Forests soak up more CO2 than thought

North American forests might be more use than expected at slowing the pace of climate change.

After a 12-year study at an experimental forest in northeastern Wisconsin, a University of Michigan team says that such forests appear to better at soaking up carbon dioxide than previously believed.

“Some of the initial assumptions about ecosystem response are not correct and will have to be revised,” says microbial ecologist Donald Zak.

To simulate the atmospheric conditions expected in the second half of this century, Zak’s team continuously pumped extra carbon dioxide into the canopies of trembling aspen, paper birch and sugar maple trees at the 38-acre experimental forest between 1997 and 2008.

Some of the trees were also bathed in increased levels of ground-level ozone, the primary constituent in smog, to simulate the increasingly polluted air of the future.

And, says Zak, both treatments produced unexpected results.

Carbon dioxide is known to make trees grow faster than normal, but it’s generally been believed that any growth spurt caused by this would be short-lived, grinding to a halt when the trees can no longer extract nitrogen from the soil.

But the researchers found that the levated carbon dioxide caused the trees to grow at an accelerated rate right through the 12-year experiment. In the final three years, they grew 26 percent more than those exposed to normal levels of carbon dioxide.

It appears that the extra carbon dioxide allowed trees to grow more small roots and thus get more nitrogen from the soil. Meanwhile, the rate at which microorganisms released nitrogen back to the soil by decomposing fallen leaves increased.

“The greater growth has been sustained by an acceleration, rather than a slowing down, of soil nitrogen cycling,” says Zak. “Under elevated carbon dioxide, the trees did a better job of getting nitrogen out of the soil, and there was more of it for plants to use.”

Zak stressed that growth-enhancing effects of CO2 in forests will eventually grind to a halt as the trees’ roots  fully exploit the soil’s nitrogen resources. No one knows how long it will take to reach that limit, he says.