Planting more trees wont fight global warming: experts

LOS ANGELES (AP)

Scientists have overestimated the potential of trees and shrubs to soak up carbon dioxide from the atmosphere, a new study said.

The reassessment casts doubt on whether planting trees is always a positive step in the fight against global warming, as President George W. Bush and others have suggested.

In the study, published in Thursdays issue of the journal Nature, Duke University scientists said trees and shrubs growing in areas of abundant rainfall are less effecive storehouses for carbon than the 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 Jackscm lead author of the study and an associate professor of biology at Duke.

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

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

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

In 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 dont see where all the carbon goes so there is a misconception that woody species store more carbon. Thats just not always the case?

Previously, studies estimated U.S. shrublands contain about 400 million tonnes of carbon. The number may be closer to 250 million tonnes, Jackson said.

That result suggests shrublands, by ab sorbing. 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 wasnt involved in the study.

However, he said he didnt consider the study definitive, given uncertainties in its measurements of the carbon contained in woody roots.

Reprinted by the Star Phoenix, August 7, 2002