The Arctic is becoming greener as plant growth increases due to warmer temperatures. More plant growth means more carbon stored as biomass, so it had been thought that this would lead to the Arctic removing enough carbon dioxide from the atmosphere to slow warming slightly.
But new research shows that the stimulation of decomposition rates in the Arctic soil outweighs the influence of the increased plant biomass. The researchers found that the expansion of forests into the tundra in northern Sweden will release carbon dioxide into the atmosphere.
Dr Iain Hartley, now at the University of Exeter and lead author of the paper, said: “Determining directly how carbon storage is changing in high-latitude ecosystems is very difficult because the majority of the carbon present is stored below ground in the soils. Our work indicates that greater plant biomass may not always translate into greater carbon storage at the ecosystem level.”
“We need to better understand how the anticipated changes in the distribution of different plant communities in the Arctic affects the decomposition of the large carbon stocks in tundra soils if we are to be able to predict how arctic greening will affect carbon dioxide uptake or release in the future.”
The researchers measured carbon stocks in vegetation and in soils, in both tundra and neighboring birch forests. They found that “the two-fold greater carbon storage in plant biomass in the forest was more than outweighed by the smaller carbon stocks in forest soils.”
In addition, using a different methodology based on measuring the radiocarbon content of the CO2 being released, the researchers found that the birch trees appeared to be stimulating the decomposition of organic matter in the soil. Which means that birch trees can contribute directly to the release of carbon, from soils, into the atmosphere.
Dr Gareth Phoenix, of the University of Sheffield’s Department Animal and Plant Sciences, said: “It shows that the encroachment of trees onto Arctic tundra caused by the warming may cause large release of carbon to the atmosphere, which would be bad for global warming.”
“This is because tundra soil contains a lot of stored organic matter, due to slow decomposition, but the trees stimulate the decomposition of this material. So, where before we thought trees moving onto tundra would increase carbon storage it seems the opposite may be true. So, more bad news for climate change.”
“The results of the study are in sharp contrast to the predictions of models which expect total carbon storage to increase with the greater plant growth. Rather, this research suggests that colonisation by productive, high-biomass, plant communities in the Arctic may not always result in greater capture of carbon dioxide, but instead net losses of carbon are possible if the decomposition of the large carbon stocks in Arctic soils are stimulated. This is important as Arctic soils currently store more carbon than is present in the atmosphere as carbon dioxide and thus have considerable potential to affect rates of climate change. It is yet to be seen whether this observed pattern is confined to certain soil conditions and colonising tree species, or whether the carbon stocks in the soils of other arctic or alpine ecosystems may be vulnerable to colonisation by new plant communities as the climate continues to warm.”