Thinning of Norway spruce

The objective of this thesis was to investigate volume and quality outcome from different thinning strategies in monocultures with Norway spruce. Two different experiments were set up at the Tonnersjoheden experimental forest in south west Sweden. In the first experiment, the combined effect of spacing and thinning type on timber quality and the spacing effect on volume production was investigated (paper I). In the second experiment, the initial growth response after thinning was investigated during three growing seasons in a 33 year old stand after removal of 0, 30- and 60% of the basal area (paper II and III). In a survey study, covering southern Sweden, the amount of thinning injuries to stems and coarse roots in spruce monocultures thinned with harvesters and forwarders was investigated (Paper IV). The total volume production was rather similar in the three spacings, 246, 226 and 232 m3 respectively (paper I). The quality of individual trees was to a large extent related to diameter at breast height and not to spacing per se. In the second experiment, heavy thinning increased soil moisture, light transmittance and soil temperature, and hence the nitrogen mineralization. The nitrogen content in the needles and the needle efficiency increased after heavy thinning but there were only small effects on those parameters for normally thinned plots. The current annual volume production after thinning showed an initial drop during the first two growing seasons but was slightly higher during the third growing season compared to the unthinned control. Heavy thinning increased resource allocation to the stem base. The basal area increment for the largest trees (100-400 stems per hectare) increased with increasing thinning intensity (Paper II & III). The risk for damage from heavy winds and wet snow showed a linear increase with thinning intensity. The frequency of injured trees was high (10-15%). The main finding is that there is a large “biological window” for silvicultural regimes in terms of their effect on total volume production but the thinning regime has a major impact of the risk for abiotic and biotic damages.