Bark beetles (Coleoptera, Scolytidae) and associated beetle species in mature managed and old-growth boreal forests in southern Finland

Abstract

We compared the assemblages of bark beetles and associated beetle species among mature and overmature managed, and old-growth Picea abies (L.) Karst. dominated mesic forests in southern Finland. We established 10, 11 and 9 sample plots in these categories, respectively, within an area of 35 × 80 km. We took the beetle samples by 10 window-flight traps in each 1 ha plot (total number of traps = 300). The species richness of bark beetles was highest in old-growth, lowest in mature, and intermediate in overmature forests. This was due to the greater amount and diversity of decaying wood in old-growth forests. Bark beetles which are dependent on deciduous trees, especially Trypodendron signatum (F.), were significantly more abundant in old-growth than in mature forests, obviously because deciduous trees have decreased in managed forests. The overall abundance of bark beetles was 23% higher in overmature and 30% higher in old-growth than in mature forests, but the differences were not statistically significant. Primary bark beetles comprised only 1% of the total catch, indicating that in non-epidemic conditions secondary scolytids are much more abundant than primary ones in old spruce forests. The abundance of bark beetles was best correlated with the amount of recently dead wood of the stand characteristics studied. Species associated with bark beetles showed patterns similar to those in bark beetles. The number of species was significantly higher in old-growth than in mature forests. The abundance of associated species was 61% higher in overmature and 89% higher in old-growth than in mature forests, although these differences were not statistically significant because of large inter-stand variation. It is thus likely that in the absence of major disturbances, the populations of primary bark beetles will stay at non-epidemic levels in old-growth forests. The species spectrum of bark beetles and their enemies could be broadened by promoting a deciduous mixture and improving the supply of dead trees in managed forests. This would also be beneficial to the conservation of species diversity in the managed forest landscape.