To assess the effects of landscape configuration on local plant species richness, we tested whether local species richness of forest understory plants is affected by the total forest area and forest edge length in the adjacent landscape. We also tested whether the landscape effect on species richness is different for forest and edge species. We estimated species richness from 113 forest sites in four regions in Northern Europe. At each site, we studied two plots, one at the edge and one in the core of the forest. Total forested area and forest edge length in circles with a 1-km radius, together with plot-specific variables of environmental conditions and temporal continuity of forests, were recorded at each plot. The amount of forest and the length of the forest edge in the adjacent landscape had a significant positive effect on local species richness of all understory plant species. As expected, edge species were positively affected by increasing length of the forest edge in the landscape, but surprisingly there was no effect of forest area on species richness of forest species. Temporal forest continuity had a negative effect on species richness of edge species but no effect on species richness of forest species. Our results suggest that forest edge length had a stronger landscape effect on understory plant species richness than forest area. Implications of these findings for the management of forest landscapes depend on priorities given to different species groups in biodiversity conservation, i.e. if emphasis is in total species richness or species richness of forest or edge species.