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Forest fragmentation increases the amount of edge habitats in landscape. Despite the profound ecological influences that edges may have, the patterns and processes of edge formation have largely remained unexplored. The aim of this study was to find answers to three questions: (1) How have the landscape structure and composition, as measured by different successional stages, changed over a period of about 50 years in forested landscapes in southern Finland? (2) What are the differences in the amount of edge and interior habitat between successive years? (3) How does the forest ownership, state or private, affect the prevalence of edges? Digitized aerial photographs taken in 1941 and 1944 (period 1), 1969 (period 2) and 1997 (period 3) were used to study the prevalence of different successional stages, area of interior mature forest and edge habitats in two nearby 10 km 2 forest areas with private and state ownership, respectively. The forest composition was similar in the two forest areas, with mature forests being replaced by younger development stages. The area of interior mature forest decreased dramatically from period 1 to period 2. Assuming an edge width of 100 m, the proportion of edge habitat increased from 30% to 61% in the state forest and from 26% to 76% in private forest. The detected changes and their possible consequences to species suggest that management of different types of edges and converging edges should receive considerable attention in the future.