Hot carbon storage

Africa’s tropical forests sequestered carbon in recent decades

Field studies reveal that Africa’s rain forests locked away, on average, more than a billion metric tons of atmospheric carbon dioxide each year during the past four decades.

TALE OF THE TAPE Four decades’ worth of field studies in Africa suggest that the continent’s rain forests sequestered an average of 340 million metric tons of carbon each year between 1968 and 2007. IMAGE CREDIT: S. Lewis

At 79 sites, none of which had been directly affected by logging or other human activities, researchers kept track for 40 years of the growth of all trees with trunk diameters larger than 10 centimeters, says Simon L. Lewis, an ecologist at the University of Leeds in England. Between 1968 and 2007, large trees at those sites sequestered about 630 kilograms of carbon per hectare (an area about 1.5 times the size of a soccer field) each year, Lewis and his colleagues report in the Feb. 19 Nature. That rate of storage, extrapolated to all of Africa’s rain forests, means that African trees sequestered more than 340 million tons of carbon each year since 1968. That amount equates to 1.25 billion tons of carbon dioxide gas, just a small fraction of the carbon dioxide emitted by humans annually.

Previous studies have shown that, on balance, tropical ecosystems worldwide absorb more carbon — about 1.5 billion tons — than they emit each year. But scientists haven’t figured out exactly where this extra carbon is being stored.

Many scientists suggest that the carbon stored in the tissues of growing trees in mature forests should balance that emitted via decomposition of dead wood there, so “it is thus surprising that undisturbed tropical forests do not seem to be at equilibrium,” Helene Muller-Landau, a botanist at the Smithsonian Tropical Research Institute in Balboa, Panama, comments in the same issue of Nature. The unexpected storage of carbon, which also has been noted in Amazonian rain forests, might result from the fertilizing effect of plant-nourishing carbon dioxide accumulating in the atmosphere because of human activity, Lewis and his colleagues suggest. As more and more carbon dioxide builds up in the atmosphere, the trees may grow larger and thus store more carbon before they die, fall and decompose.

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