Quality thinning and value development of boreal trees on spruce-dominated stands

For the first time, quality distribution of trees is introduced in a tree growth model. Consequently, the effects of quality thinning on stand development can be investigated. Quality thinning improves the financial return in all cases studied, but the effect is small. Rotation ages, timber stocks and maturity diameters are not much affected by quality thinning. Bare land valuation neither changes the contribution of the quality thinning. The reason for the small effect apparently lies in the value development of individual trees. The relative value development of small pulpwood trunks is large, since the harvesting expense per volume unit is reduced along with size increment. Such trees are not feasible objects for quality thinning, unless quality correlates with growth rate. Another enhanced stage of value development is when pulpwood trunks turn to sawlog trunks. For large pulpwood trunks, quality thinning is feasible. Existing sawlog content in trees dilutes the effect of quality thinning on the financial return. The results change if the growth rate is positively correlated with quality, quality thinning becoming feasible in all commercial diameter classes.