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 gro wth rate on fibre and pulp properties of Norway spruce (Picea abies (L.) Ka rst.) 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 r ecord variations in wood density, whereas fibre properties and kraft pulp s trength properties were measured on laboratory-made batches of unbleached k raft pulp. The main conclusion of the investigation is that a birch shelter wood has only a minor influence on the wood and fibre properties of shelter ed 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 gr owth rate than for all other trees. Tensile index was already high before r efining, 85-95 Nm g(-1), and the increase due to beating was similar for al l shelterwood densities and growth rate classes. At 2000 beating revolution s, there was a strong negative correlation between tear index and the propo rtion of fibres shorter than 0.20 mm. The volume of wood required to produc e 1 t of kraft pulp was almost identical, 5.4 +/- 0.1 m(3) t(-1), for shelt ered 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).