CO2 Fuels Forest Growth Rates

By Reid Schram
Reid Schram
Reid Schram
October 20, 2011 Updated: October 1, 2015

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 CO2 and ozone gas from 1997 through 2008. (David Karnosky/Michigan Technological University)
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 CO2 and ozone gas from 1997 through 2008. (David Karnosky/Michigan Technological University)
Northern temperate forests may be able to absorb more carbon dioxide (CO2) than previously thought and even mitigate anthropogenic global warming, according to a new study by U.S. researchers.

Their 12-year experiment was conducted on a 38-acre experimental forest in Wisconsin, and sought to understand how tree growth would be affected by rising CO2 and pollution levels.

The study showed that hardwood forests (trembling aspen, paper birch, and sugar maple) can grow an astounding 26 percent faster when the surrounding air is enriched with CO2, and the accelerated rate was not short-lived as seen in previous studies.

The increased growth corresponds with the trees’ ability to produce more small roots for seeking out nitrogen, a limiting factor in plant growth. Also, decay rates by micro-organisms increased, releasing nitrogen back into the soil more quickly.

"The greater growth has been sustained by an acceleration, rather than a slowing down, of soil nitrogen cycling," explained lead author Donald Zak, a microbial ecologist at the University of Michigan (U-M), in a press release.

"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 pointed out that the CO2-induced acceleration rate would eventually reach a limit when tree roots could fully utilize all available soil nitrogen.

To fully simulate the predicted atmosphere of the future, ozone levels were also elevated around some trees at ground level. Ozone is the main component of modern pollution, and can damage plant tissue, and inhibit photosynthesis.

Initially, ozone was found to negatively affect tree growth rates until the more ozone-tolerant individuals became dominant. By the end of the study, it had no effect.

"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.

Similarly, some genotypes and species responded better to raised CO2 levels than others.

Zak believes this study shows how important biodiversity conservation is during this time of atmospheric change.

The study was published online in Ecology Letters on Oct. 9.

 

Reid Schram