Climate Feedback

Jungle Fit!

<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.

Quirin Schiermeier

Photo: Study co-author Bonaventure Sonké and his field team measuring trees in the Dja Faunal Reserve, South-East Cameroon.

Credit: Simon Lewis

Comments

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    Doug Heiken said:

    Not to disrespect tropical forests in the slightest because they are very important to the overall global carbon cycle, but local pride requires me to say that the old-growth temperate forests in the moist “westside” portions of the Pacific Northwest, apparently store more carbon, per-acre, than any other forests in the world. (“The C densities we measured in old-growth forests of the PNW are higher than C density values reported for any other type of vegetation, anywhere in the world. … Results showed that coastal Oregon stands stored, on average, 1127 Mg C/ha, … the highest C density was at stand CH04 at Cascade Head, ORCOAST, with 1245 Mg C/ha.” Smithwick EAH, Harmon ME, Acker SA, Remillard SM. 2002. Potential upper bounds of carbon stores in the Pacific Northwest. Ecological Applications 12(5): 1303-1317.

    Here is a useful slide show clarifying many misconceptions about forests, logging, and carbon:

    http://www.slideshare.net/dougoh/forest-carbon-climate-myths-presentation/

    (For full effect click “full” in the lower right.)

    Here is a more detailed foot-noted report on forests, carbon and climate change:

    http://tinyurl.com/2n96m5

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    Geraldine Trott said:

    I have been reading the scientific statistic’s concerning climate change & don’t find it surprising or crazy or unreal. I’m no scientist or highly educated person, I’m just someone who has taken notice, of my environment, for a long time & I have felt the changers around me. I have never needed anyone to tell me what I should think, I have just used common sense & now I can’t understand why, when there is so much scientific evidence around now, why there are people & govements that think that the economy is more important then our environment? I don’t know why, we all, as a world wide web community, don’t stand up & say, “this is the planet that provides us with air to breath, water to drink, earth to grow our food, & here we are raping & pilaging it to a point, where we have become so greedy, that we are killing it, so it’s about time we stop these gready people from killing all of us & our children”. Why do people think that we will need to worry about the economy, when we are all dead from living on a dying planet, it won’t matter anymore how big your house is. What good will all that money do when there is nowhere left on this earth to live?

    I have grand children & I fear for their future. I don’t think anyone can save us now because all the “Leaders” of this world take 10 years or more to start thinking about what to do & by then it is to late. I think it’s already to late, but I would like to be proven wrong. The first thing I really feel might save us, is for every one in the world, to start today, planting trees where they have been cut down for houses or any other reason. It should be a international offense to cut down a tree unnecessarily.

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