Trees can't offset global warming

New studies show large forest areas alone may not be able to counter the effects of increased carbon dioxide in the atmosphere.

Published: Friday 15 January 1993

-- (Credit: Anil Agarwal / cse)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.
Adverse effect The scientists believe that elevated carbon dioxide may actually have an adverse effect on ecosystems. Leaves in ecosystems -exposed to elevated carbon dioxide levels contained Wce as much carbohydrates such as starch compared to those in the controlled ecosystems. The increased starch accumulation yellowed the leaves of plants such as Ficus benjwnina, which is related to the common banyan tree, and Tetrastigma wiwfianum. The leaf area index the amount of photosynthetic anschinery, particularly leaves existft on a plant, per unit area of land - in both treatments increased during the first 60 days, and then levelled out, but leaf litter production was higher under elevated carbon dioxide. The quantity of small roots increased at elevated carbon dioxide levels and the carbon dioxide loss from the soil almost doubled.

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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