Today’s paradox: higher tropical forest density leads to extra CO2

Increased forest density leads to extra CO2?Do you remember how last week we concluded so often forest climate studies have something strangely nonsensical? Here’s yet another piece of evidence to back that up.

It turns out an increase of litterfall (triangles in graph) – due to a suspected climate rainforest density feedback – leads to a substantial net increase of soil CO2 emissions.

To us that’s unexpected. And although it may be nice for our ‘paradoxical science news’ series [forgot why we even started that one] we’d much rather simply understand what’s going on at the gateways of the terrestrial carbon cycle.

Positive carbon breakdown feedback

According to research by the UK Centre for Ecology and Hydrology and other institutions, published in Saturday’s edition of Nature Climate Change, the extra litter on the forest floors leads to a process called ‘priming’ whereby the fresh carbon of dead leaves and other dead biomass stimulates microbial breakdown of the carbon.

These microbes [or their byproducts] would then also start to ‘eat away’ at older, chemically more stable carbon in the forest soils, converting that to CO2 as well.

Measurements at a rainforest test site in Panama show that a 30 percent forest density increase would translate to 0.6 tonnes of carbon per hectare of ‘lowland tropical forest’ per year.

That means you would need almost half a million hectares of forest, before you can start counting annual emissions in megatonnes CO2. But considering the size of the tropics, that’s not so difficult – besides, according to the authors, this would offset a substantial part of the carbon gain of increased living biomass density in the forests.

It is not the first time we hear of a climate feedback that acts through forest soils – and it’s also not the first time microbes are involvednot even the second.

Which is the forest density climate feedback?

There is however one other element in this study that we feel we cannot ignore. The authors link the witnessed increase in tropical forest density not to equatorial rainfall increase [our assumption] but to the good old CO2 fertilisation hypothesis, citing a not so old publication that deserves a better look, because it reached Nature in 2009. According to this study a rise in the CO2 concentration of 150ppm would lead to the above-mentioned tropical forest density increase of 30 percent.

In a couple days time we’ll get back to the subject of forest CO2 fertilisation and take a better look at that study.

© Rolf Schuttenhelm |

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