Science - February 11, 2016

Secondary forest stores a lot of carbon

Roelof Kleis

Natural second-growth forests on abandoned farmland in Latin America sequester large amounts of carbon. This is reported in Nature by an international team of researchers led by Wageningen professor Lourens Poorter. In the study Poorter and his colleagues describe the growth of 1500 patches of secondary forest on 45 locations in Latin America.

The vegetation in question is new forest growing in places where the original rainforest was felled for agriculture. This second-growth forest sequesters enormous quantities of carbon: the annual carbon intake is eleven times larger than that of the average rainforest in the Amazon. It comes to 122 tons of biomass per hectare over 20 years. Calculations show that after 66 years the forest will have stored almost as much carbon as the rainforest that was once cut down there.

This does not mean, however, that you get the old rainforest back, warns Poorter. The combination of species that regrow is different to what grew there originally. ‘There are many dimensions to the resilience of the forest. We only looked at the regrowth of the biomass. And that goes surprisingly quickly. But it will be a few hundred years before the forest has the same mix of species as the rainforest.’

For Poorter, then, the study should not be seen as sanctioning uncontrolled deforestation. ‘In virgin rainforest large amounts of carbon are sequestered. But they are almost fully grown so they hardly absorb any more new carbon. By only looking at the contribution of rainforest to carbon sequestration you miss a lot of the dynamics in tropical forests. The potential of secondary forest for carbon sequestration is enormous. Our message is twofold, then: conserve the primary forest and exploit the potential of secondary forests.’

Carbon sequestration by secondary forests does not happen on the same scale everywhere, either. The researchers discovered a link between the increase in biomass and rainfall. The wetter the climate, the faster the secondary forests grow. Based on that link, a map was made showing where secondary forest offers the most potential for carbon sequestration.

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