Arctic "greening" results in a net release of carbon dioxide.

A recent study associated with the University of Sheffield suggests that the conversion of tundra vegetation to boreal forest releases large amounts of carbon dioxide into the atmosphere, the opposite result to previous suggestions.  

During recent decades there has been a rapid change in plant communities occupying Arctic ecosystems – a scenario set to continue during the 21^st century. Most dramatically perhaps is the migration of boreal forests both uphill and northwards as a result of the gradual warming of these areas, permitting the survival of these invasive communities.

This situation, where formerly tundra heathland is converted into forest, has gained a large amount of interest due to the accompanying effects upon the carbon stored in organic matter within Arctic soils. At these northerly latitudes, carbon stored underground is actually greater than the carbon in the atmosphere (in the form of carbon dioxide, or CO₂).

The tree line of mountain-birch before the tundra environment begins further north. From shef.ac.uk  

One of the predominant species of tundra heathland (Empetrum 

hermaphroditum) (above) produces compounds that reduce the

 decomposition rate of surrounding organic matter. From fruitipedia.com

Previously it was believed that the recent encroachment of forest into such carbon-rich habitats would promote the amount of carbon stored, since trees themselves store large amounts of the element.

However, a study published this week in the Journal Nature Climate Change suggests otherwise. Iain Hartley, the lead researcher of the study, and his colleagues carried out extensive surveys into the amount of carbon above- and below-ground in birch forest and in the surrounding tundra heathland.

They found that despite the carbon stored within the birch trees, this was far outweighed by a substantial loss of carbon from underground stores compared to the tundra heathland. This outcome is caused by increased rates of photosynthesis of the birch trees (in summer at least), which increases the rate of decomposition in the surrounding soil – a process known as “positive priming” – and, in turn, increases the amount of carbon dioxide released into the atmosphere. 

In contrast to previous studies, the plots of land surveyed were not inundated with water, thus refuting the possibility that periods of drying could have removed the inhibitory effects of waterlogging on decomposition, and subsequently promote the release of carbon dioxide.

These findings, therefore, challenge previous suggestions of increased carbon storage associated with the northward migration of boreal forests. Furthermore, the indication that more carbon dioxide is being released as a consequence of the transformation of tundra to boreal forest highlights a long-term, and potentially significant, contributor to climate change that requires our immediate attention. 


"Indeed, when you consider that larger, faster growing plants could stimulate release of carbon from soil (as this work shows) and that warmer soils are likely to release more carbon anyway (because of the stimulated microbial respiration) then the potential for carbon release in a warmer Arctic is considerable and worrying" Gareth Phoenix, senior lecturer at the University of Sheffield, told me. 


He also added: "the other key point is that some of the soil carbon released by the encroaching trees is old carbon - the stuff that we would often consider to be quite recalcitrant and well locked away in the soil".

Future studies into different plant communities and regions of the Arctic are now required in order to better understand the magnitude of these soil-plant interactions.