1 We studied the migration of forest plant species using their percentage cover and frequency in 197 plots distributed over 26 transects across ecotones between ancient and recent deciduous forests in the Meerdaal forest complex in central Belgium. The recent forest stands varied in age between 36 and 132 years, and all occurred on silty, well‐drained soils. 2 The total cover, number and diversity of field layer species did not differ significantly between ancient and recent forest stands. 3 The number and cover of the ancient forest plant species and of ant‐dispersed species correlated positively with the age of the recent forest and negatively with both the duration of its former agricultural land use and the distance to the ancient forest. This implies a slow colonization of the recent forest stands by these species; all species were, however, able to migrate across the ecotones. 4 The cover of four species ( Anemone nemorosa , Lamium galeobdolon , Convallaria majalis and Polygonatum multiflorum ) declined along the transect, suggesting that they are limited by seed dispersal. Their colonization rates, calculated from the occurrence of the farthest individual, ranged from < 0.05 to 1.15 m year −1 and for other measures from < 0.05 to 0.65 m year −1 . Anemone and Lamium appeared to colonize the recent forest by establishment of isolated individuals, while Polygonatum and Convallaria expanded populations from existing patches on the border between ancient and recent forest. 5 Several forest species were able to colonize the recent forest rapidly, where some of them even reached a higher abundance, due to the increased availability of colonization sites with a higher nutrient content and a thinner organic layer.