Tillväxtreaktion hos skärmträd i högskärm av gran i Medelpad

Sammanfattning: Different methods for shelterwood cutting have been used in differ-ent parts of Europe since the middle ages. The use of shelter wood cutting in Norway spruce (Picea abies (L.) Karst.) forests in Sweden met a revival in the early 1990s as an alternative for the dominating clear-cutting system when silvicultural costs increased dramatically for the latter. Shelterwood cutting systems involve some possible problems but also a lot of positive effects. Among the problems are increased risk of wind throw and damage of the remaining shelter trees caused by the cutting operation. Positive effects are for example a valuable growth in the remaining trees and increased biological di-versity. This study is based on a total of 79 trees from 4 different treatments in a spruce shelterwood, cut in 1994, in the middle of Sweden (62,4º N). The treatments were the following; 2 shelterwood cuttings leav-ing a stand of 250 stems/ha with dominant trees respective co-dominant trees, 2 shelter wood cuttings leaving a stand of 400 stems/ha with dominant trees respective co dominant trees. From each tree 2 increment cores were taken at breast height and the an-nual year ring growth from 1984 to 2001 was calculated. The study compared the difference in growth response between dominant and co dominant trees. The response in growth caused by the release came earlier in the dominant trees than in the co-dominant. The level of reaction, measured as increase in basal area at breast height, was higher in the co dominant trees than in the dominant, after eight years 44% and 39% (co-dominant, 400 respective 250 stems/ha) respective 20% and 27% (dominant, 400 respective 250 stems/ha). There was also a difference in response between trees of different diameters. A tree that had a small diameter in 1994 when the shelter-wood cut was performed had a higher rate of reaction than thicker trees. This was evident in all treatments. The remaining shelter trees reacted to the liberating shelter cut in a good way despite their high age. A tree that was co-dominant in 1994 was not co-dominant because of poor growing capacity but because it has been pushed from the side by other trees. To maximise the economic outcome of the shelter period it is best to leave the co-dominant trees in the shelterwood. They react with a higher relative growth and yield a better value increment. Since co-dominant trees tend to be less resistant to wind throw, at least when no preparative cut has been done, this demands that the trees are ca-pable to withstand the higher wind exposure in the shelterwood.