Boreal forest fertilization leads to functional homogenization of ground beetle assemblages

Abstract

Abstract Intensive fertilization of young spruce forest plantations (i.e. ‘nutrient optimization’) has the potential to meet increasing demands for carbon sequestration and biomass production from boreal forests. However, its effects on biodiversity, other than the homogenization of ground‐layer plant communities, are widely unknown. We sampled ground beetles (Coleoptera: Carabidae) in young spruce forest plantations of southern Sweden, within a large‐scale, replicated ecological experiment initiated in 2012, where half of the forest stands were fertilized every second year. We assessed multi‐scale effects of forest fertilization on ground beetle diversity and community assembly, 4 years after commencement of the experiment. We found that nutrient optimization had negative effects on ground beetle diversity at multiple spatial scales, despite having negligible effects on species richness. At the local scale, ground beetle species had lower variation in body size at fertilized sites, resulting in within‐site functional homogenization. At the landscape scale, fertilized sites, with higher basal area and lower bilberry cover, filtered carabid traits composition to larger body sizes, generalist predators and summer breeding species. Synthesis and applications. Fertilization of young spruce plantations is a strong filter for ground beetle assemblages, leading to functionally homogeneous communities in the short term, without changes in species richness. The large‐scale functional impoverishment of carabid communities because of fertilization may have negative consequences on system resilience and on ecosystem service provision by this functionally diverse group. Large‐scale establishment of nutrient optimization threatens ground beetle diversity in young conifer plantations, underlining the risks of introducing more intensive management schemes in already heavily managed forest landscapes.