The Effects of Forest Management and Natural Disturbance on Greenhouse Gas Exchange in Boreal Forests
Disturbance usually leads to increased availability of organic material and increased decomposition of soil organic matter, which means larger carbon dioxide (CO2) emissions. There is also a risk for increased nitrous oxide (N2O) emissions and nitrogen leaching to the ground water because of increased nitrogen availability. In addition, boreal forest soils normally take up methane (CH4) from the atmosphere and there is a risk that this uptake of CH4 is reduced or even turned into an emission following a severe disturbance.
Previous studies have shown that clear-cuts, which is the result of the dominating harvesting method in Sweden, causes large emissions of CO2 during 10-20 years. Our own measurements also indicate significant emissions of the powerful greenhouse gases methane and nitrous oxide from clear-cuts. It is important to understand how general this picture is and if there are alternative management strategies that maintains a high forest productivity but with smaller negative impacts on the climate and the environment.
At our research station Norunda in Uppland , we are studying how forest thinning, clear-cutting and stump harvesting affects the greenhouse gas exchange in long-term (> 5 years) experiments. In co-operation with Gothenburg University, we have established another long-term experimental site on a clear-cut in Skogaryd, Västra Götaland. We also have a mobile measurement system that we are moving around on new clear-cuts with different degrees of wetness in southern Sweden.
We established an experimental site in Rumperöd, Skåne in 2013 in order to investigate the carbon balance of a forest that has not been clear-cut for at least 12 generations. The forest has been managed by a selective cutting strategy since the 1950s. This means that the large initial emissions of carbon dioxide and other greenhouse gases on clear-cuts can be avoided but an important question is if forest productivity is high enough to make selective cutting a viable alternative to clear-cutting on a national level. Results from these measurement sites are compared to data from our measurement sites Nimtek, Lappland (natural spruce and pine forest) and (a typical southern spruce forest) Hyltemossa, Skåne .
The storm Gudrun hit Sweden in January 2005 and felled stem-wood equivalent of one years harvest in Sweden. Eddy covariance measurements of carbon and water fluxes started at Asa in Småland, Sweden, during the following summer. Asa is from 2015 part of an intense-fertilization program, looking at how the growth of the forest can be increased.