<img alt=“Lewis.jpg” src=“http://blogs.nature.com/climatefeedback/Lewis.jpg” width=“300” height=“375” align=“right” hspace="10px"" />Tropical forests which (still) cover around 10% of the global land area contain more carbon per hectare than any other form of vegetation. It’s obvious from that that their growth or decline has a huge impact on the global carbon budget.
Cutting down forests will add carbon to the atmosphere, no matter which kind of land cover replaces the jungle. But what’s happening in tropical forests that have long been undisturbed by logging, storms or fire? Theoretically, the carbon balance of such old-growth forest – if tree growth and death are in equilibrium, that is – should be next to zero.
But apparently it’s not. In a paper in Nature today (subscription), a team led by Simon Lewis of Leeds University in Britain reports that tree biomass in intact African forests increased between 1968 and 2007. Across 79 plots monitored in ten countries large living trees added an average 0.63 tonnes of carbon per hectare each year. Scaled up to the continent, and including roots, smaller trees and dead wood, African forests seem to have stored 340 million tonnes of carbon per year during recent decades. Previous studies suggested that Amazonian forests are accumulating biomass and carbon at a similar rate. Globally, intact tropical forests seem to take up 1.3 billion tonnes of carbon per year – equivalent to almost 20% of annual carbon dioxide emissions worldwide.
If these figures hold up, they may help explain the gap between fossil fuel emissions and measured atmospheric carbon dioxide concentrations. The global rain forest belt accounts for more than half of the land carbon sink that is required, simplistically speaking, to reconcile the amount we emit with the amount that stays in the atmosphere.
Is that good news? Yes, in that tropical forests’ ability of taking up carbon is a welcome “ecosystem service” that reduces the rate of increase of atmospheric CO2. There’s no indication that the jungle, if adequately protected, will cease any time soon to act as a large carbon sink. The long-standing notion that old-growth forests are carbon-neutral was only recently challenged.
But there’s no free lunch. “Securing this service will probably require formalizing and enforcing land rights for forest dwellers, alongside payments for ecosystem services to those living near forested areas,” the authors write.
The question remains why old-growth forests do not seem to be at equilibrium. One possible explanations, notes Helene Muller-Landau of the Smithsonian Tropical Research Institute in Balboa, Panama, is that forests are still recovering from major disturbances, natural or man-made, in the past few centuries or even millennia.
“Far from being pristine wildernesses little influenced by their human inhabitants, many areas were cleared or otherwise intensively used in past,” Muller-Landau writes in the accompanying News and Views article here. ”These disturbances are almost certainly contributing to carbon accumulation in many tropical forests today.”
A second possibility is that excess atmospheric carbon dioxide is fertilizing tropical tree growth, allowing trees to grow larger before they die.
Whatever explanation is correct – it might well be a combination of both – one thing is sure: Keeping intact the remaining areas of undisturbed tropical forest wouldn’t be such a bad idea at all.
Photo: Study co-author Bonaventure Sonké and his field team measuring trees in the Dja Faunal Reserve, South-East Cameroon.
Credit: Simon Lewis