GAP STRUCTURE, DISTURBANCE AND REGENERATION IN A PRIMEVAL PICEA-ABIES FOREST

We investigated patterns of disturbance and recovery in Fiby urskog, a primeval spruce (Picea abies) forest, situated south of the border between the Boreo-nemoral and Boreal regions in East-central Sweden. The main types of disturbances are storm damage, fungal infection and insect attacks. The response of the different tree species varied and the mode of tree-fall depended on the different combinations of disturbance agents. The DBH distributions of gap creators and gap-border trees were almost the same. There was a high age diversity (100-240 yr) among the fallen trees. We concluded that all canopy trees (DBH > 20 cm) had the same probability of being felled by storms, irrespective of their age and DBH. According to an estimate along transect lines, gaps made up 31% of the spruce forest area. Individual gap sizes ranged from 9 m2 to 360 m2, but 83% of the gaps were < 150 m2. The varied age structure of logs in individual gaps indicated that gap enlargements were common. 96 tree-falls were observed on four days with an hourly mean wind speed > 12.0 m/s; all trees fell in the direction of the wind. However, when we consult the 30-yr record (1959-1989) of the mean hourly wind speed > 12.0 m/s, it is clear that the pattern of storm-directions does not match the pattern of orientation of fallen logs. The present disturbance regime and the predominance of small gaps were more favourable for the regeneration of Picea abies than of light-demanding tree species. In one large, 2900 m2 gap, not crossed by the transects, all the major tree species had established within 7 yr, suggesting that classical succession in the sense of complete species replacement or 'relay floristics' did not occur. Our observations seem rather to fit the 'initial floristic' model. Estimates of turnover time ranged from 170 to 228 yr, depending on the method used.