The soils of western European forests may be accumulating carbon, because tree biomass has been expanding in these forests already for decades, and the more numerous and larger trees can produce more litter. We calculated the carbon budget of soils and trees in the forests of 14 EU countries plus Norway and Switzerland from 1950 to 2040 by integrating forest resource information (inventory data from 1950 to 1990 and a forest resource forecast from 2000 to 2040), biomass allocation and turnover information, and a dynamic soil carbon model.
The carbon stock of the soils increased throughout the studied period. In 1990, the soil carbon sink was 26 Tg per year. This is 32 or 48% compared with our two estimates of the tree carbon sink for that year. Until 2040, the soil carbon sink was estimated to increase to 43 Tg per year. This would already be 61 or 69% compared with the tree carbon sink that year. In 1990, the soils contributed most to the total forest carbon sink in central Europe, where the soil carbon sink was almost as large as the tree carbon sink. The soils were least important in southern Europe, where the soil carbon sink was less than 25% compared with the tree carbon sink. In the future, the contribution of the soils to the total forest carbon sink was estimated to increase everywhere except in southern Europe.
The soil carbon stocks increased mainly because litter fall from living trees increased while the other sources of soil carbon, i.e. the residues of harvests and natural disturbances, varied less. This litter fall was also the largest source of soil carbon accounting for 70-80% of the total. The soil carbon stocks in these forests could thus be most effectively controlled by forest management actions, such as the choices of harvest regimes or tree species, which especially affect the litter production of living trees. According to an uncertainty analysis, we may have overestimated the soil carbon sink by 35% or underestimated it by 50% throughout the studied period. The largest uncertainties were related to calculating the litter production of living trees and decomposition in soil.