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Aspects of precommercial thinning in heterogeneous forests in southern Sweden

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The overall objective of the work underlying this thesis was to suggest and evaluate possible strategies for the tending of young heterogeneous stands of Norway spruce, Scots pine and birch in southern Sweden. Heterogeneity was defined as variation in species composition, height distribution and spatial arrangement of the trees. The influence of stand density after precommercial thinning and timing of thinning on the diameter of the thickest branch was studied for naturally regenerated Scots pine. The branch diameter was found to decrease with increasing number of remaining stems after precommercial thinning. However, leaving very dense stands (> 3000 stems ha-1) resulted only in a minor reduction of the branch diameter. Late precommercial thinning, compared to early, reduced the branch diameter. The influence of the precommercial thinning regime on the crown ratio (living crown length/tree height) was also analysed. To be able to simulate the influence of different management options on the development of the young forest, single-tree growth models was developed for Scots pine, Norway spruce and birch. Height growth and diameter was estimated as a function of tree height, stand and site variables. Growth reduction due to competition was estimated using individual, distance independent indices as well as expressions of the overall stand density. In the third study the influence of stand structure after precommercial thinning on the development of mixtures between Norway spruce and silver birch was simulated. The aim was to identify mixtures that allowed both species to develop well until the first commercial thinning. By leaving birches with an average height slightly greater than spruce at precommercial thinning, a large proportion of competitive birches were available at first commercial thinning, at the same time as the relative diameter distribution of spruce in the mixture was equal to that of a pure spruce stand of the same density. The height difference between the species as well as the species proportion had a decisive impact on volume production. In the fourth study different precommercial thinning strategies were identified and applied to a heterogeneous stand including Scots pine, Norway spruce and birch. Stand development and economical returns over a rotation was estimated using a set of empirical models. The aim of the long-term strategies was: (i) a conifer dominated stand with focus on high production, (ii) a conifer dominated stand with focus on high timber quality, (iii) to preserve the heterogeneous stand structure, (iv) a mosaic pattern by tree species, (v) to reduce the precommercial thinning cost, without jeopardizing the future stand development. The difference in total volume production was found to be relatively small between the strategies. The lowest production was found for the strategies promoting species mixture at tree level (iii) and group level (iv). The net present value was highest for the strategy aiming at high production (ii) and lowest for the strategy aiming at preserved heterogeneity (iii). The minimal precommercial thinning (v) was a less profitable alternative, mainly because of an expensive first commercial thinning. Differences in timber quality were not considered in the simulations. The case study illustrates the possibilities for influencing the structure of a heterogeneous stand through precommercial thinning, as well as the limitations imposed by the initial stand structure.

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