By ANDREW BRIDGES, AP Science Writer

LOS ANGELES (AP) - Scientists have overestimated the potential of trees and shrubs to soak up carbon dioxide from the atmosphere, according to a new study.

The reassessment casts doubt on whether planting trees is always a positive step in the fight against global warming ( news - web sites), as President Bush ( news - web sites) and others have suggested.

In the study, published in Thursday's issue of the journal Nature, Duke University scientists say trees and shrubs growing in areas of abundant rainfall are less effective storehouses for carbon than native grasslands they have steadily replaced across much of the western United States.

Vegetation stores carbon that otherwise might trap heat in the atmosphere, driving up temperatures and leading to climate change. Previous studies have ignored what was going on below ground, said Robert Jackson lead author of the study and an associate professor of biology at Duke University.

In wet locations, replacing grass with shrubs and trees actually can lead to a decrease in the amount of carbon locked up in organic matter mixed in the soil, Jackson said. The amount can be enough to offset any gains achieved above ground.

"The study suggests that we need to look very closely at what's below ground before we add up just what's stored above ground in tree trunks," Jackson said.

Scientists studied six pairs of adjacent western grasslands. In one of each pair, trees and shrubs had cropped up sometime in the last 100 years.

In the drier sites, the invasive growth led to an increase in the amount of carbon locked up in the soil. In wetter areas, however, the opposite was the case, Jackson said. It is not clear what caused the change.

"Grasses are deceptively productive," Jackson said. "You don't see where all the carbon goes so there is a misconception that woody species store more carbon. That's just not always the case."

Previously, studies estimated that U.S. shrublands contain about 440 million tons of carbon. The number may be closer to 280 million tons, Jackson said.

That result suggests shrublands, by absorbing carbon from the atmosphere, do less to balance emissions from the burning of fossil fuels than previously thought, Jackson said.

"It would not surprise me at all if they were absolutely spot-on right," said Steve Pacala, a Princeton University professor ecology, who wasn't involved in the study. However, he said he didn't consider the study definitive, given uncertainties in its measurements of the carbon contained in woody roots.

The study helps dispel the notion that humans can plant their way out of global warming, said Daniel Becker, director of the Sierra Club ( news - web sites) global warming and energy program.

"We are going to need to tackle the industrial sources of emissions head-on rather than just plant a bunch of trees," Becker said.

As part of his administration's strategy for curtailing carbon dioxide emissions, Bush has proposed tax incentives for farmers who plant trees.

Copyright © 2002 The Associated Press.