Unstable winters and pollution: Northern nature under stress

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From the field experiment in summer. Photo and front photo: Jarle W. Bjerke, NINA.

Text & video Jon Schärer

How will northern Norwegian ecosystems tolerate unstable winters and nitrogen pollution?

– Little is known about how winter climate change affects northern ecosystems, and knowledge of the impacts of multiple events is particularly lacking, says postdoc Stef Bokhorst at NINA (The Norwegian Institute for Nature Research) in Tromsø.

To understand how climate change will affect northern ecosystems and nature-based industries, three related projects are being executed to elucidate various aspects of winter climate change.

The ongoing project “WINNIT”   (derives from the words “winter” and “nitrogen”), aims to address this issue by combining these two drivers in field experiments running over several years in a full-factorial set-up.

This video gives highlights from the field experiment.

– During wintertime, we expose field plots to realistic winter warming events, while during the growing season, we add realistic nitrogen loads to the plots. Through this, we will quantify the vitality and community development of Arctic ecosystems and their trophic levels, Bokhorst says.

– More importantly, we aim to determine the mechanisms behind species responses of both pressures and combined, through gene expression, thermal tolerance and stress metabolism levels.

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From the field experiment at Håkøya in winter. Photo: Kjell-Sture Johansen, NORUT.

Long-range nitrogen

Such warm spells are extreme in their nature, as they enforce major temperature changes on ecosystems due to the loss of insulating snow in winter. In addition, the High North appears to be a sink for long-range nitrogen pollution transport from European (and other) emission centers. Polluted air masses carrying large quantities of nitric acid are carried over the Arctic and deposit large nitrogen doses in fragile and typically nutrient-limited ecosystems.

Although both winter climate change and nitrogen pollution are receiving more research attention this is not addressed in concert. Therefore, NINA in collaboration with Bioforsk, UiT – the Arctic University of Norway and international partners, received funding from the Research Council of Norway for studying this combination of drivers of ecosystem change.

Senior researcher Jarle W. Bjerke leads the project, and Stef Bokhorst works as a postdoc on it.


First results

After the first winter warming event, the warmed plots and the control plots differed in their carbon flux. The warmed plots had reduced photosynthesis compared to the control plots. However, after two weeks, photosynthetic rates were similar in warmed and control plots, indicating that the winter warming imposed only a little stress to the ecosystem. There were no effects of the nitrogen addition. The winter warming and nitrogen treatment will be repeated two more times, and we expect to see stronger longer-term effects.


«WINNIT – Winter disturbance and nitrogen deposition: Unraveling the mechanisms behind ecosystem response to combined effects of climate and pollution» – a project financed by the Research Council of Norway and FRAM – High North Research Centre for Climate and the Environment.

For more information:



Postdoc Stef Bokhorst, stef.bokhorst@nina.no

Project leader Jarle W. Bjerke, e-mail jarle.werner.bjerke@nina.no