Open forest successional stages and landscape heterogeneity promote wild bee diversity in temperate forests

Recent studies have emphasized forests as crucial habitat for wild bees. In Europe, most forests are managed following the principles of close-to-nature silviculture, which combine timber production and nature conservation. However, open late and early successional stages within these forests are largely missing, which could be important for wild bees. This highlights that close-to-nature silviculture alone might not be sufficient to conserve bees within temperate forests. Open structures such as canopy gaps and road verges in forests could improve habitat for bees. To provide management recommendations for wild bee conservation in temperate forests, we analyzed how components of bee beta diversity varied between forest management types and tested how open structures, namely clear-cuts, canopy gaps, and forest road verges influenced bee abundance, richness, and diversity. In addition, we analyzed the abundance and percent of red-listed bee species at different scales. Bees were sampled using 90 pan traps on 45 (1 ha) plots in 2019 and 2020 in the Black Forest, Germany. Plots were selected in 15 triplets each consisting of three management types related to different successional stages: unmanaged, close-to-nature, and small clear-cut. Beta diversity was not consistently nested highlighting the importance of different management and successional stages within the landscape to support bees in forests. Abundance, species richness, and Shannon diversity of bees were highest on clear-cuts, compared to unmanaged- and close-to-nature plots. At landscape scale, wild bee abundance increased with canopy openness while wild bee diversity increased with landscape heterogeneity. Abundance- and percent of red-listed bee species increased with the length of forest road verges. We advocate creating habitats at local scales which offer flowering and nesting resources by providing canopy gaps. At landscape scale, heterogeneity created through different forest successional stages is needed to conserve the entire community of wild bees.