The world's populations of wild insects are declining. These insects have an important key function as pollinators for the plant communities around them, both wild plants and human crops. Without them, both people and nature will suffer.
“One of the main causes of the decline in wild bees is habitat loss as a result of human activities,” says PhD candidate Mari Steinert.
The main objectives of Steinert’s doctorate were to examine the conservation value of power-line clearings as habitat for insect-pollinated plant and wild bee communities in forest landscapes and to identify effects of different maintenance clearing practices.
Cover large areas
In Norway, power lines cover a distance of 11,000 km across the country. In forest areas, the clearings cover an area of 200 square kilometers. To prevent the trees from growing tall and reaching the high-voltage lines, they are cleared every 5-12 years.
They therefore remain in early stages of juvenile forest. After cutting the trees, there is more light and space available for short vegetation which support a higher abundance of herbs and nectar-rich plants.
Previous research has shown that there is a higher biological diversity in the power line clearings compared to adjacent forest areas.
Steinert’s research shows that power line clearings can act as alternative habitats for plants that rely on pollinating insects, and pollinators such as solitary bees and bumblebees.
"The power line clearings are particularly important habitats for species associated with natural or semi-natural habitats characterized by early succession, because many of these species are suffering from habitat loss and fragmentation" she says. Maintaining early successional habitats in the landscape may provide a high diversity of flowering plants which can sustain diverse wild bee communities.
The habitat quality in the clearings can be enhanced with proper management.
“If we manage them properly, the extensive network can serve as an alternative and important habitat for many plant and insect species,” Steinert comments.
Cutting the vegetation had a positive effect on plants and wild bees compared to the uncut plots, regardless of woody debris removal. Cutting and removing the biomass had a positive effect on plant and bee communities under specific environmental conditions.
In sites at lower elevations, with intermediate to high productivity and where there were source habitats in the landscape removal of woody debris led to a higher richness and functional diversity of plants which promoted a higher functional diversity of wild bees.
Variations between solitary bees and bumblebees
For solitary bees, woody debris removal promoted a strong positive effect which was sustained three years after cutting. Bumble bees seemed to thrive both when vegetation was cut and left to decay on the ground and when woody debris was removed from the clearing. But bumble bees may fly over long distances to collect pollen and nectar, and may not have responded to the local management practices.
Late-season food source
Steinert also found that the powerline clearings are valuable open canopy habitats for bumble bees in managed forests and that the power-line clearings provide essential late-season foraging resources, particularly
heather (Calluna vulgaris). The power-line clearings attracted a higher diversity of bumble bees to the clearing compared to the adjacent forest habitats. Only in spring when billberry was in bloom, there were similar visits to the clearing and forest habitats.
Important with open habitats
Steinert’s dissertations shows that open habitats in managed forests are important and that less dense forest stands are important for wild bee conservation.
“To conserve native plants and wild bee species, the populations need to be maintained at a landscape level,” she says.
Hence, management practices should be implemented in areas enabling connectivity between open early successional habitats in the landscape.
Recommendations for overall wild bee conservation would thus be to implement a mosaic of woody debris retention and removal in power-line clearings, to support diverse wild bee communities. Together, the findings of her thesis represent a foundation for developing pollinator-friendly management strategies in power-line clearings in forest landscapes and suggest how power-line clearings should be included in integrated conservation plans.
Steinert defends her thesis, "The conservation value of power-line clearings for native insect-pollinated plants and wild bees in boreal forest landscapes", Friday June 12, 2020. You can follow the disputation via Zoom. Read more about how here.