Prescribed fires and retention trees help to conserve beetle diversity in managed boreal forests despite their transient negative effects on some beetle groups

Controlled burning and green-tree retention have been suggested to alleviate the negative effects of forestry on species diversity in boreal forests, but the ecological impacts of these measures are poorly known. We studied experimentally the response of four ecological groups of beetles - saproxylics, herbivores, species on ephemeral resources, and litter-dwelling species - to different harvesting intensities and controlled burning in Scots pine-dominated forests. The study included four levels of green-tree retention (0, 10, 50 m(3) ha(-1), and no harvesting) with burning on 12 of the 24 study sites, covering similar to 4 ha each. A beetle data of 153 334 individuals representing 1142 species were collected during one pre-treatment (2000) and two post-treatment years (2001-2002), using window traps. Species richness increased in all four groups after harvesting, with and without burning, and there were major community-level changes. The species richness of saproxylics and herbivores continued to increase in the second post-treatment year on burned sites, whereas it decreased on many unburned sites. The assemblages were strongly affected by the treatments, but higher volumes of green-tree retention maintained them closer to the pre-treatment structure. Although some ecological groups, such as species on ephemeral resources, experienced substantial turnover as a result of burning, populations of species that initially declined recovered. Since the increase in the saproxylics was evident, and the population reductions of other species were transient, we recommend the controlled burning with reasonable volumes of green-tree retention to reduce negative effects of forestry on insect diversity.