From the air, they look like a cross between unexplained Midwestern crop circles and the megaliths of Stonehenge. But these tall structures loom out of a forest.

Arranged in a loop, the 100-foot-high by 100-foot-wide assemblages are releasing carbon dioxide, a colorless, odorless miasma that wafts through the loblolly pines they encircle.

The 50-foot-tall pines, natives of the Deep South, are subjects in an experiment by scientists at Duke University who are using this engineered micro-climate as a kind of time machine to find out how these trees are likely to react as carbon dioxide builds up in the atmosphere and temperatures climb.

Since 1800, carbon dioxide levels have increased from 280 parts per million (ppm) to 370 ppm, a result of burning fossil fuels such as coal and oil. That level is expected to soar to about 560 ppm by the year 2050.

Carbon dioxide is a potent "greenhouse gas": It traps heat from the sun in the atmosphere, which in turn raises the planet's temperature. Carbon dioxide is also critical to the growth of plants. Without it, they cannot convert sunlight to energy.

The Duke experiment, called FACE (Free-Air Carbon Dioxide Enrichment), is funded by the U.S. Department of Energy and the Forest Service, which are also supporting similar forest projects in Wisconsin and Tennessee. Duke's FACE project, which began in 1996, is the largest and longest-running.

The Duke scientists selected loblollies as their subject because of their fast growth rate and widespread distribution and importance across the region.

Loblolly pines are to the Southeast what sugar maples are to New England or redwoods to California. In the Deep South, the word loblolly means a wet depression in the ground. Early settlers slogging their way through Southern bottomlands called the pines they saw there loblollies.

The loblolly shares the Southern pine ecosystem with the longleaf pine, the red-cockaded woodpecker and the nine-banded armadillo.

"Loblollies have provided us with a major timber resource in the South, served as wind breaks on our farmlands, even graced our homes as Christmas trees," said Ram Oren, director of the Duke project.

Researchers working in the Blackwood division of Duke Forest near Chapel Hill, N.C., the location of the experiment, have found that pines in the carbon dioxide-enriched rings are growing and reproducing much faster than other loblollies.

That, said Oren, is not necessarily good news for the loblollies themselves or the Southern ecosystems of which they are a part.

Some scientists believe that the faster growth may be beneficial to the environment, as trees sequester or store the atmospheric carbon dioxide in their tissues and thereby ameliorate global warming. That outcome is highly unlikely, said Oren, adding, "What's concerning is how this could affect the Southern forest as a whole, and the plants and animals that live there."

If the pines do mature and spread more quickly, they might crowd out slower-growing hardwood species such as oaks and hickories, trees perhaps less likely to be affected by rising carbon dioxide levels.

"This could have a cascading effect on a host of species," Oren said. Animals such as black bears and blue jays that rely on acorns from hardwoods might decline as the pines outcompete the hardwoods. "Eventually," he said, "the ecosystem might be dominated by loblolly pines."

In the FACE rings with high carbon dioxide, "it's as though we had put the loblollies inside a greenhouse," said Heather McCarthy, a biologist at Duke. "Within two years of their exposure to higher levels of carbon dioxide, the trees' needles hit a major growth spurt. The pines reached maturity faster than their counterparts. They produced cones at younger ages than their cousins growing in lower carbon dioxide conditions. And the cones contained many more seeds" -- as much as 300 percent more.

McCarthy placed buckets beneath the pines in the experimental areas. Like old-time Southern weavers who harvested loblolly pine needles for intricate coiled baskets, she collected needles shed by the pines. She also sampled needles on the branches.

She found that the pines exposed to high levels of carbon dioxide had many more needles, at least 17 percent more, than untreated pines.

"It's clear that high carbon dioxide levels are causing these loblolly pines to 'make' more needles," McCarthy said.

But the faster growth may not continue indefinitely, because rising carbon dioxide levels and temperatures are likely to have other effects, including reducing rainfall.

The trees' growth is likely to diminish over time, Duke scientist Ashley Ballantyne said, "due in part to the hot, dry weather that may be more common then."

Loblolly growth in the Blackwood division, where Ballantyne did his research, "seems to be related to soil moisture -- water available for use by the trees -- in addition to carbon dioxide," he said. "Increasing temperatures will likely cause a decline in soil moisture, eventually suppressing tree growth."

In gold rush terms, rising carbon dioxide creates a loblolly boom, but decreasing soil moisture ushers in a bust.

McCarthy and Ballantyne presented their findings in August at the annual meeting of the Ecological Society of America.

Loblolly pines, they say, are also known as oldfield pines because they are the first trees to move into former farmland. That trait can put another obstacle in the way of loblolly growth, said Oren and David Ellsworth, a plant ecologist at the University of Michigan who is also conducting research at the North Carolina site.

Over much of the pines' Southern range, intensive agriculture has stripped soils of nitrogen, leaving little to sustain the growth of trees.

"Loblollies exposed to high levels of carbon dioxide are able to maintain growth increases only when nitrogen is added to such nutrient-depleted soils," said Ellsworth, who published his research in the November 2004 issue of the journal Global Change Biology.

He and Oren believe that short-term increases in loblolly pine tree growth are not sustainable over the long term in a low-nutrient environment such as the Southeast.

"So we can't look to loblollies as an answer to rising carbon dioxide levels," Oren said.

Such findings are damping the hopes of some that planting forests or speedier growth of vegetation could counteract the apparently inexorable rise in carbon dioxide. Said Oren:

"One lone pine can't do it all."

Towering above a stand of loblolly pines, an array of PVC pipes releases carbon dioxide into the air as part of an experiment assessing the effects of elevated levels of the greenhouse gas on ecosystems. Gassed pines dramatically increase needle and seed production.