Abstract
Bumblebees (Bombus spp.) are efficient pollinators of insect-pollinated flowering plants. Young spruce plantations may provide important floral resources for bumblebees, but the flower preferences of bumblebees in these plantations have not been studied yet. Understanding preferences is important for managing these plantations effectively to preserve and protect floral resources for bumblebees. In this study, I focused on bumblebee preferences, seasonal changes in flowering, and grazing influences on flowering in spruce plantations in southeastern Norway.
In the summer of 2021 and 2022, data were collected from 24 plots (12 per site) during five field surveys across two study sites in Norway, both young spruce plantations. This included counting flowers and bumblebees visiting them. Six plots at each site were grazed by cattle and ungulates, the other six were fenced from grazers but previously grazed. I used the preference index to determine flower preferences of bumblebees. The flowering peak of the most preferred species was analysed with Generalized Linear Mixed Models. I used Canonical Correspondence Analyses to determine the influence of grazing and season on the composition of visited flowers.
Overall, bumblebees showed a strong preference for Cirsium palustre, followed by Filipendula ulmaria, and Epilobium angustifolium at one site, and Rubus idaeus and Solidago virgaurea at the other. Some of these most preferred species peaked early or mid-season, while others showed a relatively stable flowering throughout the summer. The composition of all visited flowers was found to be significantly related to Julian date in all CCAs, while graminoid and dwarf shrub heights were only significant predictors in grazed plots. Several visited flowers were negatively related to grazing, although the effect appeared to vary between species and sites.
My results showed that young spruce plantations can provide valuable floral resources for bumblebees, if open areas are maintained and food supply is supported throughout the season. Additionally, it is important that grazing is carefully managed at a local scale with low intensity to protect these floral resources.
Key words: boreal forest, bumblebees, Bombus spp., Cirsium palustre, flower availability, preferred flowers, visited flowers, young spruce plantations
