Some good news for those worried about climate change: The trees in African rainforests are gobbling up ever more carbon dioxide and thereby mitigating the buildup of the greenhouse gas in Earth's atmosphere. The finding underscores the importance of protecting the rainforests, say the authors.
Trees take in CO2 as they grow, and when they die, their decay releases it back into the air. In theory, these fluxes are balanced in a mature forest, so the trees are neither a net sink--as the storage process is called--nor a source of CO2 to the atmosphere. But about 10 years ago, researchers discovered that the old-growth rainforests in the Amazon were growing enough to remove a significant amount of CO2 from the air (Science, 16 October 1998, p. 439).
No one knew if the same thing was happening in Africa, home to a third of the world's tropical rainforest areas. To find out, an international team led by ecologist Simon Lewis of the University of Leeds in the U.K. has been monitoring the effects of CO2 buildup there. In this week's issue of Nature, the researchers report data from 79 plot surveys scattered across 10 African countries that included records from 1968 through 2007. Limiting their surveys to the oldest trees, those with trunks 10 centimeters in diameter or greater, they examined how the trunk size changed over time. The result: Trunks in mature forests have been expanding, adding on average 0.63 metric tons of carbon per hectare per year.
That's roughly the rate seen in the Amazon and suggests that old rainforests across the tropics are taking up carbon consistently. Incorporating these first data from Africa, researchers estimate that tropical old-growth forests across the world sock away about 1.2 billion metric tons of CO2 per year, or about 5% of the world's output from fossil-fuel burning.
Ecologist Helene Muller-Landau of the Smithsonian Tropical Research Institute in Balboa, Panama, says the current growth spurt of tropical rainforests, which seems puzzling on the surface, might be due to their recovery from wildfires or from deforestation by our human ancestors that occurred centuries or even millennia ago. Whatever the cause, however, she says, "there's no question we've gotten lucky," having this extra source for carbon storage. How long the luck will hold is unclear. As Lewis cautions, "these trees cannot continue growing bigger forever."