New studies show large forest areas alone may not be able to counter the effects of increased carbon dioxide in the atmosphere.
IT IS WIDELY believed that higher
carbon dioxide levels in the atmosphere will stimulate plant growth
and thus help crops and forests fix
more carbon - a positive feedback
effort that may partially help control
the global warming problem. But two
recent studies indicate'forests may
not be as efficient in filtering out carbon dioxide as many people
think. Researchers at
the University of Basel
in Switzerland have
found that elevated carbon dioxide levels in
the atmosphere only
lead to a greater "carbon turnover" and not
to greater "carbon
sequestering" by terrestrial ecosystems like
forests (Science, Vol
257, No 5077).
Christian Kbrner and John.Arnone III constructed four model humid tropical ecosystems in polyethylenecovered houses and exposed two to higher levels of carbon dioxide and two to ambient conditions. Both treatments resulted in vigorous plant growth, but exposure to elevated carbon dioxide levels did not increase plant growth any more than exposure to ambient levels.
The absorption of carbon dioxide by the canopy increased in the elevated carbon dioxide experiments, but the scientists found that this was offset by the subsequent increase in carbon dioxide lost frorn'the soil. The elevated carbon dioxide did not lead to increased plant growth and carbon sequestering from the atmosphere as compared to ecosystems exposed to ambient carbn dioxide levels, but led only to more rapid carbon turnover.
The scientists warn that carbon dioxide fertilisation could promote osses of soil carbon and the release 'of soil nutrients over a period of time. The deleterious levels of starch produced in leaves at the top of canopies under elevated carbon dioxide levels may also cause changes in the structure of plant communities.
Another recent analysis done by Titus D Bekkering of the Netherlands shows national forestry action plans introduced in some countries will fail to counter the effect of greenhouse gases if deforestation is not stopped first (Ambio, Vol 21, No 6). Only a few of the forestry action plans of 13 countries, including Argentina, Bolivia, Tanzania and Indonesia, implemented under the Tropical Forestry Action Plan (TFAP) have formulated strategies to halt deforestation.
The study found that even if reforestation efforts in 11 of the 13 countries are stepped up, the total area reforested -annually would be much smaller than what is needed to have a global impact on carbon dioxide levels in the atmosphere. New plantings would fix about 12.4 million tonnes of carbon annually, compared to more than 282 million tonnes of carbon released every year by the 11 countries.
Bekkering, currently advisor to a social forestry project in northern Pakistan, doubts that the huge costs involved are justifiable simply for the sake of fixing carbon. He says, "Forestry alone will not solve the greenhouse effect."
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