Forest edge quantification by line intersect sampling in aerial photographs

There is a need for accurate and efficient methods for quantification and characterisation of forest edges at the landscape level in order to understand and mitigate the effects of forest fragmentation on biodiversity. We present and evaluate a method for collecting detailed data on forest edges in aerial photographs by using line intersect sampling (LIS). A digital photogrammetric system was used to collect data from scanned colour infrared photographs in a managed boreal forest landscape. We focused on high-contrast edges between forest (height ≥ 10 m) and adjoining open habitat or young, regenerating forest (height ≤ 5 m). We evaluated the air photo interpretation with respect to accuracy in estimated edge length, edge detection, edge type classification and structural variables recorded in 20 m radius plots, using detailed field data as reference. The estimated length of forest edge in the air photo interpretation (52 ± 8.8 m ha^?1; mean ± standard error) was close to that in the field survey (58 ± 9.3 m ha^?1). The accuracy in edge type classification (type of open habitat) was high (88% correctly classified). Both tree height and canopy cover showed strong relationships with the field data in the forest, but tree height was underestimated by 2.3 m. Data collection was eight times faster and five times more cost-effective in aerial photographs than in field sampling. The study shows that line intersect sampling in aerial photographs has large potential application as a general tool for collecting detailed information on the quantity and characteristics of high-contrast edges in managed forest ecosystems.