Finnish Forest Research Institute  Metla

Media release 18.11.2005

Dissertation: Forests in Finland have been a carbon sink since the 1970s

Forest vegetation and mineral soils in Finland have been a carbon sink since the 1970s. The net primary production (NPP) by forests in Finland has increased during the past 82 years. However, the carbon pools represented by vegetation and soil fluctuate a lot, often increasing and then decreasing from one year to the next, which could be explained by cuttings and temperature variation. These results were presented in the doctoral dissertation defended by Aleksi Lehtonen at the University of Helsinki.

The UN Framework Convention on Climate Change requires participating countries to report not only their emissions of greenhouse gases (GHG) but also changes in carbon stocks, because the forest carbon balance significantly affects the GHG concentrations and the greenhouse effect. Due to the Kyoto Protocol and the targets set for GHG reduction, an extensive and reliable system for estimating the forest carbon balance is essential, since forest carbon sinks will be credited in exchange for GHG emissions. So far, reporting of forest carbon sinks in Finland has concerned only trees and no testing has been performed to show how representative the models are.

The role of Finnish forests in the national GHG balance necessitates extensive calculation - it is essential to take into consideration all carbon stocks, including trees, litter, soil and dead wood. It is important to determine changes in these carbon stocks, particularly if silvicultural methods and recommendations are to be changed to enhance carbon sequestration. As part of research for his doctoral dissertation, "Carbon stocks and flows in forest ecosystems based on forest inventory data", Aleksi Lehtonen has developed methods for determining forest carbon stocks and flows on a national scale based on forest inventory data.

Lehtonen's research results indicate that the carbon stocks both in trees and soil have increased during the study period, 1922 - 2004. In the 1920s the tree carbon stock was 510 teragrams (Tg), whereas in 2004 it was 780 Tg. In other words, the stock has increased by 270 Tg during the study period. Concurrently the soil carbon stock increased by 110 Tg, being 960 Tg in 2004. The results also showed great variation in vegetation and soil carbon stocks, with the direction of the carbon flux frequently changing from one year to the next. The variation was mainly due to annual changes in temperature and cuttings. For instance, since cuttings performed during the 1950s and '60s exceeded the increment, it temporarily decreased the tree carbon stocks and increased cutting residues but decreased the quantity of litter produced by living trees in the 1960s.

The annual NPP of the forests in Finland increased during the 82-year period by 0.1 kg m-2, the annual amount being 0.4 kg m-2. The vegetation and mineral soils have together represented a carbon sink since the 1970s, with the exception of a couple of years when more carbon was released than bound.

The time series for the carbon stocks were calculated using a combination of biomass expansion factors (BEF), litter production and a soil model.

To allow biomass estimation for trees, BEF factors were developed for Finland. Furthermore, uncertainties of the BEF factors were also determined in the study. Also the carbon flow from branches to soil was estimated by using the new models. The estimation methods of both biomass and branch litter were tested using independent measuring data.

The developed BEF factors and branch litter factors were applied to the National Forest Inventories performed between 1922 - 2004 in order to determine forest carbon stocks and their annual changes. The main sources of litter (litter in the biomass, cutting residue and mortality) were taken into consideration in the calculations and their effect on soil carbon stock was modeled in a dynamic soil decomposition model.

Julkaisu: Lehtonen, Aleksi 2005. Carbon stocks and flows in forest ecosystems based on forest inventory data. University of Helsinki, Department of Forest Ecology

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