Publikasjoner

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1995

Sammendrag

The project Quality of wood with selection cutting and shelterwood cutting is one of the projects in the research program Forest ecology and multiple use. The purpose has been to analyse wood quality in uneven-aged forests. The material originates from seven experimental plots with selection forests. Of these plots two are located in Nordland, three in Nord-Trndelag, one in Oppland, and one in stfold. Five mature trees were harvested from each plot. The sample trees should be big enough to give sawnwood of the dimension 50x200 mm from the butt log, 50x150 mm from the second log (the middle log), and 50x100 mm from the third log (the top log). The project is divided into three parts:Detailed analyses of annual growth ring width and density were done on trunk discs from different tree heights.Mechanical, technological, and physical properties were measured on small, clear specimens together with anatomical characteristics.Timber in standard structural sizes was sorted both visually and mechanically before testing for modulus of elasticity and static bending strength in the laboratory. At stump height (1 % of tree height) mean annual growth ring width increases from 0.7 mm near the pith to 2.6 mm near the bark. The rate of increase in annual growth ring width is strongest near the pith, and declines towards the bark. Basic density is showing the opposite development, and declines from ca 450 kg/m3 near the pith to ca 350 kg/m3 near the bark. In 20, 40, 60 and 80 % of tree height it was found very small variations in density from the pith towards the bark. Mean basic density for the different experimental plots varies from 350 kg/m3 to 379 kg/m3. Mean for the whole material is 365 kg/m3. The corresponding figures for annual growth ring width show variation in mean values for the plots from 1.38 mm to 2.01 mm, with a mean for the whole material of 1.64 mm. For small, clear specimens the following mean values were found: Modulus of elasticity in static bending 13.4 GPa, static bending strength 79.9 MPa, compression parallel to grain 39.7 MPa, impact bending strength 35.9 kJ/m2, and static hardness in radial direction 2281 N, in tangential direction 2014 N, and in longitudinal direction 2741 N. Standard structural timber was sorted visually and in two different types of stress grading machines. Mean modulus of elasticity is calculated to 11.4 GPa, and mean static bending strength to 42.0 MPa. Results from the present material show that selection forests dont have the same development in annual growth ring width and density as spruce planted on high site index in Eastern and Western Norway. At stump height the annual growth rings near the pith are very narrow, and the growth rings are getting wider towards the bark. Basic density shows the opposite development. Near the pith basic density is high, and it declines towards the bark.Of the experimental plots at our disposal six plots of a total of seven were localized far north or at high altitudes, and therefore the plots are less representative for the main productive spruce forest areas in Norway. Besides, the site index is considerably lower for the present material from selection forests than the materials used for comparison (G11-17 against G23-26). Still we can conclude that wood quality from these experimental plots of selection forests show more uniform annual growth ring width and density, both in stem cross section and in tree height than wood from planted forests. It is not possible to separate juvenile wood on the basis of density and annual growth ring width in wood from selection cutting. Even if the stem form is not specially good, the analyses show that it is possible to get very good wood quality, particularly regarding strength properties, in selection forests which are near the timber line of the natural growth area for spruce in Norway.

1994