Thinning intensity and long-term changes in increment and stem form of Norway spruce trees

The effects of thinning intensity on the basal area and volume increment, as well as on the stem form and crown height, of individual Norway spruce (Picea abies (L.) Karst.) trees were investigated in 21 long-term thinning experiments in southern and central Finland. The measurement period was on average 28 years and the thinning intensity ranged from heavy thinning (45% of basal area removed) to no thinning. Basal area increment of the trees increased with increasing thinning intensity, but the absolute thinning effect decreased with decreasing tree size within the same plot. In contrast, thinning increased the relative basal area increment of smaller somewhat more than that of larger diameter trees. Thinnings increased the volume of the largest trees. During stand rotation it was possible to produce 400–600 larger trees ha^-1 on the heavily thinned plots compared to the unthinned plots. On the other hand, the average stem diameter of the standing and removed trees remained lower on the heavily thinned plots because of the early removal of thinned trees. Height/diameter ratio decreased with increasing thinning intensity, but the differences in stem taper were also related to tree age. In addition, lower stand density slowed down the rate of crown rise. The stand age at the time of establishment of the experiments had no major effect on the growth reactions. Thus, heavy thinning results in a higher number of larger-sized stems at the expense of poorer stem form and lower average stem diameter of all the trees (standing and removed combined).