Thinning intensity and growth of Norway spruce stands in Finland

The effects of thinning intensity on the growth and yield of Norway spruce (Picea abies (L.) Karst.) were investigated in long-term thinning experiments on mineral soil sites in southern Finland. The measurement period was on average 27 years, and the intensity of the thinnings from below ranged from heavy thinning (45 per cent of basal area removed) to no thinning. Total stem volume increment and merchantable volume produced per hectare were the highest on the unthinned plots, but light and moderate thinning (<30 per cent removed) produced almost as much. Heavy thinning (>30 per cent removed) reduced the volume increment by about 10 per cent. However, a part of the total production of unthinned plots was lost through natural mortality. On the thinned plots, natural mortality was considerably lower compared with the unthinned plots. The average diameter increment of all the trees, as well as the diameter of the largest trees, clearly increased with increasing thinning intensity. In contrast, dominant height increment was not affected by thinning. The stand age at the time of establishment of the experiments had no major effect on the growth reactions after thinning. Thus, heavy thinning results in earlier thinning yields and a higher proportion of larger-sized stems at the expense of a somewhat lower total yield.