Bark beetle outbreak and biodiversity in commercial spruce plantations: Responses of four model groups

During the last decades, a massive bark beetle outbreak has caused Norway spruce (Picea abies) diebacks across Central Europe, including the Czech Republic. This situation is much studied with respect to the dynamics of the pests, operations in forest planting and economic and societal impacts. In stark contrast, studies of biodiversity consequences were largely restricted to mountain protected areas within P. abies' natural distribution, omitting the situation in planted forests of lower elevations. We surveyed vascular plants, butterflies, moths, and saproxylic beetles in dieback-affected spruce plantations in three representative submontane regions of the Czech Republic. We related the quantitative characteristics of the sampled assemblages, as per their species composition, to forestry interventions (dead, sparse, salvage logged, and replanted stands), the representation of vegetation layers, plant species composition (for the insect taxa), and local dead wood supply. Vascular plants did not respond to forestry interventions, reflecting more likely such circumstances as humidity and soils of the sites. The highest numbers of butterflies preferred clear-felled clearings, which were abundantly occupied by grassland species; butterfly species composition responded to the representation of vegetation layers. The highest numbers of moths were found in dead stands, owing to moths developing on decaying plant tissues or lichens; the assemblages composition responded to the composition of vegetation. The highest diversities of saproxylic beetles were found in sparse stands with retention of some non-spruce trees, but contrary to expectations, they responded only weakly to dead wood supply, probably because dead wood was abundantly present at study plots. The transient habitats created by post-dieback operations were species rich, hosted assemblages unusual for recent Central European landscapes, and contained high representations of red-listed species. The diebacks thus contribute to preserving biodiversity at landscape scales. We understand that the post dieback habitats, and associated assemblages, will eventually succumb to forestry operations. To retain some of the biodiversity benefits in future forest plantations, we recommend retaining at least some amount of open structures and spontaneously developing wetlands. Prolonging terms for afforestation and stabilisation of planted forests or support of non-spruce trees would be also beneficial.