Partial cutting in mixed boreonemoral forests as a restoration approach to increase insect diversity

Several studies have revealed an alarming negative trend for richness and abundance of insects in European agricultural landscapes . Intensified land use is one underlying cause of insect decline, including overgrowth of wooded pastures and meadows with dense, secondary forest. Restoration cutting in mixed boreonemoral forest may be a way to restore temperate broadleaved forests with semi-open canopy, but effects on biodiversity need to be tested. We designed a Before-After Control-Impact experiment in 11 40–80 year old mixed forests on abandoned agricultural land in the boreonemoral zone of Sweden and Norway and performed restoration cutting by the removal of ca 22 % of the total volume timber (mainly spruce Picea abies but also some birch Betula spp) from 1 ha thinned plots. Our results show a significant increase in the total number of individuals and species of insects following the experimental treatment. Species richness of flower-visiting insects and Syrphidae increased significantly as a result of restoration cutting, while abundance increased for all insect groups. The treatment also had a significant positive effect on species richness of vascular plants, including number of flowering species. There was a positive relationship between the increase in plant species richness and the richness and abundance of insects. We suggest that restoration cutting by the removal of spruce in mixed recent boreonemoral forests may help ameliorate insect decline in the agricultural landscape by improving habitat conditions through increased light influx and richness of flowering plants. • Removing spruce in mixed recent boreonemoral forest favors biodiversity of insects. • Vascular plant diversity, flowering, and the diversity of flower visiting insects also benefit. • Restoration cutting jointly contributes to wood production and climate adaptation.