Tree species influences diversity of ground-dwelling insects in afforested fields

• Early successional afforested fields harbor species rich arthropod assemblages.
• Agricultural legacy is still evident in arthropod assemblages after 20 years.
• Tree species has a strong influence on ground beetles and ants.
• Diversity of ground beetles and ants increases from pine to spruce to birch.
• Restoration of old fields by afforestation benefits native forest diversity.

Impaired natural ecosystems, such as agricultural lands, are restored toward original or other target stage. Because the long agricultural history has changed the physical, chemical and biological features of the soil, afforested fields can harbor novel species assemblages and interactions. Our overall aim was to quantify the diversity of ground beetles and ants in early successional afforested fields. In a large scale field experiment, we compared plots that had been afforested 25 years ago by planting monocultures of birch, pine or spruce trees. A total of 4080 carabid individuals representing 41 species, and 131,933 ant individuals representing 15 species were recorded. Both the carabid and ant assemblage included forest and open-habitat species, but were dominated by generalists, which is typical for early successional habitats. Tree species had a strong influence on carabid and ant assemblages. Species composition among pine plots was more homogeneous than among spruce or birch plots. The diversity of ground beetles and ants increased from pine to spruce to birch plots, most likely due to positive influence of leaf litter. Our results indicate that early successional afforested fields can harbor species rich arthropod assemblages. The diversity and species composition of these assemblages are influenced by tree species, already at early successional stages, and are likely to become more prominent as succession proceeds. Thus, the tree species used in afforestation is one important factor for consideration, if field afforestation has biodiversity objectives.