Retreating sea-ice may lead to declined growth of tundra shrubs – University of Copenhagen

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06 September 2017

Retreating sea-ice may lead to declined growth of tundra shrubs

A GREENER ARCTIC?

For long, the commonly accepted consensus has been that reduction of the Arctic sea-ice leads to favorable conditions for Arctic tundra vegetation, and has thus generally been linked to the recently observed increase in plant productivity. A recent article published in the scientific journal Biology Letters by Mads Forchhammer of University Centre in Svalbard (UNIS) and the Greenland Perspective initiative suggests otherwise, and shows the relationship between sea-ice and tundra vegetation to be more complex than previously assumed.

The current consensus of the scientific community is that the Arctic is greening, as the amount of sea-ice is reduced. The loss of near-coastal sea-ice results in an increase in temperature along the Arctic Ocean coast lines. Multiple studies, hereunder studies utilizing satellite imagery of the Arctic as well as on site warming experiments, further support the idea that a warmer Arctic results in better growing conditions for the plants of the region thus strengthening the coupling of decreasing sea-ice and increasing tundra vegetation.

Birch subshrub growing in Adventdalen, Spitsbergen, Svalbard. Foto: Mads Forchhammer.

Birch subshrub growing in Adventdalen, Spitsbergen, Svalbard. Foto: Mads Forchhammer.

Forchhammer is challenging this notion. Using data from three different species of willow and birch from the tundra of Greenland and Svalbard in a plant growth model, he finds a different picture, which shows a more complex relationship between ice and vegetation growth. Using his model, Forchhammer finds an overall negative influence of summertime sea-ice retreat on the long-term annual growth of the plant species examined.

Furthermore, the influence of the sea-ice is shown to decrease northwardly, with the northernmost plant samples showing the least response to sea-ice reduction. The plants’ growth here proves to be more dominated by other factors, including the growth of previous years. Forchhammer also shows that age is an important factor in the calculation, with the younger plants being more dominated by the climatic changes than the older plants of the same species.

Forchhammer’s study differs from other studies in that it focuses the attention of just a few species rather than large areas of vegetated tundra, as well as comparing growth of these species across multiple latitudes. This way, it is shown that different plants behave and respond differently to the retreat of the Arctic sea-ice, and that the response also varies on a spatial scale, suggesting that other, local environmental conditions also play a part in the relationship.

Forchhammer points to the fact that multiple factors may influence growth of plants in the Arctic region. Herbivorous grazing by caribou and musk oxen and drought stress induced by rising temperature in an already dry environment as well as local variations in permafrost preservation and soil humidity are such factors, and Forchhammer stresses the point of including environmentally dominated factors as well as species-specific factors when evaluating the growth responses to climatic changes.

Western plains of Adventdalen. Foto: Mads Forchhammer.

Western plains of Adventdalen. Foto: Mads Forchhammer.