One of the largest environmental threats believed to be facing us today is global warming due to the accumulation of green house gases (GHG). The concentrations of GHG in the atmosphere are a result of the net strength of different sinks and sources. Forests, in this context, are of particular interest because of their dual role as both sinks and sources. Most forests are net sinks for CO2 but others, such as drained forests, may be significant sources of both CO2 and N20. Consequently, it is essential to understand the fluxes of GHG between drained forests and the atmosphere in order to obtain accurate estimates of national GHG budgets. The findings reported in this thesis and the accompanying papers are based on dark chamber flux measurements of soil GHG fluxes and modelled annual net primary productions in five drained forest sites and two undrained sites situated on organic soil. Temporal variations in forest floor CO2, release could be explained, to a large extent, by differencies in temperature and groundwater level. The within-site spatial variation in soil GHG fluxes could only be explained to a very small extent by distance to tree stems. Much of the among-site variations in soil CO2 and CH4 release could be caused by differences in the mean annual groundwater table, while N20 emissions were strongly correlated to the carbon-to-nitrogen ratio of soil organic matter. Most poorly drained forested areas are probably net sinks for GHG as the CO2 uptake by trees more than compensates for the soil GHG emissions. However, the total drained forested area in Sweden was estimated to be a net source of GHG. The CO2 release from decomposition of soil organic matter stored before drainage was estimated to be substantial. Corresponding to 15% of the CO2 release from the consumption of fossil fuels. / <p>On the day of the public defence of the doctoral thesis the status of the articles I and II was: Conditionally accepted; article III was: Submitted and articles IV and V was: Manuscript. The title of article III was on the day of the public defence "Can the distribution of trees explain the spatial variation in N2O emissions from boreal forest soils?".</p>