We used a nation-wide forest dataset from the Swedish National Forest Inventory (NFI) and the Survey of Forest Soils and Vegetation, covering an area of 400,000 km2 of production forests. The inventory uses a regular sampling grid with a randomly selected starting point covering the whole country, with each tract being surveyed once every 5 years. The tracts, which are rectangular in shape and are of different dimensions in different parts of the country, consists of 8 (in the north) to 4 (in the south) circular sample plots. The circular plots have different radii (5-20 meters) to ensure that the variables recorded characterize the ongoing forest dynamics and management. We used only plots on ‘productive forest’ (average production of standing volume, stem volume over bark >1 m3 ha–1 year–1), which had not been harvested, cleared, or thinned during the five years before the survey. To be included in our analyses, the plots had to be located on only forested land, i.e. not including a
ny river, road, grassland, etc. We excluded plots where the biomass production of any of the focal tree species was greater than the 99% percentile (which is standard procedure in analyses of data from the NFI). This procedure excluded plots with unrealistically high values for tree biomass production. For the current study, we utilized data on the following ecosystem service and on forest age, to represent the succession.
Wood production was estimated as the yearly change in tree biomass (kg m-2 year-1), calculated over a period of 5 years for all tree individuals higher than 1.3 meters. For plots where biomass was measured in 1999-2002, the baseline for wood production was thus measurements of biomass in the years 1994-1997 (referred to as 2000), and correspondingly, for year 2010 we used data from 2008-2012 and 2003-2007 (referred to as 2010). We excluded plots that had been harvested, cleared, or thinned within the two periods of measuring biomass for calculating production, e.g. 2008-2012 to 2003-2007. The sample size for wood production was 4,582 plots.
Bilberry production was measured as the percentage of the forest plots covered by bilberry (Vaccinium myrtillus). The cover of bilberry is strongly correlated with the annual bilberry production. We used data from 2,257 plots inventoried 1999-2002 (referred to as 2000), and 2009-2012 (referred to as 2010).
Soil carbon storage was measured as the amount of carbon (g m–2) in the topsoil, which consisted of either purely organic horizons, i.e. mor layers (63%) or peat layers (21%), or less frequently of minerogenic A-horizons (16%). We could utilize data from 2,051 plots inventoried 1999-2002 (referred to as 2000), and 2009-2012 (referred to as 2010).
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