Sedge grows tall in 'greenhouse' effect Some plants flourish with carbon dioxide

December 23, 1990|By Phillip Davis | Phillip Davis,Sun Staff Correspondent

SHADY SIDE -- In a brackish marsh hard by the Chesapeake Bay, scientists are beginning to suspect that the "greenhouse effect" may not be the environmental disaster it was once thought it might be.

Many plants, it seems, love increased amounts of carbon dioxide -- the gas that cars, power plants and factories spew into the air by the billions of tons.

Researchers at the Smithsonian Environmental Research Center here have spent $1.2 million and five years discovering something greenhouse farmers have known for decades: More carbon dioxide means faster plant growth and reproduction.

The findings are significant, though, because few experiments have sought to measure this effect under natural conditions.

Using a complex system of open-topped enclosures, the researchers have found that when Olney Three-Square -- a common marsh sedge that covers most of this marsh -- is exposed to high levels of carbon dioxide, it grows almost 75 percent to 100 percent faster than plants a few feet away that had not been similarly exposed.

"[Most] plants getting increased carbon dioxide grow taller and last longer," said Bert Drake, who is directing the experiment at the Smithsonian center here. "By the end of the growing season, if we add it all up, there is something like a 15 to 20 percent increase in above-ground production, but like an 85 percent in below-ground production."

The results, which are being replicated in several similar experiments in the Kansas plains, the for

ests of Tennessee and in the Arctic, will have far-reaching implications, said Fakri Bazzaz, a professor of botany at Harvard University who has followed the Smithsonian experiment.

The new research indicates that crops such as potatoes, wheat and soybeans will be affected both in their growth rates and how nutritious they are. (Corn falls into a category of plants that are not affected.)

Even more intriguing will be the effect of the changing atmosphere on large-scale systems such as forests, marshes and plains.

"One can say now with a degree of certainty that the composition of plant communities will change, drastically in some cases, with the rise in carbon dioxide in the atmosphere," Mr. Bazzaz said.

Some plants, which do not respond as well to these changes, he said, could be pushed aside by faster-growing species.

Carbon dioxide in the air acts as an insulator, trapping the Earth's

heat close to the surface -- the so-called "greenhouse effect."

But the gas has other effects that are potentially as significant as its role in global warming, Mr. Drake said.

Standing on a boardwalk that stretches out from the marsh's edge over the waving sea of salt grass and sedge, he pointed out one effect: In the past four years, the salt grass -- which does not respond to increased carbon dioxide -- has retreated in the face of resurgent growth from the sedges.

The center's monitoring devices, like those elsewhere, have noted a measurable increase in carbon dioxide in the air over the past four years alone -- and that could possibly explain the sedge's retreat.

Yet despite the enormous implications for plant life around the world, according to Mr. Drake and other researchers, the effects of higher carbon dioxide levels haven't been studied extensively in the wild.

"Most of the research has been done with agricultural plants in a greenhouse setting, and not in an intact ecosystem," said Richard Norby, a scientist at Oak Ridge National Laboratory's carbon dioxide information system in Tennessee, who is

experimenting with carbon dioxide and poplar saplings.

"Bert Drake, though, has a more or less intact ecosystem there," he said.

Mr. Drake, paddling out on the marsh's outskirts in a canoe, said, "This is one of the least interesting environments on Earth to me."

But it is an almost perfect natural laboratory, he said.

The salt grass and the sedge -- with their differing response rates -- are the two main plants growing in the marsh.

The simplicity of this ecosystem allows the scientists to chart more easily the effects of increased carbon dioxide.

In one corner of the marsh, they're doing this by surrounding 30 clumps of the grass with enclosures that look like miniature greenhouses but are open to the air and rain.

In half the enclosures, a small, black, plastic tube spews out a steady stream of carbon dioxide gas, effectively doubling the concentration found in ordinary air.

The enclosures are constantly monitored by nearby computers.

Not only does the "gassed" sedge grow faster, scientists found, but it also continued to grow two weeks longer in the fall than the grass growing in normal conditions. And it used water 30 percent more efficiently to boot.

Faster growth means that the exposed plants have pulled in carbon dioxide at a faster rate, helping cleanse the air.

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