Our regenerating forests may not be the panacea for mitigating global climate change that some experts once expected, say researchers in the journal Science.

By sponging up vast amounts of the carbon dioxide released into the atmosphere from natural and human sources, regenerating forests such as those in the Northeast were long considered to be a stable “sink” for carbon, offsetting carbon releases from an increasing number of sources – automobiles, factories, and fires.

The theory is that as open land regrows trees and as existing trees age (as is the case in the Northeast), those trees require extra carbon dioxide from the atmosphere to photosynthesize and get larger. The atmosphere, in turn, happens to have ever more carbon dioxide to spare, thanks to the combustion of fossil fuels. As trees take up some of this excess carbon dioxide while photosynthesizing, they are expected to grow faster and lock up the excess carbon in their wood while emitting only oxygen as a byproduct. Experiments have confirmed that this so-called “carbon dioxide fertilization” and subsequent “carbon sequestration” are valid concepts. The U.S. and other nations even proposed planting trees and simply letting forests regrow as a main strategy for meeting emissions reductions targets, such as those set forth in the Kyoto Protocol.

But the results of research conducted by Christian Koerner and his colleagues from the University of Basel in Switzerland may reverse current thinking on the validity of counting on this carbon sink. In an attempt to simulate the expected atmosphere of the future – with nearly 50 percent more carbon dioxide than today’s atmosphere contains – Koerner’s team applied two extra tons of carbon dioxide per day, over the growing season, to a 500-square-meter patch of mixed forest in Switzerland for four years and then measured the results.

Four years later, the team found no greater net growth in the biomass of carbon-fertilized trees compared to trees in a control plot. While some trees, like poplar, showed higher growth rates the first year, by the end of four years, their growth rate was normal, despite the presence of an enriched carbon environment. Other trees’ growth rates never changed at all.

“Our data suggest that trees do not grow faster if provided with more carbon dioxide. They simply pump more carbon through their body and release it through root and soil microbe respiration within a few days,” says Koerner.

Adding to the concern of climate modelers, who count on forests as reliable carbon sinks in their projections, is another recent study indicating that high temperatures may actually cause forests to emit carbon dioxide.

Philippe Ciais and his colleagues from the Laboratory for Climate Sciences and the Environment in France studied the ramifications of the heat wave that plagued Europe in 2003. They found that during that summer’s drought, the grasslands and forests of the affected regions released as much carbon, mostly through the death and decomposition of trees and plants, as they had stored over the previous four growing seasons combined, or half a billion tons per year – more than three times the industrial emissions of the United Kingdom during 2003.

These two studies present important caveats to our current understanding of global warming. If growing forests don’t function as a carbon sink they way modelers have predicted, the rate of carbon dioxide buildup in the atmosphere will be higher than anticipated. More carbon dioxide means more gas to trap heat near the surface of the earth, which leads to more heating of Earth’s atmosphere, according to most scientists. And if forests are indeed net emitters of carbon dioxide in warmer conditions, the cycle of warming could increase at unprecedented rates.

Koerner’s team, meanwhile, continues to study the carbon fertilization phenomenon and notes that there are many factors in their study, such as its time scale, that may not accurately reflect reality. And the results may not apply “across the pond.” At the very least, these two studies suggest that where global warming is concerned, nothing is for certain.

 
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