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Clear-cut forest areas emit greenhouse gases

The forest and forest management are a hot topic in the climate debate, but there are still uncertainties in the mapping of the forest's function as a carbon sink or carbon source. In a new study, led by Patrik Vestin at the Department of Physical Geography and Ecosystem Science, the researchers have made measurements of greenhouse gases at clear-cutting and found that there are large net emissions.
Clear-cut area in boreal forest.

Measurements were started one year after clear-cutting, immediately after soil preparation and planting, and included the three most important greenhouse gases (carbon dioxide CO2, methane CH4, nitrous oxide N2O) on a clear-cut 2010-2013. The study was performed on two different types of surfaces to investigate differences between wetter and drier parts of a clearing.

Water and vegetation affecting gas flows

At both measuring sites, significant emissions of all three greenhouse gases were observed, but the amount of water in the soil was decisive for the respective gases' respective flows. This affected the total greenhouse gas balance both directly and indirectly. As vegetation grew faster on moister soil, it contributed to reduced carbon dioxide emissions when the vegetation started. But the total carbon balance with net emissions was maintained in part by the wetter soil instead emitting more methane gas, which is a stronger greenhouse gas.

Climate calculations may be misleading

Both measuring surfaces had increased water saturation as a result of the trees being removed, and it is probable that this caused the forest to change from methane sink to methane source after felling. Something Patrik Vestin, research leader for the study, thinks should be taken into account in forestry from a climate point of view in the future:

— Roughly speaking, in individual years’ methane and nitrous oxide could each contribute up to 10% each to the total greenhouse gas budget on these surfaces. This applies from a 100-year perspective, which is often taken into account when the greenhouse gas effect on the climate is to be modeled.

But Patrik Vestin thinks that one should think about whether the calculations should be done differently:

— If one were to consider intensifying forestry to meet the demand for fossil-free fuel or products in the short term, within about 20 years, one should expect another "global warming potential" for methane and nitrous oxide, and then triple the effect of methane emissions. This is because methane has a relatively short lifespan, which means that the effect from a certain methane emission is estimated to be greater in 20 years than if the effect is calculated on a 100 years, he says.

For more recent research on the effects on clear-cutting, read more in these articles:

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Department of Physical Geography and Ecosystem Science
Lund University
Sölvegatan 12
S-223 62 Lund
Sweden

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