Forests acting as a carbon source : analysis of two possible causes for Norunda forest site

Only 40% of the total CO2 emitted by human activities and natural sources remain in the atmosphere, since the rest is taken up by plants and oceans. In particular temperate and boreal forests are usually significant sinks of carbon, even if they are very susceptible ecosystems and they might turn into sources in consequence of the predicted temperature increase. That is one of the reasons why a network of researches about Nordic ecosystem has been developed in the last years. Actually some forests already act as sources of carbon, and this is the case of the ecosystem considered in this study: Norunda forest has been releasing carbon to the atmosphere since scientists started measurements in 1994, through a 100 meters tower. The aim of the project is to give an explanation of this particular behaviour, by analysing two hypotheses. The first one regards the morphology of the area of the tower, which is supposed to be an inflow area collecting Dissolved Organic Carbon (DOC) from the surroundings. The second one concerns the responsibility of forest soil in emitting CO2 after human exploitation. Topography of the area was analysed, then some of the streams and lakes in the catchment area have been sampled two times to get DOC content of the water, and discharge has been calculated. Role of the forest soil was analysed by using a simulation model, which gave responses to the drainage carried out at the beginning of the last century and to the repeated harvests occurring every one hundred years. The source area of measurements was found not to be an inflow area. However analyses on DOC revealed that streams and lakes in Norunda drainage basin are supersaturated in carbon, which means that water directly releases CO2 into the atmosphere. Moreover, a certain quantity of carbon has been found to flow out the basin during the sampling time, and if it will be confirmed by long term analyses this can be seen as another cause of carbon release. The simulation model showed that draining and harvesting a forest site can strongly influence forest soils delicate balance, reached after thousands of years. Drainage, in fact, lowers the water table and places more oxygen at microorganism’s disposal. Respiration and decomposition rate increase and then CO2 is released to the atmosphere, while soil carbon diminishes. Repeated forest harvests lead to a significant long-term decline of C soil storage as a consequence of removal of biomass from the site. This study demonstrates that inside a forest CO2 fluxes are determined not just by plant respiration-photosynthesis cycle, but even soil and water have an important role. 10% of Norunda heterotrophic behaviour is explained by forest draining and harvesting, and another part has to be quantified from direct CO2 emissions by streams and lakes. Other hypotheses will be investigated in future studies