Steeped in Greenhouse Gas, Pine Trees Deviate

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 traitcan 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."