Can powerline clearings be managed to promote insect-pollinated plants and species associated with semi-natural grasslands?

Powerline clearings are common in urban and semi-urban landscapes worldwide. The clearings could potentially promote flowering species associated with open-canopy habitats, and provide alternative habitat for species dependent on semi-natural grasslands, but the empirical evidence for this hypothesis is weak. We carried out a large-scale field study of understorey plant communities at 51 sites in powerline clearings and adjacent boreal forest. At each site, we placed four 4 m × 5 m plots with five 1-m2 subplots in the clearing centre and a parallel set of plots 100 m into the adjacent forest. Habitat characteristics were sampled at plot and site level and from digital maps. The clearings had a higher richness of species associated with semi-natural grasslands, and a higher richness of shade-tolerant and shade-intolerant forb species. Total plant richness and diversity were higher in the clearings than in the forests. The magnitude of difference in species composition between clearings and forest, and plant community properties in the clearings, depended on the environmental context (age of the clearing, productivity, shading by tree regrowth, landscape fragmentation, climate). Our results indicate that over time, powerline clearings develop into novel habitats. Low-productive sites were dominated by Ericaceaous dwarfs shrubs. Richness of forbs and semi-natural grassland species in the clearings increased with productivity. Richness of shade-intolerant forbs and semi-natural grassland species increased with fragmentation of the surrounding landscape. We conclude that frequent clearing of productive sites can enhance plant biodiversity - including richness of insect-pollinated species - and mitigate the loss of species associated with semi-natural grasslands.