From: Tom Marshall, Planet Earth Online
Published June 27, 2012 05:55 AM

Would more trees in the Arctic absorb carbon, or cause more to be released?

Trees colonising formerly open tundra as the climate warms could cause Arctic ecosystems to release vast amounts of stored soil carbon into the atmosphere, a new paper argues.

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Many climate models have assumed that trees taking over the Arctic, and the enormous increase in plant biomass this would bring, would cause these landscapes to absorb much more carbon than they did before, helping restrain the effects of climate change.

But this study suggests that's far from certain. In Scandinavia at least, when tundra heath turns into birch woodland it seems it could release much of the carbon stored in the soil into the air. This will more than counterbalance the fact that a forest holds around twice as much carbon in its biomass. So far from holding climate change in check, accelerated tree growth, and colonisation of treeless landscapes, could speed it up.

The question matters a great deal, because Arctic soils hold a huge amount of carbon - much more than is in the whole atmosphere at any given time. If all this carbon were released in a short period, the effects on the climate would be profound. And because global warming helps the birch trees get a foothold on tundra heathland, this could turn into a vicious circle whereby warming causes more carbon release, leading in turn to further warming.

'Our study questions the idea that high-latitude environments will be big carbon sinks' says Dr Iain Hartley of the University of Exeter, lead author of the paper in Nature Climate Change. 'It's hard to know the magnitude of these effects at the moment, but this certainly suggests that our understanding of the effects of environmental change in the Arctic needs to change - greater plant biomass doesn't always mean greater carbon storage.'

Arctic trees via Shutterstock.

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