Today´s paradox: more vegetation means more CO2 – in tundra that is

Birches can’t hurt the climate. They’re made of carbon and – just to be sure – they’re even painted white!

Well, at least on the Swedish tundra that’s no guarantee…

birches decrease tundra carbon store
Photo credit (CC): Cassi Saari. (Hold your chainsaws, these birches grow at Chicago Botanic Garden.)

Probably best thing is to chop these birch trees up and send the wood south to that Småland furniture farm, a Scottish team of climate researchers says. In fact, extrapolating their newfound insights east to northern Siberia and west to Alaska, that may prevent emissions of one third of the total tundra carbon store (which is estimated at 500+Gt).

What we did know about the tundra

Arctic warming does not just cause sea ice decline in the Arctic Ocean, it also has a changing impact on the bordering tundra biome. Among direct effects are decreased (seasonal) snow cover and permafrost melting. The latter can lead to the sudden release of stored methane but also – because without a frozen layer the peaty soils dry out in summer – significant CO2 emissions, possibly sped up by tundra wildfires.

Next to changes in ice and water, there is also already a noticeable vegetation response going on, as frost, snow cover and cold air are limiting factors for plant life in the tundra biome, all of which are now diminishing.

Therefore it only makes sense the taiga biome will creep north. And as one degree warming equals to a poleward climatic migration of somewhere around 100 kilometers – large parts of the tundra biome will be pushed off the Eurasian and North American continent, into the coastal seas of the Arctic Ocean, drowning those lemmings for sure.

Small trees popping up from nowhere

It is however not just a gradual process along neatly defined geographical borders. Earlier this month a group of Finnish researchers reported strong vegetation changes within the tundra biome, with a shrub landscape turning to one with small trees, over the last few decades.

And what was it again with trees? Good for the climate because they store carbon – and, indeed, you definitely scored one there, not always so good for the climate, because they tend to lower the Earth’s albedo. This goes especially for these taiga pine forests, which are not only dark-coloured by themselves, but also tend to stick through the spring and autumn snow cover [remember albedo is never a concern in winter, as there is no sun to either reflect or absorb during the long polar night].

Trees bad, reindeer good for albedo

Indeed the current vegetation increase lowers the Arctic tundra albedo and thereby speeds up local warming, another Finnish study concluded last April. [From that study by the way we also learned reindeer grazing is important to keep the tundra nice and reflective – so for instance maintaining the multimillion caribou herds on the Alaska North Slope is about more than just preserving a back-up Serengeti for wildlife documentary film crews…]

But then there is this new study. The tundra vegetation increase would not just hurt the local albedo – it would also promote CO2 emissions. That at least we learn from a new publication in Nature Climate Change by a group of Scottish researchers, led by the University of Stirling.

Higher plant productivity as part of faster carbon cycling

tundra carbon soils, birch trees vs heathOf course what one would expect to see under increased plant growth, is increased carbon retention, not CO2 emissions – but that of course would only be the case if the vegetation increase would actually lead to larger amounts of biomass.

And in case of the studied bit of North-Swedish [they still have a tiny bit left] tundra that does not seem to be the case, the authors state. In a tundra heath landscape the combined* amount of carbon stored in live and dead biomass and organic soils is over 7 kilograms of carbon per square meter. For tundra birch forest it is just over 4.6 kg – counting down till bedrock.

That means birch forestation could decrease the total tundra carbon store by more than a third.

[*) In both tundra landscapes the total amount of carbon stored underground, including organic soils and root systems, is larger than the part above ground (living and dead biomass, shrubs and trees combined). The above-ground carbon store is of course considerably larger in the birch landscape.]

Local tree growth is not just a result of the faster biological cycle, it also contributes to it, the researchers say. During the middle of the growing season the high plant activity stimulates the decomposition of older soil organic matter, something the authors refer to as ‘positive priming.’

“We suggest that, as more productive forest communities colonize tundra, the decomposition of the large C stocks in tundra soils could be stimulated. Thus, counter-intuitively, increased plant growth in the European Arctic could result in C being released to the atmosphere, accelerating climate change.”

© Rolf Schuttenhelm |

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