Early tree succession and species diversity along forest : gap gradient ten years after restorative partial cuttings with burning and dead wood increment

Natural disturbances are important for regeneration and biodiversity of forests. Restoration imitates natural disturbances in order to return ecosystems towards its natural state. Knowledge is lacking about the efficiency of restoration, as well as early tree successions. The objects of this study were to investigate the effects of fire and coarse woody debris (CWD) increment on early tree succession and tree species diversity along forest – gap gradient 10 years after restorative partial cuttings in southern Finland. Seedling densities and heights, microhabitat distributions and tree species diversity were compared between four restoration treatments; unburned treatments with 5 and 60 m3 CWD ha-1, and burned treatments with 5 and 60 m3 CWD ha-1. Total seedling densities were tested with two-way analysis of variance. Species diversity was analyzed by species richness and Shannon index, which were tested with two-way analysis of variance. In general, the burned treatments included higher seedling densities and higher seedlings. The different amounts of dead wood revealed differences in deciduous tree species. The density of birch was higher in the treatments with 60 m3 CWD ha-1 and the heights of birch and other deciduous tree species were higher in burned treatment with 60 m3 CWD ha-1. Pine and birch were abundant in the burned treatments. The densities of spruce were very high in the unburned treatment with 5 m3 CWD ha-1, but fairly low in the other treatments. The seedling densities and heights were the lowest in the forest. Seedling densities did not vary significantly between the edge and gap segment. Microhabitat distributions of the coniferous seedlings differed between the unburned and burned treatments, but no clear differences were observed between the low (5 m3 ha-1) and high amount (60 m3 ha-1) of CWD. Microhabitat distributions of seedlings between forest, edge and gap did not differ noticeably, except with spruce in the burned treatments between the edge and gap. Tree species diversity of regeneration was slightly higher in the burned than in the unburned treatments. Different amounts of CWD did not affect the tree species diversity. The tree species diversity was significantly lower in the forest than in the treatment area, but it did not differ between the edge and gap. The results indicate that early tree succession is likely to be more abundant after burning than without. Fire as a restoration tool can be suggested for the regeneration of pine and birch. Dead wood appears to be insignificant for early tree succession. The limited size of restoration areas does not impede the tree succession, since the densities and heights of seedlings were generally the highest near the forest edge. Fire might increase tree species diversity.