Diverging effects of two contrasting tree species on soil and herb layer development in a chronosequence of post-agricultural forest

The restoration of forest ecosystems on former agricultural land faces numerous problems. Recolonisation 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 20–30 y, whereas soil pH hardly changed under poplar plantations. Carbon (C) and nitrogen (N) soil content increased with stand age, while C/N ratio depended on tree species but remained constant in time. Multivariate analysis revealed that vegetation development depended on tree species, isolation and stand age. Poplar stands were characterised by rough Arrhenaterion grassland species while oaks had low understory cover and consequently had no indicator herb species. No tree species effect was found on the cover of strict forest species and ancient forest species. Oak favoured acid tolerant forest species, while poplars favoured light demanding and acid intolerant forest species.Our results lead to the conclusion that tree species can be important drivers of vegetation development in post-agricultural forests, in particular through their impact on soil development and light availability for the understory. Therefore it is important that goals for soil development and understory restoration are taken into account when tree species are selected for afforestations. Acidifying tree species may have irreversible effects, permanently excluding acid intolerant species. This is especially important when aiming to safeguard acid intolerant forest species from acidification in ancient forest by creating post-agricultural forest as refugee zones.When a species rich herb layer with acid intolerant species are aimed at, selecting a tree species with a good decomposable litter like poplar is a necessity and suppressing competitors by a shady overstory of the tree or shrub layer. Soil acidifying species like oak might be more suitable to enlarge forest habitats that mainly contain acid and shade tolerant species.

► Tree species as ecosystem engineers for forest restoration.
► Chronosequence of post-agricultural forests with oak and poplar.
► Acidification under oak reaching Al buffer after 30 y, stable situation under poplar.
► Poplar characterised by Arrhenaterion grassland, oaks had low understory cover.
► Each tree species favours the colonisation of different forest plant species.