Effects of intermixing native and non-native conifers in beech forests on ground beetle diversity and biomass

Effects of intermixing native and non-native conifers in beech forests on ground beetle diversity and biomass -Over the past decades, the North American Douglas fir (Pseudotsuga menziesii) has been increasingly planted in central Europe as a possible alternative to Norway spruce, due to an expected better adaptation to climate change and higher resilience to forest pests.However, mixed results have been reported on ecological impact, especially concerning effects on forest-associated biodiversity.In this context, particularly the effects of planting non-native tree species in local-scale mixtures with native tree species or mosaic mixtures of pure stands in the landscape remains poorly understood.We sampled ground beetles (Coleoptera: Carabidae) and analysed their biomass and diversity patterns in five stand types, comprising monocultures of the non-native conifer Douglas fir, the native conifer Norway spruce (Picea abies), and the deciduous European beech (Fagus sylvatica), as well as mixtures of beech with each of the two conifer species, across four sites in Lower Saxony in Germany.Our results show that non-native conifer stands supported the highest ground beetle biomass.Furthermore, admixing conifers with beech can promote stand-level ground beetle diversity and biomass compared to pure beech stands, and that coniferous stands particularly benefitted ground beetle species with larger body size.However, pure stands of beech showed higher turnover of ground beetle species across stands than the more homogeneous species composition across pure stands of the two conifer species, indicating that forest management benefitting landscape-level biodiversity needs to be aware of potential scale-dependent effects of tree species identities on biodiversity.