U. MICHIGAN (US) — North American forests appear to have a greater capacity to soak up heat-trapping carbon dioxide gas than previously thought.
The results of a 12-year study at an experimental forest in northeastern Wisconsin challenge several long-held assumptions about how future forests will respond to the rising levels of atmospheric carbon dioxide blamed for human-caused climate change, says Donald Zak, professor of ecology at the University of Michigan.
“Some of the initial assumptions about ecosystem response are not correct and will have to be revised,” adds Zak.
For the study, researchers 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, 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 polluted air 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. 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, published in the journal Ecology Letters, the trees bathed in elevated carbon dioxide continued to grow at an accelerated rate throughout the 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 says. 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 says. “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.”
Growth-enhancing effects of CO2 in forests will eventually “hit the wall” and come to a halt, Zak says. The trees’ roots will eventually “fully exploit” the soil’s nitrogen resources. No one knows how long it will take to reach that limit.
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 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 says.
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 says. “Biodiversity matters, in this regard.”
Researchers from the University of Idaho, U.S. Forest Service, and Michigan Technological University contributed to the study that was funded by grants from the U.S. Department of Energy and the U.S. Forest Service.
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