Thinning intensity and long-term changes in increment and stem form of Scots pine trees

The effects of thinning intensity on the basal area and volume increment, as well as on the stem form and crown height, of Scots pine trees (Pinus sylvestris L.) were investigated in 37 long-term (on the average 25 years) thinning experiments in Finland. The intensity of the thinnings from below ranged from heavy thinning (42% 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. Thinning increased the relative basal area increment of smaller trees somewhat more than that of larger ones. Depending on the stand age at the time of establishment of the experiments, it was possible to produce 400–800 larger trees per ha on the heavily and moderately thinned plots compared to the unthinned and lightly thinned plots. The average stem diameter of the standing and removed trees remained lower on the heavily thinned plots because of the early removal of trees in thinning. Thus, heavy thinning results in a higher number of larger-sized stems at the expense of somewhat poorer stem form and lower average stem diameter of all the trees (standing and removed combined).