Power line clearings are edge-creating disturbances in landscapes world-wide, but there have been few studies on their bordering vegetation. Our aim was to quantify edge effects on plant communities along such clearings in Norway and to identify factors that influence these edge effects.
We surveyed understorey plant communities on either side of the power line clearing–forest edge at 51 sites, along four parallel transects at each site. Each transect had four plots located, respectively, in the clearing centre, clearing edge, forest edge and forest. We quantified the magnitude of edge effects (MEE) on either side by comparing edges with their corresponding ‘non-edge’ reference habitats. We also measured differences in species composition across the edge (clearing edge vs. forest edge). Habitat characteristics were sampled at plot and site level and from digital maps. % cover of vascular plants were registered (see Eldegard et al 2015 for description of study design and methods).
Differences in species composition were greater between clearing centres and clearing edges than between forests and forest edges. Differences in species composition across the edge increased with edge contrast and forest productivity. Edge effects on species composition into the forest were smallest along north-facing edges, whereas those in the clearings increased with power line age.
Species richness increased slightly towards the edge in forests but decreased considerably towards the edge in clearings. The direction and MEE on either side differed among functional groups. Edge contrast and edge aspect were the prime factors influencing the MEE into forests, whereas in clearings, these were influenced principally by tree regrowth in the clearings and by forest productivity.
Synthesis and applications. Edge effects on plant communities bordering power line clearings were determined by factors that can be influenced by planners and managers. For existing power lines, management plans should differentiate between the following: (i) clearings through high conservation value forests, where edge effects into the adjacent forest should be limited; (ii) clearings that can act as replacement habitat for cultural landscape species, where maintaining open-canopy habitats should be prioritized; and (iii) ‘business-as-usual’ clearings, where continuing the current practice of cutting every 5–10 years is recommended.