What was done
The authors used official forestry statistics on standing tree volumes and harvests to estimate the standing stock of tree components for each year between 1926 and 2000 throughout all of Sweden for the country's two most dominant tree species - Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) - which together constitute 80.6% of the country's standing tree stock. For each of these tree components they estimated annual litter production, the decomposition of different litter fractions in the soil, and ultimate turnover rates, with soil carbon stocks for all sites evaluated to a depth of one meter or to bedrock, if bedrock was encountered at less than a meter depth.

What was learned
Agren et al. report "there was a steady increase in carbon stocks in Swedish forests in the period 1926-2000," such that forest ecosystems in 2000 contained 35% more carbon than they did in 1926, with tree biomass being responsible for nearly three-quarters of the total increase in carbon stocks. For comparative purposes, they note that the total amount of carbon sequestered amounted to just under half that emitted to the air by the burning of fossil fuels in Sweden.

With respect to the soil component of the carbon sequestered, the Swedish scientists arrived at a value of 1.7 Tg/year, or just under a quarter of the total sequestered carbon (7.1 Tg/year). In addition, they say "the current soil carbon stock is not in equilibrium with the current rate of litter production," as "the increase in soil carbon stocks since 1926 is only 33% of what would be required to be in equilibrium with current tree biomass." This state of affairs, however, is only to be expected, as Agren and Bosatta (1998) have calculated that for forest litter typical of coniferous needles and Swedish climatic conditions, "50% of the steady state value is reached after ca. 200 years but 15,000 years are required to reach 80%."

What it means
Carbon stocks in Swedish forests are growing significantly faster than one might expect, especially in light of the fact that from 1951 to 2000, the forested area of the country actually declined by 1%. Hence, we likely have global warming, atmospheric CO2 enrichment, nitrogen deposition and management factors to thank for the increase, which as noted above has compensated for just under half of the country's carbon emissions from the burning of fossil fuels. What is more, we can expect forest extraction of carbon from the air to continue well into the future in Sweden, as Agren et al. note that "the time since the last glaciation has probably not been long enough to reach a steady state," and since the factors that determine steady state appear to be changing in ways that continue to increase the magnitude of that elusive equilibrium condition.

Reference
Agren, G.I. and Bosatta, E. 1998. Theoretical Ecosystem Ecology - Understanding Element Cycles. Cambridge University Press, Cambridge, UK.