fuel their photosynthesis. But forests also produce CO2 when soil microbes break down carbon-laden twigs, branches, and fallen leaves.

To improve our understanding of both CO2 flux in North America and the global carbon cycle, Penn State meteorologist Kenneth Davis is studying CO2 above forests in northern Wisconsin and the Upper Peninsula of Michigan. The work is part of the Chequamegon Ecosystem-Atmosphere Study, or ChEAS, pronounced "cheese"—"It's the Wisconsin connection," Davis says with a grin. He and his research team, along with colleagues from other universities and federal agencies, have placed sampling equipment on towers in the Chequamegon-Nicolet National Forest. This remote, lightly settled area has a diversity of trees, from spindly alders to massive hemlocks. The tallest eminence is a red-painted steel 447-meter TV tower: while Wisconsin Public Television broadcasts News- Hour With Jim Lehrer and Reading Rainbow, instruments at six levels on the tower gulp in air and check its CO2 content.

According to Davis, sampling CO2 near and over the oceans "helps scientists establish background levels and calculate global annual carbon gradients. It avoids the daily extremes that occur over land, where fluctuating CO2 levels make accurate analysis difficult." On land, CO2 levels fall during daylight hours as photosynthesis outstrips decomposition; at night, photosynthesis shuts down and temperature inversions trap CO2 near ground level, concentrating the gas. Says Davis, "The tower gets us above the commotion."

Air samples are continuously piped to the base of the tower, where an infrared gas analyzer measures their CO2 content every 0.2 second. To determine flux, Davis combines the infrared readings with measurements from sonic anemometers, which record wind speed in three dimensions. "After analyzing the data, we can say, "Here's a burst of high-CO2 air going up'—that represents emission. Later, we look for a corresponding downdraft of CO2-poor air. By totalling the contributions of updrafts and downdrafts over time, we can tell whether the forest is emitting carbon or storing it."

Davis credits the current global strategy of marine measurements with doing "a beautiful job of measuring an absolute amount of CO2 in the atmosphere." Scientists have been charting CO2 increase on both micro and macro levels. The gas is accreting at about 1.5 parts per million per year, for a net annual increase worldwide of around 4 gigatons. (A gigaton is 1 billion metric tons, approximately equivalent to 1 billion English tons.) Fossilfuel burning produces 6 to 7 gigatons of CO2 yearly, while deforestation in the tropics, with its accompanying burning and rotting of vegetation, adds 1 to 2 gigatons. On the other side of the equation, about 2 gigatons dissolve annually into the world's oceans, while another 2 gigatons are stored by terrestrial ecosystems, including half a gigaton in North America.

Davis wants to determine where and at what rate carbon is being stored in—or given off by—the extensive forests of the Upper Midwest. The TV tower sensors "sample a roughly 5-by-5-kilometer domain," a mosaic of hardwood trees of different species, with wetlands supporting dense alders and cedars covering 30 to 40 percent of the acreage. It turns out that this particular area actually produces more carbon than it stores, "which surprised us," says Davis, "since it seems to run counter to what's going on across the continent." Three shorter towers in the ChEAS program have been set up in an upland northern hardwood forest; an alder wetland; and a stand of never-logged old-growth trees, mainly maples and hemlocks. All three of these sites store more carbon than they release.

Why should the tall-tower site generate excess carbon? "It's been drier than normal in the Midwest in recent years," says Davis, "and it's possible that the margins of nearby wetlands are drying up and releasing CO2." In contrast, the old-growth stand is "storing about 100 grams of carbon per square meter per year," suggesting that the tract is still increasing in biomass. A mature forest, ecologists predict, should release about as much CO2 as it stores. Most North American forests were logged within the last hundred years and are now growing back, which implies that they're soaking up CO2.

As regrown forests continue to mature, "It's possible that we'll lose our terrestrial sink in North America," Davis says. "Should we expect that in 50 years this important sink will weaken, leading to accelerating rates of CO2 accumulation in the atmosphere?

"It makes sense to get to know a place really well before extrapolating to a larger area," Davis says. "Right now, we're working at a scale that's large for ecologists but small for atmospheric scientists. The two scientific communities can talk at this scale."

Kenneth J. Davis, Ph.D, is associate professor of meteorology in the College of Earth and Mineral Sciences, 512 Walker Bldg., University Park, PA 16802; 814-863- 8601; Research associates Chuixiang Yi and Scott Richardson also work on the Chequamegon Ecosystem- Atmosphere Study, as do graduate students Martha Butler, Dan Ricciuto, Ankur Desai, and Weiguo Wang and postdoctoral researcher Natasha Miles. The study has been funded by the National Institutes for Global Environmental Change, the U.S. Department ofEnergy, the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the National Science Foundation.

Last Updated September 01, 2003