Emulation of natural disturbances and the maintenance of biodiversity in managed boreal forests: the effects of prescribed fire and retention forestry on insect assemblages

Intensive forest management and effective prevention of natural disturbances have caused major changes in forest ecosystems and many forest habitats and species are now threatened. Retention forestry and prescribed burning are methods that aim to bring the legacies of forest management closer to those of natural disturbances and to maintain biodiversity in managed forests. In this thesis, the effects of these two methods on biodiversity are explored by studying retention tree dynamics, and changes in the assemblages of beetles (Coleoptera) and flat bugs (Heteroptera: Aradidae, Aradus). The study is based on a 10-year long, replicated field experiment on boreal pine dominated forests in eastern Finland. The data includes 2758 individually marked retention trees on 24 sites, which were followed individually over a 10year period. Species inventories from the experimental sites resulted in a dataset that included 468 species and 60,879 identified individuals. Prescribed burning without logging, the method considered to be close to natural fire in a forest ecosystem, caused a long-term increase in species richness and differences in functional and phylogenetic compositions of saproxylic beetle assemblages compared to harvested stands. In the harvested stands, species richness increased strongly in the shortterm but returned over the 10-year period to the pre-treatment level in the retention stands and to lower levels in the clear-cut stands. In the short-term, the functional-phylogenetic diversity of saproxylic beetles changed from a random to a clustered pattern after burning and retention harvests, whereas clear-cutting resulted in random communities. This implies that the species composition in retention harvests became phylogenetically more related and consisted of species with similar resource preferences, i.e. favoring open habitats and early decay stages. Ground beetles (Carabidae) are effective colonizers of open habitats, and their species richness increased after fire and logging. Applied retention tree levels (10 m3 ha-1 and 50 m3 ha-1, representing 3.5 % and 17 % of the pre-harvest volume, respectively) were not able to maintain ground beetle assemblages found in uncut forests. Burning increased species richness of pyrophilous and rare and Red-Listed (RRL) beetles and flat bugs but this effect was transient. Neither burning nor tree retention resulted in increased damage from pineshoot beetles (Tomicus spp.) when compared with traditional clear-cutting. The main findings of this thesis emphasize the difference between the assemblages of burned forests and the assemblages of clear-cut stands but shows that prescribed burning and retention forestry can be beneficial to conserve and maintain insect diversity. Furthermore, this thesis suggests that relatively high retention levels are needed due to the high post-harvest mortality of the trees. This is particularly important for the long-term availability of habitats that retention trees provide and low retention tree levels are unlikely to maintain diverse fauna on harvested sites.