Green-tree retention and controlled burning in restoration and conservation of beetle diversity in boreal forests

Abstract

The main aim of this thesis was to demonstrate the effects of green-tree retention and controlled burning on beetles (Coleoptera) in order to provide information applicable to the restoration and conservation of beetle species diversity in boreal forests. A methodological aspect was also included, in the form of an examination of the sampling of forest beetle communities. A large-scale field experiment involving 24 forest sites was established in eastern Finland, where harvesting intensity was manipulated together with burning treatments. The beetle data collected during one pre-treatment year and two post-treatment years covered altogether 201 501 individuals representing 1235 species. The main findings were: 1) Harvesting with or without burning increased the species richness, but it often began to decrease again in the second post-treatment year. 2) Many species of beetle colonized the sites effectively after the treatments, particularly the burned sites. 3) The richness of red-listed and rare saproxylic (deadwood-dependent) species was higher at burned than unburned sites, an effect which was not caused solely by pyrophilous species as many other species showed a similar pattern. 4) Higher levels of green-tree retention seemed to increase the richness of saproxylics, including red-listed and rare species, at burned sites in the second post-treatment year. 5) The abundance of red-listed and rare saproxylic species was higher at burned sites, and the pyrophilous species in particular showed population increases after fire. 6) Higher tree retention levels maintained the assemblages closer to the pre-treatment structure. The assemblages of saproxylic species were distinctly affected by the treatments and also differed between the two post-treatment years. 7) Harvesting with or without burning had a marked effect on herbivores, but they recovered by the second post-treatment year in burned areas. 8) Species dependent on ephemeral resources were the least affected by the treatments. 9) Burning and harvesting was detrimental for litter-dwelling species, but they seemed to recover quickly. 10) Ecological classification of the material collected in traps is important for revealing ecological patterns. 11) Large collections are needed to obtain representative samples of beetle communities in boreal forests. The results show that the negative effects of timber harvesting on beetle diversity in boreal forests can be alleviated by increasing the green-tree retention volumes and by controlled burning. Many red-listed and rare saproxylic species seem to benefit particularly from the burning of harvested sites with retained trees. Unharvested burned sites seem to support rather different species assemblages from harvested ones, however, emphasizing the importance of fire as a restoration tool in conservation areas. Controlled burning and green-tree retention do not solve all the problems related to commercial forest management, but they will clearly benefit a significant part of the ecosystem, including perhaps the most species rich and one of the most endangered species groups, the saproxylic beetles.