The restoration of forest ecosystems on former agricultural land faces numerous problems. Recolonisadon of forest species is hampered by the modified habitat quality and by the isolation from source populations. Tree species are ecosystem engineers that can modify soil and light conditions and can therefore act as a possible catalyst for understory recovery. Therefore, we set out to study the effects of tree species on herb layer development. For this purpose, a chronosequence of post-agricultural oak (Quercus robur and Quercus petraea) and poplar (Populus x euramericana and Populus x interamericana) plantations on silt and sandy silt soils was selected. The selected tree species are frequently planted and have contrasting characteristics (e.g. in terms of litter quality, shade casting ability and growth rate). Under plantations of oak, soils acidified with increasing stand age and dropped into the aluminium buffer range after only 20-30y, whereas soil pH hardly changed under poplar plantations. Carbon