Conservation of solitary bees in power-line clearings: Sustained increase in habitat quality through woody debris removal

Declining populations of wild pollinators have increased the interest in the management of human-modified landscapes for pollinator conservation. Modified landscape features, such as power-line clearings, may provide valuable habitats with increased floral resources for insect pollinators and solitary bees in particular. However, whether the effects of costly habitat interventions on bee communities in power-line clearings are sustained over time, is poorly understood. We conducted an experiment in 19 sites in power-line clearings across southeast Norway where the woody vegetation was; 1) cut and left to decay, 2) cut and removed, or 3) uncut. We assessed the temporal effects of habitat interventions and environmental factors on species richness, abundance, functional groups of nesting preference, body size, tongue-length, and species composition. Species richness and abundance of solitary bees increased substantially with time in both the cleared treatments. But compared to leaving the woody debris to decay, woody debris removal more than doubled estimated bee richness and abundance in areas with a higher forb richness and increased the trait variation in areas with high functional trait variation of forbs, dead wood, and available source habitats. The positive effect of woody debris removal on the species and functional trait diversity of bees was sustained during three years post-treatment, but the effect was dependent on abiotic and biotic environmental conditions. Over time the sites increasingly attracted bees from the regional species pool, showing how power-lines may benefit bee populations at a regional scale, through maintenance of temporary refugia in a forested landscape. The current routine practice (cut and left to decay) improves habitat quality for many bee species through alterations to early succession. However, our results indicate that the more costly woody debris removal is a more effective conservation measure for solitary bee communities over time and is to be recommended when funds are available. Removing the woody debris after routine maintenance clearing at specific sites may further enhance the diversity and persistence of bee populations over time, contributing to more diverse and stable bee communities.