Inherent properties of Scots pine forest residues harvested in South Norway
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Biomass from forestry sector is able to provide an important contribution to meet the government’s targets for increasing bioenergy use. Traditionally it has been stem wood which is used as raw material for energy. For a deeper understanding of trees, knowledge is required not only of the stem wood, but also of the branches and tree tops. Research complex on Norway's industrially important tree species - Scots pine (Pinus sylvestris L.) forest residues (stem wood, stem bark, branches, top of trees) moisture content, basic density and effective heating value were analysed in 2 sampling plots, depending on site index quality in South Norway forest. The vertical dependence of bark proportion was observed. The pattern was similar for both site indexes, lower bark content in the bottom part of stem, slightly increasing approximately to 35 % of the tree height and so increasing towards the tree top. The vertical dependence of bark thickness of Scots pine trees showed an increasing trend towards the top. Bark was significantly thicker near the base than near the top. Bark proportion may be a relevant aspect for the utilization of Scots pine forest residues as potential biomass feedstock. Considerable variations in qualitative properties between stem wood, stem bark and branch wood of Scots pine along the stem were observed. The basic density of stem wood was in range 308.2-418.3 kg/m3, of stem bark 265.1-364.2 kg/m3 and of branch wood 400.5-579.2 kg/m3. The basic density of stem wood was higher in the lower part of stem, vertically decreasing towards to tree top. Contrary, the basic density of stem bark decreased to 40 % height and then slightly increasing again towards the top. The axial dependence of basic density in stem bark was different than the one in stem wood, more regular. Branch density decreased moderately within the axial direction along the crown. Scots pine branch wood exanimated in this study was denser in the bottom part crown towards less denser branches positioned in the top crown section. More else a clear variation pattern was apparent in basic density variations along the branch. Density declined from the branch base outward first rapidly and then levelled. The highest basic density was found for the branch base. There was not found relationship between basic density of stem wood, stem bark, branch wood and site index quality of stands. The average moisture content of stem wood and stem bark harvested in early spring season increased axially from the base toward tree top, within significantly more pronounced variations on the tree base compare to tree top. Stem bark had relatively higher moisture content compare to stem wood. The moisture content in stem wood was in range 39.9-90.2 %, in stem bark 42.8-94.3 % and in branch wood 43.5-62.8 %. The effective calorific value of stem wood was in range 5.09-5.40 kWh/kg, of stem bark 5.11-5.51 kWh/kg and of branch wood 5.16-5.49 kWh/kg. The vertical dependence trend of effective calorific value for Scots pine stem wood was similar to that for stem bark. Regular decreasing pattern towards the top was observed. We observed significantly higher calorific value at level p<0.05* in stem bark than in stem wood. Elevated effective calorific value of stem bark and branch wood make these materials a valuable industrial energy source for bioenergy in Norway.