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A clear-cut forest provides essential information for climate research

The clear-cutting shown from above. Please observe that the clar-cut is shown at a higher speed in the video than in reality. Video: ICOS Norunda.

The machines have started rolling in at Norunda and are cutting down trees, one by one. Norunda is Sweden’s oldest measuring station for greenhouse gases and it is unique. Following several decades of being surrounded by a hundred-year-old forest, it will instead be surrounded by a clear-cut.
- We expect a significant increase in greenhouse gas emissions, says professor Natascha Kljun at CEC.

The Norunda field station is located about 30 kilometres north of Uppsala. Greenhouse gases in the atmosphere have been measured here continuously since 1994, and the data provide important information for research projects. Since 2014, Norunda has been part of the European research infrastructure ICOS.

A 100-metre-high mast rises at the site and is well equipped with technical equipment, for measuring e.g. carbon dioxide, methane, wind direction, and wind speed. The mast has been surrounded by an old forest plantation consisting mainly of pine and spruce. The trees are about one hundred years old and ready for felling. A spruce bark beetle infestation followed the record dry year of 2018 and increased the need for action. At the end of July this year, clear-cutting began. It will be completed by the end of 2022. 

- We are getting used to it. Personally, I am sad when such an old forest disappears, but as a scientist, this is very exciting. We can rely on a long time series, and now we can measure what happens to the carbon cycle in connection with the clear-cutting, says Irene Lehner, research engineer at Norunda field station.

From carbon sink to a carbon source, even in summer

The measurements already show clear changes when the measured air comes from the clear-cut area. Usually, the carbon dioxide content is around 410 ppm, but now it has risen to about 500 ppm for short periods, with a few peaks above that. It is a significant increase, and Natascha Kljun, professor at CEC and scientific principal investigator of the ICOS Norunda station, expects more changes in the future.

- Normally, forests absorb carbon dioxide during summer, and atmospheric concentrations decrease. At Norunda, this uptake will be erased, and instead, carbon dioxide will be released all year round during the first few years after clear-cutting. We will be able to monitor the processes closely with our continuing measurements, she says.

Methane can be released from the soil

Even at ground level, much is changing. The water table is high in the area, and the combination of no trees left to soak up water and heavy logging machines compacting the soils can lead to wet grounds with many water pools. That might affect soil microbes that break down dead tree roots. Methane can then be released, resulting in further increases in greenhouse gases in the air.

- The forest floor has acted as a small methane sink. Depending on groundwater levels, we expect to see methane release next spring instead. And methane is a much more potent greenhouse gas than carbon dioxide, says Natascha Kljun. How quickly we can see the changes in methane also depends on whether it will be a dry or wet year in 2023. 

During the following year, soil scarification will be carried out, and young pine seedlings will be planted. It will probably take 10-15 years before they have grown enough to turn the forest into a carbon sink.

- We now have an excellent opportunity to see how the carbon cycle changes with clear-cutting, replanting and tree growth. This will help us to calculate how much the forests can help to slow down climate change in the future, says Natascha Kljun.