Gasping for breath

Entire tree species face the threat of extinction from the increasing concentration of atmospheric carbon dioxide

Published: Sunday 15 May 1994

-- (Credit: David Danmen / PANOS PICTURES<)SWELLING concentrations of carbon dioxide in the atmosphere are threatening the diversity of tree life on Earth, say 2 US ecologists (Science, Vol 263, No 5149).

O L Phillips of the Missouri Botanical Garden, USA, and the late A H Gentry (1945-1993) analysed independent surveys of tropical forests worldwide and discovered an increase in forest turnover, a measure of the number of trees coming up and dying out in an area.

The increase in turnover rates, the scientists foresee, will seriously affect the diversity of tree species. There will be an increasing predominance of light-hungry plants and climbers and some slow-growing, shade-tolerant trees will eventually die out. Further, the light-demanding trees have less dense wood than the shade-lovers. Thus mature tropical forests will sequester less carbon per unit area and may ultimately act as sources, not sinks, for atmospheric carbon.

Phillips and Gentry suggest that the most plausible explanation for this increased turnover is the rising carbon dioxide concentration in the atmosphere. This increase, they guess, may boost tree productivity, encouraging a rapid turnover. Their argument gains credence as observations reveal that the build-up of atmospheric carbon dioxide coincided with the period of enhanced turnover in humid tropical forests.

Of the 19 forest sites Phillips and Gentry examined, 14 showed significant increase in turnover rates -- upto one-and-a-half times the earlier values. The turnover decreased in a few sites but the scientists say that this decrease was statistically insignificant.

However, linking global warming and the rise in atmospheric carbon dioxide to the changes in biodiversity has been a rather difficult task. For one, no two plant species (out of a possible 500,000) respond similarly to the laboratory-controlled increases in temperature and carbon dioxide; second, surveys of tropical forests on a global scale and over a long period are rarely carried out.

Ideally, say ecologists S L Pimm of the University of Tennessee at Knoxville and A M Sugden, editor of Trends in Ecology & Evolution, data are required on the turnover of species, not just individual trees in these tropical forests. "Then," they say , "we could test directly whether the increased turnover of individuals leads to increased turnover in species and that in turn to a decrease in diversity."

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