Trädföryngring i låga gropar uppkomna vid brand i boreal naturskog

Abstract: This study was conducted in the "Kåtaberget" forest reserve in Västerbotten in the summer of 1999. In the summer of 1995, a prescribed burning was performed in the reserve. In some places where coarse dry woody debris (logs and stumps) was present, the fire consumed the wood, as well as parts of the humus layer, creating deep-burned patches. The low intensity ground fire also created dead woody debris by burning off living trees with open fire scars, as well as standing dead trees. In this study, the post-fire tree seedling establishment was examined, with special attention given to the former presence of woody debris. The study also includes a survey of the areal extent of deep-burned patches, as well as an examination of tree seedling establishment in these patches, compared to other burnt ground. Furthermore, the amount of woody debris caused by the fire was examined. Four years after the fire 14,5 logs/ha had been added, which corresponds to 8,3 cubic metres/ha, with most logs in the diameter classes 20-29 cm and 30-39 cm. This pattern differs from the mortlity pattern, i.e. decreasing mortality with increasing diameter. The difference could be due to the fact that larger trees have fire scars from previous fires to a higher extent than smaller trees. Such fire scars may burn out and cause the trees to fall. The tree establishment was considerably higher in deep-burned patches, which had 27000 seedlings/ha, compared to other burnt ground, which had 2800 seedlings/ha. This can be explained by the more favourable conditions for seedling establishment in the thinner humus layer, such as lower competition, more moisture and higher nutrient concentration. However, the average seedling height was the same for both microhabitats. The deep-burned patches constituted only 0,3% of the reserve area. This means that their importance for tree regeneration might not be that significant as the high seedling establishment indicates. There was an especially large difference in seedling establishment for deciduous trees between the deep-burned patches, which had 7,7 seedlings/ha, and the other burnt ground, which had 0,2 seedlings/ha. Burnt patches with no, or a thin, humus layer are believed to have been important sites for the regeneration of deciduous trees in the natural boreal forest landscape.