Wild bees benefit from structural complexity enhancement in a forest restoration experiment

Structural complexity enhancement (SCE) is a forest restoration approach aiming to maintain or increase biodiversity in managed, secondary forests. Whether the diversity of wild bees also benefits from such restoration measures in temperate forests is not well studied. We conducted a restoration experiment in secondary spruce forests on twelve 50 × 50 m plots in the Black Forest National Park in Baden-Wurttemberg, Germany. SCE was carried out on half of the study plots (six) in 2016 comprised create deadwood through girdling and uprooting of 20 spruce trees per plot and thus initiating small gaps. The other six plots were used as control plots with no SCE. Wild bees were sampled in June 2018 and 2019 using coloured pan traps. We hypothesized that creation of deadwood influences bee abundance and richness, especially of aboveground-nesting bees, and thus would also change bee community composition at the plot scale. Furthermore, we assessed the influence of bilberry (Vaccinium myrtillus L.) cover in the plots and areas of meadows and roads of the intermediate surrounding on wild bees. Estimated species richness was higher on restored plots than on control plots. Deadwood originating from restoration increased the number of aboveground-nesting, solitary and cuckoo bee individuals. In particular standing deadwood had a positive impact on bee individuals. Increased bilberry cover and nearby meadow areas increased abundance and richness of the bee community. We conclude that creation of deadwood in coniferous forest is a promising restoration measure to promote solitary, aboveground-nesting bees. Increased light availability through restoration, which promotes bilberry flowering, is also beneficial for bees.