Atmospheric Ambiguities

August 1, 2001


Results from continuing experiments near Duke—where forest plots grow in the higher atmospheric levels of carbon dioxide expected by the mid-twenty-first century—suggest that trees and soil may not sop up much of the extra gas over the long term under real-world conditions.
Future air apparent:research towers release tomorrow's atmospheres to pine grove

Future air apparent:research towers release tomorrow's atmospheres to pine grove.

One of two articles in a May issue of the research journal Nature shows that while twenty-year-old loblolly pine trees began growing up to about 25 percent more wood after becoming continually exposed to 1.5 times current levels of carbon dioxide (CO2), that initial growth spurt dropped back to only marginal gains after the first three years. Researchers found they were able to enhance wood production as much as 74 percent at a nearby experimental site by providing extra nitrogen fertilizer as well as CO2 to trees growing in nutrient-poor soils. But growth did not increase at all without the supplemental nitrogen.

“That suggests that CO2 effects on tree growth in pine forests will be highly variable and depend greatly on site fertility, perhaps to the point of not responding at all on the nutritionally poorest sites,” concluded the article’s eleven authors, led by Ram Oren, an associate professor at Duke’s Nicholas School of the Environment and Earth Sciences.

Another variable was moisture. An early growing-season drought in 1999 delayed growth in the elevated CO2 plots at the Duke Free Air Carbon Enrichment (FACE) study site, the scientists found. Even with extra nutrients and CO2 provided, drought in 1999 reduced sequestration of carbon by more than 25 percent relative to the moist growing season in the year 2000.

The FACE experimental research site at Duke Forest is designed to mimic effects that extra CO2—expected from continued industrial and vehicular emissions and other human activities—will have on typical forest ecosystems.

An initial ring of computer-controlled towers, later joined by another three, was designed to immerse 30-meter-diameter circles of pine forest in constantly regulated equivalents of a mid-twenty-first century atmosphere. Another three plots received no extra CO2, but instead served as experimental “controls” for comparison.

Oren’s group also compared the FACE experimental results with a separate study of twelve trees growing in low-nutrient soils at the U.S. Department of Agriculture’s Forest Service Southeast Tree Research and Education Site (SETRES) in North Carolina’s Sandhills region. Half of those SETRES trees received 1.5 times normal CO2 concentrations through open-topped growth chambers.

In addition to Department of Energy and U.S. Forest Service funding, support came from the National Science Foundation.

Many scientists suspect the enhanced carbon dioxide is already beginning to warm Earth’s climate by trapping solar energy through a so-called “greenhouse effect.” Policy makers are thus searching for ways to reduce CO2 levels. One suggested way is to lock up some of the extra gas in growing wood, or in the soil humus created by microbes from fallen leaves or pine needles.

Some earlier experiments had suggested that the extra atmospheric carbon dioxide might itself have a “fertilizing” effect on forests, causing them to lock up extra amounts of CO2 and thus mitigate global warming impacts.

In a second article in the same issue of Nature, William Schlesinger, James B. Duke Professor of Biogeochemistry and the Nicholas School’s new dean, joined John Lichter of the biology department and environmental studies program at Bowdoin College in Brunswick, Maine, to examine whether the FACE experiment would also store extra carbon dioxide in soils in the CO2-treated pine forest plots.

Comparing soils archived before the onset of the FACE experiment with those collected later, the pair found significant increases in the production of dead plant debris within the forest floor of the CO2-enriched plots. Because the CO2 used in the FACE treatment has less-than-normal concentrations of the carbon-13 isotope, the scientists could also employ isotope ratios as a marker to trace the chemical origins of carbon in the developing soil.

Using this information, Schlesinger and Lichter deduced that while the initial bursts in wood production also produced extra leaf litter, very little of the carbon originating from FACE-emitted CO2 entered the mineral soil over the three-year study period. “In the absence of significant changes in the carbon content of the mineral soil, these data suggest only a limited potential for long-term soil carbon sequestration in this forest in response to rising CO2 in Earth’s atmosphere,” their Nature article concluded.