For more than a decade, the U.S. government has spent millions of dollars pumping elevated levels of carbon dioxide into small groups of trees to test how forests will respond to global warming in the next 50 years.

Some scientists believe they are on the cusp of receiving key results from the time-consuming experiments.

The U.S. Department of Energy, however, which is funding the project, has told the scientists to chop down the trees, collect the data and move on to new research. That plan has upset some researchers who have spent years trying to understand how forests may help stave off global warming, and who want to keep the project going for at least a couple of more years.

"There has been an investment in these experiments and it's a shame we are going to walk away from that investment," said William Chameides, an atmospheric scientist at Duke University, where one of the experimental forests is located. "There is no question that ultimately we want to cut the trees down and analyze the soil. The question is whether now is the time to do it."

Ronald Neilson, a U.S. Forest Service bio-climatologist in Corvallis, Ore., said the experiments should continue because they still have potential to answer key questions about how rainfall and fertility affect how much carbon a forest will store long-term — essential to understanding how forests may soften the blow of climate change.

But the Energy Department, following the advice of a specially convened panel of experts, believes that chopping down the trees and digging up the soil will allow the first real measurements of how much carbon the leaves, branches, trunks and roots have been storing, said J. Michael Kuperberg, a program manager with the agency.

Ending the experiments will also allow the funding to be devoted to new research that will look at the effects of higher temperatures, changes in rainfall and variations in soil fertility, Kuperberg said.

Ending research could be mistake: scientists

"What we are trying to do here is balance the time to get optimal results out of the existing experiment with our desire for a new generation of experiments that we feel is more likely to realistically represent future climate scenarios," Kuperberg said.

Some scientists, though, believe ending the long-term research may be a mistake.

"If we stop these experiments now, it could cost many years to get back to this point, time we may not have," Kevin Lee Griffin associate professor of environmental sciences at Columbia University, wrote in an e-mail.

The research program, Free Air CO2 Enrichment (FACE), consists of rings of tall white plastic pipes with holes along their length that emit once-liquefied carbon dioxide in carefully metered doses.

The loblolly pines planted in 1994 at Duke in North Carolina are located behind gates several miles from campus.

There are also experiments at Oak Ridge National Laboratory in Tennessee and the Harshaw Experimental Forest in Wisconsin. The carbon dioxide levels around the trees are about 50 per cent higher than current levels — the amount expected 40 to 50 years from now.

The Department of Energy's Office of Biological & Environmental Research has informed those managing the experiments that their current research will be phased out by 2011. They are to get the definitive measurements on how tree growth, which represents stored carbon, was influenced, and should design new experiments to get rolling by 2012.

Trees reaching height of pipes

The panel found that the current experiments had a useful life of 10 to 12 years, and in a few more years the results would become invalid, in part because the trees were nearly taller than the pipes delivering the carbon dioxide.

Results so far indicate that elevated levels of carbon dioxide make forests grow more quickly, said Ram Oren, associate professor of ecology at Duke University's Nicholas School of the Environment and Earth Sciences and principal investigator on the experiments there.

But unless forests are on fertile ground — hard to come by because of development — growth will be in leaves, needles and fine roots, which die off and decompose in a year or two, releasing the carbon dioxide back to the atmosphere, Oren said.

The Duke experiment recently began looking at fertility, and a couple of more years will give them better data on how forests react differently to drought and plentiful rainfall, he said.

"To stop an experiment that cost $55 million, $10 million before it reaches its real conclusion makes no sense to me," Oren said.

Rich Norby, who oversees the tree experiment at Oak Ridge, said he had thought it had run its course, but emerging trends indicate the new wood growth from increased carbon dioxide tapers off because of limitations of nitrogen — fertilizer — in the soil.

"This comes up in all sorts of long-term experiments — when is the right time to say, 'Enough,' " Norby said. "There's no good answer to that."