Old trees in young forests : Biodiversity management in planted conifer forests in southern Sweden

Sweden’s long history of intensive forest management has made conservation measures in the forest landscape necessary to counteract the loss of biodiversity. Retention forestry has been systematically practiced since 1993 and consists of the preservation of different structures and habitats to create and maintain suitable habitats for species that do not cope well with clear-cutting. Many of these stands are now entering thinning. The aim of this thesis was to evaluate how choices in forest management contribute to biodiversity with the example of tree retention in thinning or through the choice of rotation length. First, gap cutting around old oaks (Quercus robur) in a Norway spruce (Picea abies) forest was investigated as an example of retention management in southern Sweden (I + II). We found that oaks contributed to the diversity of the stand due to their specific associated species. The removal of Norway spruce next to the oaks at the time of thinning boosted oak vitality and increased the species richness and abundance of vascular plants and saproxylic beetles due to increased temperature and light exposure, especially oak-associated beetles. In another study, the effect of forest age was investigated on four taxa: birds, bryophytes, lichens, and vascular plants in Norway spruce and Scots pine (Pinus sylvestris) stands (III). The older stands, both of Norway spruce and Scots pine, had a distinct species community of understorey species and epiphytes. In Norway spruce stands, older stands typically had more deadwood, which could have favoured bryophytes and birds by providing specific habitats and resources. Older Scots pine stands had the highest lichen species richness, but the implementation of other understorey species was impaired by the dominance of a few competitive dominating species. Finally, we explored the potential of using a canopy height model (CHM) to find and map retention trees (IV). We concluded that this method is a cost-effective solution to map and characterise past retention efforts to facilitate forest management and ensure that old trees are kept throughout the present and subsequent rotations. Our results indicate that this method can discern retention areas to an accuracy of 66%. The findings in this thesis provide knowledge on how the management of retention trees and the choice of rotation length can support biodiversity and provide guidelines for forest management so that their positive effects on forest biodiversity can be maintained over time.