Fibre properties of Norway spruce of different growth rates grown under birch shelterwoods of two densities

The effects of birch shelterwood density (0, 300, and 600 trees/ha) and growth rate on fibre and pulp properties of Norway spruce (Picea abies (L.) Karst.) understorey (growing at 1500 trees/ha) were evaluated for a trial in the boreal coniferous forest, 58 years after establishment of the stand and 21 years after establishment of the trial. Microdensitometry was used to record variations in wood density, whereas fibre properties and kraft pulp strength properties were measured on laboratory-made batches of unbleached kraft pulp. The main conclusion of the investigation is that a birch shelterwood has only a minor influence on the wood and fibre properties of sheltered Norway spruce and that the resulting consequences for kraft pulping are moderate. Length-weighted mean fibre length was significantly affected only by growth rate. It was 1.75 mm, or 6-13% lower, for trees showing a low growth rate than for all other trees. Tensile index was already high before refining, 85-95 Nm g-1, and the increase due to beating was similar for all shelterwood densities and growth rate classes. At 2000 beating revolutions, there was a strong negative correlation between tear index and the proportion of fibres shorter than 0.20 mm. The volume of wood required to produce 1 t of kraft pulp was almost identical, 5.4 ± 0.1 m3 t-1, for sheltered and unsheltered spruce. Norway spruce growing without shelter produced more pulp per hectare in all fibre length classes, but the difference was greatest, 56-59% compared with sheltered spruce, for the longest fibres (i.e., longer than 3 mm).