Length and classification of natural and created forest edges in boreal landscapes throughout northern Sweden

Forest edges have numerous implications for structure and function of forest ecosystems. Previous studies on edge quantity have used broad classifications. However, edge influence is driven by the contrast in vegetation structure between adjoining ecosystems, and thus we need detailed site-specific data to assess the role of edges in forests. We studied the variability of sharp edges in 28 boreal landscapes (4 km × 4 km) across an 830 km gradient throughout northern Sweden. Our objectives were: (1) to compare the length of natural and created edges, (2) to classify edges in detail by edge origin, maintenance and forest attributes, and (3) to examine relationships between length of edge and landscape variables. Data were collected using stereo-interpretation of high spatial resolution colour infrared aerial photographs, in combination with line intersect sampling and plots. The length of edge varied from 12 to 102 m ha−1 among landscapes, with an overall mean of 54 m ha−1. Created edges dominated most landscapes (mean 33 m ha−1) and had greater variability than natural edges (mean 21 m ha−1). Maintained edges (e.g. roads, agricultural land) were more abundant than regenerating edges caused by logging. Thirty percent of total edges adjoined narrow linear features. Seventy percent adjoined wider patches and showed high variability (35 classes). Overall, high-contrast edges towards mature forest dominated, i.e. ones that may experience strong edge influence. The amount of edge increased with percent of landscape affected by disturbance, and decreased with latitude and elevation. This study shows that edges are both abundant and highly variable in boreal forests and that forestry is the main driver behind edge creation. Detailed classification of edges based on site-specific forest and patch attributes may help to estimate edge influence at landscape level, and can guide experimental design for examining the impact of edges on structure and function of forest ecosystems.

► We quantified length and characteristics of forest edges from aerial photographs.
► Edges were classified in detail by origin, maintenance and forest variables.
► Length of both natural and created edges varied strongly among landscapes.
► Length of edge increased with disturbance by forestry and varied geographically.
► Edges have many implications for structure and function of boreal forest ecosystems.