The results from a 12-year study on forests provide researchers with surprising new evidence: forests have a greater capacity to soak up heat-trapping carbon dioxide gas than previously anticipated. The implication is that forests could help slow the pace of human-caused climate warming more than most scientists had thought.

The experiment was led by microbial ecologist Donald Zak, a professor at the University of Michigan School of Natural Resources and Environment, and the Department of Ecology and Evolutionary Biology in the College of Literature, Science, and the Arts.

Experimental Forest in Wisconsin

Aerial view of 38-acre experimental forest in Wisconsin. Credit: David Karnosky, Michigan Technological University

“Some of the initial assumptions about ecosystem response are not correct and will have to be revised,” said Zak. To simulate atmospheric conditions expected in the latter half of this century, Zak and his colleagues continuously pumped extra carbon dioxide into the canopies of trembling aspen, paper birch and sugar maple trees at a 38-acre experimental forest in Rhinelander, Wisconsin.

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

CO2 and Ozone

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. Climate researchers 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.

Zak said that their results indicate that the extra carbon dioxide allowed trees to grow more small roots and “forage” more successfully for nitrogen in the soil. Additionally, 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.”

But at some point the growth-enhancing effects of CO2 in forests will eventually “hit the wall” and come to a halt because the trees’ roots will have “fully exploited” the soil’s nitrogen resources. Zak said that no one knows how long it would take to reach that limit.

More Surprising Results

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

Ground-level ozone can 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 trees. 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.

Retaining Our Forests

The provocative evidence from the work of Zak and associates suggests that foresters and forestry management agencies need to reconsider policies. Although clearcutting has long been frowned upon, it is still practiced in some areas. A 2002 report from the US Forestry Service states that in the South alone, 5.2 million acres were harvested annually, with 39 percent being clearcut. Measurement discrepancies among these regions may account for some of these differences. Overall, including seed tree and salvage cuts, about half the timber harvests in the South are made by clearcutting.

Maintaining our forests, harvesting sustainably, replanting continuously and starting new growth wherever possible would benefit the planet tremendously, if the U.M. study were to be taken seriously. Rather than clear-cutting and reducing forest populations, humanity would be wise to preserve the trees that are so vital to our common existence.

This does not mean that the continued release of toxic chemicals into the air from factories or other atmospheric pollutants should continue unchecked, assuming that “the trees will fix the problem.” Instead, implementing policies that further reduce all areas of concern in the global warming arena would allow the forests to contribute to restoring the natural balance more effectively. Next time you’re out hiking to admire the soul-stirring autumn colors of trees, stop to ponder what marvelous, silent allies they truly are.

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