Science in the Triangle

Higher concentrations of carbon dioxide are pumped into four of the experimental rings. Photo: Will Owen

Late in 2010, an epic ecological experiment in the Triangle will begin drawing to a close when carbon dioxide stops pumping from four massive rings of towers in the Duke Forest. Since 1996, more than 250 scientists at Duke and dozens of other institutions have measured the response of this forest ecosystem to the elevated amounts of carbon dioxide expected in the Earth’s atmosphere in the future. They’ve measured tree and plant growth, photosynthesis, leaf size, soil composition, root growth, and water use in the plots bathed in elevated carbon dioxide and in three other “ambient” control plots.

The first, prototype ring was built in 1994; six more came in 1996 (three controls and three experiments). Each ring consists of 16 metal towers in a 30-meter diameter. Computer-controlled instruments in the experimental rings bathe the interior of the plot in carbon dioxide. It’s called Free-Air CO2 Enrichment, or FACE. As opposed to “chamber studies,” in which plants are studied in carefully controlled growth chambers or greenhouses, the rings are open to nature. That means that mammals and insects can circulate freely and that natural events like hurricanes, ice storms, and droughts affect the research site.

Shannon LaDeau, who studied seed and pollen production at the site as a Ph.D. student, fondly calls it the EcoCircus, referring to both the ring-shaped sites and the riot of instruments, leaf-collection baskets, and colored flags staking out individual research groups’ claims to a particular layer of soil or stand of plants. LaDeau is one of at least 25 scientists who conducted Ph.D. research at FACE. “One of the really big bonuses of that site and others like it,” she says, “is that people are coming at it from different directions—biogeochemistry, biology, and so on.” There was an integration of ideas, she says, that “doesn’t happen naturally when scientists go out and choose their own site and do their own thing.”

I talked recently with Ram Oren, Nicholas Professor of Earth System Science at Duke and co-principal investigator for the project since 1998. He explained that the Department of Energy–funded project will enter a final phase this fall when the carbon dioxide is turned off. A scientific team will follow the trees for two more years to see how they respond to the “severe diet” that will be imposed on them when they are no longer receiving the added carbs.

Oren reminded me that when the experiment began, it was already well documented that trees grew faster under higher levels of carbon dioxide, especially when they were well nourished and watered. Retired Duke ecologist Boyd Strain and his students and colleagues had already established this in studies in which trees were isolated in growth chambers and treated with different regimes of carbon dioxide, nutrients, and water.

The FACE experiment was intended to test how entire ecosystems, not just trees, responded to additional carbon dioxide. In particular, researchers wanted to know if trees and soils would store or sequester extra carbon dioxide, keeping it from the atmosphere where it would contribute to a warmer climate.

The early major findings of the experiment were that, similar to the chamber studies, plants in the forest did indeed grow faster when exposed to extra carbon dioxide, especially in the presence of plentiful water and nutrients. And the ecosystem did store more carbon, but mostly in plant stems, not in soil as had been predicted.

Photo: Lisa M. Dellwo

A second wave of findings showed that the continuing growth response of plants to a “high-carb” diet depended on the native fertility of the site. Trees in fertile areas responded strongly to the carbon dioxide treatment and continued a higher growth rate, but trees in infertile areas didn’t retain their original growth response.

That’s important in the real world, because our most fertile soils tend to be cultivated for agriculture, leaving forests in less fertile areas. So we cannot expect trees to retain extra carbon in the forests of the future, says Oren.

While some scientists were studying tree growth and soils, others were finding that poison ivy has a remarkable response to higher CO2 conditions. Not only did it grow two times as fast as poison ivy in ambient conditions, but it produced much more toxin per leaf.

Shannon LaDeau, who conducted pollen studies, told me that the trees exposed to extra CO2 reached reproductive maturity at a younger age and smaller size. For those of us who suffer allergies, that is a bit ominous. While pine pollen—the yellow-green stuff that bathes the Triangle every spring—is not technically considered an allergen, other trees with true allergy-causing pollen may well have the same response as the pines, LaDeau says.

In 2004, when this photo was taken, the towers were still higher than the treetops. Now the trees have outgrown the towers. Photo: Lisa M. Dellwo

When the Department of Energy announced two years ago that it would cease funding the FACE project in Duke Forest—and similar projects elsewhere—Oren said that the project had not reached its true conclusion. He still believes that. But funding aside, there is a technical reason the project is drawing to a conclusion: the trees have outgrown the towers. When the experiment began, anyone who entered the site or who happened to fly over it could see the rings of towers clearly above the canopy. Now, trees that measured ten meters in 1996 are 21 meters high, and the towers have receded into the canopy.

In addition to the generation of ecological scientists trained at the site and the more than 250 papers reporting on the response of the ecosystem to elevated CO2, Oren believes that an important legacy of the FACE experiment will be the data gathered there over 17 years. Very few experiments last that long, and the accumulated data from FACE is being made available to computer modelers who will use it for years into the future to test and extrapolate responses to future climate change on a larger scope.