2 September 2005Discovery ChannelRossella Lorenzi
Plants may not suck up as much carbon dioxide as previously thought, Swiss scientists warned in a study published in the current issue of Science. If the team's findings hold, it may mean that global warming is happening at a faster rate than predicted.
Found in nature, carbon dioxide (CO2) is also the product of combustion of carbon-based fuels: the colorless, odorless, tasteless gas is formed when carbon, a solid, is burned in the open air.
Absorbing and trapping heat near the surface of the Earth like the glass panes of a greenhouse, carbon dioxide has long been recognized as a major agent of global warming.
According to some estimates, CO2 emissions will be almost 40 percent higher by the end of the decade than they were in 1990.
Scientists have long believed that plants would thrive on the gas excess, since they take in carbon dioxide and release oxygen during photosynthesis. Basically, atmospheric CO2 would increase the so called "CO2 fertilization effect," stimulating vegetation to grow faster and store more biomass carbon.
But according to Christian Koerner, a plant ecologist at the University of Basel, a carbon dioxide rich future will not favor the greening of planet Earth — what will be left, will only be the greenhouse effect.
Koerner and colleagues sprayed a 500-square-meter patch of a multispecies forest near Basel, Switzerland, with pure carbon dioxide for four years.
The forest featured 32 to 35-meter-tall middle-aged trees.
"Tall trees in a natural forest had not previously been studied because of technical difficulties. We solved the problem with a technique called web-FACE that releases pure CO2 through a fine web of tubes woven into tree canopies with the help of a construction crane," Koerner said.
The trees were exposed to two tons of extra carbon dioxide each day during the six month annual growth season. The experiment simulated an atmosphere loaded with almost 50 percent more CO2 than exists today.
After four years, Koerner and colleagues found no evidence of enhanced biomass growth in stems or leaves.
"Our data suggests 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," Koerner told Discovery News.
Not all species reacted in the same way to the excess of CO2.
"One species, Fagus, showed a sharp growth stimulation in the first year. However, after four years this response was gone. Other species never showed a growth," Koerner said.
The experiment still has drawbacks and can't lead to wide conclusions yet. For example, the time scale is too short to fully estimate the plants' reaction to carbon dioxide excess and there might be still the possibility that a fraction of the extra carbon is stored below ground rather than in the trees.
"Despite whether the results are widely applicable, Koerner's study puts a big warning flag to the climate community," Josep Pep Canadell, executive director of the global carbon project in Canberra, Australia, told Discovery News.
"If Koerner's results were generalizable, we would then have a much less efficient terrestrial biosphere to soak up CO2 than we are currently assuming in our model predictions of climate change over the next 100 years," he continued.
"The consequence is that atmospheric CO2 growth would accelerate at a faster pace than currently predicted and so would do global warming."
