The importance of forest forage resources and a landscape perspective, managing bumblebees (Bombus) in Swedish forest-farmland

Abstract: The Swedish forest has been transformed during the last hundred years, from semi-open forest of multiple tree species to dense production forest of mainly spruce and pine. This has led to alterations of species composition in the forest floor, reducing coverage of dwarf shrubs like Vaccinium myrtillus (European blueberry). Bumblebees (Apidae: Bombus spp.) forage from V. myrtillus flowers and it forms together with other early flowering plant species a foundation for bumblebee colony establishment in spring. Against this background I examined how resources of V. myrtillus in the landscape affect bumblebees. I also studied the influence of season on preferred bumblebee forage habitat, comparing forests with road verges in the open landscape. The research was conducted in spring and summer of 2021 in the county of Södermanland in Sweden. Bumblebees were sampled in forests and road verges in 20 study landscapes, dominated by a forest-farmland mosaic. Bumblebee abundance and species richness was found to increase with coverage of V. myrtillus shrubs in forests during flowering of V. myrtillus in spring and with coverage of flowers in forests and road verges in summer. Bumblebees were also found to mainly forage in forests in spring and in road verges in summer and approximately the same bumblebee species occurred in both habitats. I conclude that bumblebees moves in between the forest and the open landscape in seasons, controlled by availability of flowers in the habitats. It is further concluded that V. myrtillus flowers is an important forage resource for bumblebees in spring. This calls for a landscape perspective managing bumblebees in forest-farmland landscapes in Sweden, recognizing the importance of forest forage resources as well as forage resources in the open landscape. A forest management that acknowledge the need for forest floor conditions supporting dwarf shrubs like V. myrtillus and summer flowering species is further recommended in order to preserve bumblebees in Swedish forest-farmland landscape.