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 carbon dioxide 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.
The amount of water in the soil, both soil moisture and groundwater levels, is an important factor that is probably directly linked to both forest production and methane flows. The standing forest is mostly a sink of methane to the ground. When the ecosystem is disrupted (storm or felling), it reduces the transpiration from trees and evaporation from the soil, which often results in higher water table. We have seen that logging seems to lead to the ecosystem going from methane uptake to large methane emissions. We are now studying whether this is a general phenomenon, and therefore we move around a mobile system for measuring carbon dioxide and methane flows between clearings on dry ground, medium moist soil and damp / wet ground for comparing the measured values.
Stations and measurements
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. In southern Sweden, we also have the mobile measurement system that we are moving around on new clear-cuts with different degrees of wetness .
We established an experimental site in Romperöd, 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 Hyltemossa, Skåne (a typical southern spruce forest).
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.