Tropical forests hold more living biomass than any other terrestrial ecosystem. A new report in the journal Nature by Lewis et al. shows that not only do trees in intact African tropical forests hold a lot of carbon, they hold more carbon now than they did 40 years ago--a hopeful sign that tropical forests could help to mitigate global warming.

In a companion article, Helene Muller-Landau, staff scientist at the Smithsonian Tropical Research Institute, says that understanding the causes of this African forest carbon sink and projecting its future is anything but straightforward.

Growing trees absorb carbon. Dead, decomposing trees release carbon. Researchers expect growth and death to approximately balance each other out in mature, undisturbed forests, and thus for total tree carbon stocks, the carbon held by the trees, to remain approximately constant.

Yet Lewis and colleagues discovered that on average each hectare (100 x 100 meters, or 2.2 acres) of apparently mature, undisturbed African forest was increasing in tree carbon stocks by an amount equal to the weight of a small car each year. Previous studies have shown that Amazonian forests also take up carbon, although at somewhat lower rates.

"If you assume that these forests should be in equilibrium, then the best way to explain why trees are growing bigger is anthropogenic global change - the extra carbon dioxide in the atmosphere could essentially be acting as fertilizer" says Muller-Landau.

"But it's also possible that tropical forests are still growing back following past clearing or fire or other disturbance. Given increasing evidence that tropical forests have a long history of human occupation, recovery from past disturbance is almost certainly part of the reason these forests are taking up carbon today."

Muller-Landau, who directs a project to monitor carbon budgets in forest study sites worldwide as part of the Smithsonian's Center for Tropical Forest Science and the HSBC Climate Partnership, advises that this newfound sink shouldn't be taken for granted, or presumed to continue indefinitely.

"While we still can't explain exactly what is behind this carbon sink, one thing we know for sure is that it can't be a sink forever. Trees and forests just can't keep getting bigger. Tropical forests are buying us a bit more time right now, but we can't count on them to continue to offset our carbon emissions in the future."