Effects of forest management on greenhouse gas fluxes in a boreal forest

Forest ecosystems cover 31% of the terrestrial land area and store large amounts of carbon in biomass and in soils. The 2015 Paris Agreement recognizes the importance of sinks and reservoirs of greenhouse gases (GHGs) in forests and the importance of enhancing them through sustainable forest management policies. Managing forests for climate mitigation purposes is, however, complex since management affects the climate and the environment in many different ways. The aim of the work presented in this thesis was to investigate and quantify short-term effects of forest management on GHG fluxes in a boreal forest in central Sweden.We simulated a selective cutting system through thinning of a mature mixed Norway spruce and Scots pine forest. During 2007-2016, we measured fluxes of carbon dioxide (CO2) and methane (CH4) with chambers and we used the eddy-covariance (EC) technique to measure CO2 fluxes at the stand level. We established four experimental plots at a soil scarified clear-cut. Tree stumps were removed from two of these plots. During 2010-2013, we measured fluxes of CO2, CH4 and N2O (nitrous oxide) with the flux-gradient technique and with chambers (CH4) at these plots.Forest management had clear effects on GHG fluxes. Increased soil moisture and a raised groundwater level after thinning reduced soil CH4 uptake in the thinned stand and caused the clear-cut and stump harvested plots to switch to net sources of CH4. Clear-cutting and stump harvesting resulted in large emissions of GHGs, but fluxes were dominated by CO2. The degree of wetness and vegetation development had an effect on the relative contribution of the different GHGs. Stump harvesting had significant effects on CO2 and N2O fluxes at dry plots and the results suggest reduced mitigation potential of using stumps for bioenergy production. Substantial N2O emissions at the dry stump harvested plot was an additional cause for concern. Thinning had an effect on both daytime uptake of CO2 and ecosystem respiration, which resulted in reduced net CO2 uptake at the stand level during growing seasons. On an annual basis, decreased ecosystem respiration and increased CO2 uptake by ground vegetation had larger impacts and the net effect on annual NEE was minor.Combined, the results presented in this thesis highlight the importance of accounting for all greenhouse gases when considering short-term effects of forest management on the climate. The results indicate that a silvicultural system that avoids net emissions of GHGs during a clear-cut phase and maintains a constant NEE might have a stronger mitigation potential than conventional even-aged forestry