Density–growth relationships in thinned and unthinned Norway spruce and Scots pine stands in Norway

Drawing on national forest inventory data representing a wide range of stand densities, site qualities, ages, and management practices in Norway, this study (1) analyzed the stand density–growth relationships in Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) dominated stands and (2) evaluated the influence of site index, stand age, and thinning on the density–growth relationships. We developed nonlinear growth models that described the periodic annual basal area or volume increment as a function of initial basal area, site index, and stand age. The results showed that for both species, basal area growth increases with stand density although degressive at higher densities. Volume growth increases nearly linearly with stand density. Nevertheless, as demonstrated by the similar shapes of the growth curves, the pattern of the density–growth relationships did not vary considerably by site index classes, by stand ages, and between thinned and unthinned stands. Thinned stands grew about 10% more than unthinned stands of the same density, site index, and age. Our models showed a much stronger decrease of volume growth with decreasing density than previously published results from thinning experiments for the two species in Fennoscandia. The study also demonstrated the suitability of national forest inventory data for analyzing the density–growth relationship.