Thinning intensity and growth of Scots pine stands in Finland

The effects of thinning intensity on the growth and yield of Scots pine (Pinus sylvestris L.) were investigated in longterm thinning experiments on mineral soil sites in southern and central Finland. The measurement period was on an average 25 years, and the thinning intensity ranged from heavy thinning (42% of basal area removed) to no thinning. Total stem volume increment and merchantable volume produced per ha were the highest on the unthinned plots, but light thinning (<20% removed) produced almost the same increment. When the relative basal area of a plot decreased to below 75–80% of that on the unthinned plots, the volume increment decreased considerably. Moderate and heavy thinning decreased the volume increment by about 15% and 25%, respectively. On the thinned plots, natural mortality was considerably lower than that on the unthinned plots. The average diameter increment, as well as the diameter increment of the largest trees, clearly increased with increasing thinning intensity. The dominant height increment decreased slightly with increasing thinning intensity. The absolute increment reactions after the treatment were somewhat greater on sites of high fertility, as well as in the experiments established at the first commercial thinning stage, than on relatively infertile sites or in the experiment established later. Thus, heavy thinning results in earlier thinning yields and higher proportion of larger-sized stems at the expense of a somewhat lower total yield.