MADISON, WISCONSIN--Some scientists and policy-makers claim forests can absorb enough carbon dioxide to cut the risk of further global warming. But at least some forests may not be up to the job. Rather than storing most of the extra carbon in long-lasting woody parts like trunks and branches, trees in an experimental forest in Tennessee instead make tiny roots that quickly degrade in the soil--sending the CO2 right back into the atmosphere.
; after a few years, the trees don't absorb much.
Researchers have debated for decades how much CO2 forests can soak up and turn into wood. It's already clear that young trees grown in partial enclosures respond to high CO2 with a growth spurt, stepping up photosynthesis and making more leaves and wood. But what's true for a stand of saplings may not be true for a forest, says physiological ecologist Rich Norby of Oak Ridge National Laboratory in Oak Ridge, Tennessee. That's because as young trees grow up and run low on water, nutrients, and light, they may not be able to soak up all the extra gas.
To find out how much CO2 mature trees can absorb, 4 years ago Norby and colleagues built towers to pump carbon dioxide into the canopies of four stands of young sweetgum trees. As Norby reported here 9 August at the annual meeting of the Ecological Society of America, most of the extra carbon went into wood in 1998, the first year the towers were turned on, with the gassed-up sweetgums accumulating 35% more carbon than trees in a control plot. But 2 years later, extra wood formation was down to 7%. Instead, more than twice as much carbon ended up in the fine roots--thin structures that fall off and die each year. Soil organisms quickly break down the root debris to carbon dioxide, which then escapes into the air.
Forest ecologist Adrien Finzi of Boston University, who conducts a similar experiment in a pine forest in North Carolina, calls the results "really interesting," but cautions that they may not hold true in other forests. He and Norby agree that forest soil could potentially store some of the carbon, and that the real answers will come by tracking what happens below the ground as well as above it.