Future forests may soak up more carbon dioxide than previously believed

October 13, 2011
An aerial view of the 38-acre experimental forest in Wisconsin where U-M researchers and their colleagues continuously exposed birch, aspen and maple trees to elevated levels of carbon dioxide and ozone gas from 1997 through 2008. Credit: David Karnosky, Michigan Technological University

North American forests appear to have a greater capacity to soak up heat-trapping carbon dioxide gas than researchers had previously anticipated.

As a result, they could help slow the pace of human-caused climate warming more than most scientists had thought, a U-M ecologist and his colleagues have concluded.

The results of a 12-year study at an in northeastern Wisconsin challenge several long-held assumptions about how future forests will respond to the rising levels of blamed for human-caused climate change, said University of Michigan microbial ecologist Donald Zak, lead author of a paper published online this week in .

"Some of the initial assumptions about ecosystem response are not correct and will have to be revised," said Zak, a professor at the U-M School of Natural Resources and Environment and the Department of Ecology and in the College of Literature, Science, and the Arts.

To simulate atmospheric conditions expected in the latter half of this century, Zak and his colleagues continuously pumped extra into the canopies of trembling aspen, paper birch and sugar maple trees at a 38-acre experimental forest in Rhinelander, Wis., from 1997 to 2008.

Some of the trees were also bathed in elevated levels of ground-level ozone, the primary constituent in smog, to simulate the increasingly of the future. Both parts of the federally funded experiment---the carbon dioxide and the ozone treatments---produced unexpected results.

In addition to trapping heat, carbon dioxide is known to have a fertilizing effect on trees and other plants, making them grow faster than they normally would. and ecosystem modelers assume that in coming decades, carbon dioxide's fertilizing effect will temporarily boost the growth rate of northern temperate forests.

Previous studies have concluded that this growth spurt would be short-lived, grinding to a halt when the trees can no longer extract the essential nutrient nitrogen from the soil.

But in the Rhinelander study, the trees bathed in elevated carbon dioxide continued to grow at an accelerated rate throughout the 12-year experiment. In the final three years of the study, the CO2-soaked trees 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 "forage" more successfully for nitrogen in the soil, Zak said. At the same time, the rate at which microorganisms released nitrogen back to the soil, as fallen leaves and branches decayed, increased.

"The greater growth has been sustained by an acceleration, rather than a slowing down, of soil nitrogen cycling," Zak said. "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 "hit the wall" and come to a halt. The trees' roots will eventually "fully exploit" the soil's nitrogen resources. No one knows how long it will take to reach that limit, he said.

The ozone portion of the 12-year experiment also held surprises.

Ground-level ozone is known to damage plant tissues and interfere with photosynthesis. Conventional wisdom has held that in the future, increasing levels of ozone would constrain the degree to which rising levels of carbon dioxide would promote tree growth, canceling out some of a forest's ability to buffer projected climate warming.

In the first few years of the Rhinelander experiment, that's exactly what was observed. Trees exposed to elevated levels of ozone did not grow as fast as other . But by the end of study, ozone had no effect at all on forest productivity.

"What happened is that ozone-tolerant species and genotypes in our experiment more or less took up the slack left behind by those who were negatively affected, and that's called compensatory growth," Zak said. The same thing happened with growth under elevated carbon dioxide, under which some genotypes and species fared better than others.

"The interesting take home point with this is that aspects of biological diversity---like genetic diversity and plant species compositions---are important components of an ecosystem's response to climate change," he said. "Biodiversity matters, in this regard."

Explore further: Global warming may affect the capacity of trees to store carbon, study finds

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3 / 5 (2) Oct 13, 2011
Wait a minute.

We just had an article a few weeks ago saying forests RELEASED more CO2 than previously believed.

Which is it?
1 / 5 (3) Oct 13, 2011
Great, until this nonsense is uttered: "...effects of CO2 in forests will eventually 'hit the wall' and come to a halt. The trees' roots will eventually 'fully exploit' the soil's nitrogen resources."

No option for there to be a symbiotic cycle / reactionary adjustment, like the others you arrogantly dismissed as possibilities which this very experiment threw right back in your face?

Come on guys, what's the problem?
not rated yet Oct 13, 2011
What about the monocultures of all food production lands? have they conducted a similar experiment on maize/corn , canola, soy etc?
2.6 / 5 (5) Oct 14, 2011
This is a no-brainer ,simple logic , After millions of years if plants had not developed the ability to adjust to changing conditions do you really think they would be here now? The whole too much carbon/global warming farce is as dead as the flat earth at the center of the solar system idea is.
1 / 5 (1) Oct 14, 2011
CO2 is not even the main problem. think about methane, sulfur etc. and other contamination like mercury, radiation etc.
Aerosols that damage the ozone layer for ex.
Carbon structure like fullerines by car exhausts that cause brain damage when breathed in.

At one point the forest will be fully grown and it wont absorb much CO2 anymore. Remember that it is "just" a carbon sink like the ocean and ice sheets. So massive land use is still a major issue carbon wont reach the deep soils anymore and forests will reach there maximum potentional sooner or later, plus melting ice sheets and damaging ocean, CO2 will sooner or later become a problem to human life.
not rated yet Oct 14, 2011
I wonder how long it will take for the rest of the forests to grow back, if we stop cutting them down, that is? Merely as an experiment, mind you.
1 / 5 (1) Oct 14, 2011
"The polar bears will be fine." - Freeman Dyson

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