Vindfällning, tillväxt och plantuppslag i en 13-årig granskärm i Medelpad

Abstract: The interest for alternative forest regeneration methods increased significantly during the 1990´s after 50 years dominance of the clear-cutting system (Holgén & Hånell 2000). One reason for this was that clear-cutting was being more and more questioned by different envi-ronmental organizations and because the forest sector for economical reasons wanted to find cheaper ways to regenerate forests. In the beginning of the 1990´s two shelterwood experi-ments were established in a joint effort by Svenska Cellulosa Aktiebolaget (SCA) and the Dept of Silviculture, Swedish University of Agricultural Sciences (SLU). The experiments include various forms of cutting and shelterwood densities and represent productive spruce forests in central and north Sweden (Roggsjön in the province of Medelpad and Skikkis-jöberg in the province of Västerbotten, respectively ). The experiment next to Roggsjön has been reported earlier in three MSc theses, and is also the focus of this thesis. The experiment includes two adjacent spruce stands with elements of birch and is located east of the lake Roggsjön (62, 4˚ degrees N, 216 above sea level) about 60 km west of Sundsvall. The stands are managed and administered by SCA. At the time of the shelterwood cutting, in early 1994, both stands were about 90 years old and contained approximately 300m3/ha (Andersson, 1995). The experiment was designed in randomized blocks with two blocks and four shelterwoods: 400 Dominant trees (400 H), 250 Dominant trees (250 H), 400 Co- dominant trees (400 MH), 250 Co- dominant trees (250 MH). Thirteen years later windthrow damage were extensive among the co- dominant trees, fore-most in the sparser shelterwood (250 MH), where 70 % of the trees had blown down. In the denser shelterwood (400 MH) approx. 40 % had blown down whereas the windthrow dam-ages where fairly small in the shelterwoods with dominant trees, (approx. 10 %). A strong correlation was stated between strength of cutting and redundancy. In all shelterwoods the spruce regeneration met the requirements of the Swedish Forestry Act (SVL) concerning natural regeneration. The mean height was over one meter in all the shelterwoods, which partly was due to the high amount of advance spruce regeneration. The occurrence of de-ciduous trees was high in all the shelterwoods except for the densest one (400 H). The mean height of the deciduous trees was significantly lower in the shelterwoods with dominant trees. The species composition in the field and bush layers varied greatly between the shelterwoods with dominant and co-dominant trees. Under the cover of the dominant trees there was hardly any bush layer whilst under the co- dominant trees there were in places plenty of raspberry and rosebay. High occurrence of raspberry and rosebay were found to have a nega-tive effect on the height increment of the spruce regeneration. In the field layer the dwarf-shrubs where found almost exclusively under the cover of the dominant trees whereas bracken and low-herbs were found under the co- dominant ones. In the bottom layer the main part of the moss indicated mesic-dry conditions, and sphagnum was found exclusively under the dominating shelter trees.