Tree species determine the colonisation success of forest herbs in post-agricultural forests: Results from a 9 yr introduction experiment

The limited dispersal and recruitment of ancient forest plant species (AFS) hampers the restoration of herb layer communities in post-agricultural forests. Recruitment limitation is often caused by the high availability of soil nutrients, in particular phosphorous (P), which can aggravate competitive exclusion of AFS by nutrient-demanding species. We monitored the survival, growth and reproduction of six AFS with variable ecological amplitudes for light and soil acidity according to expert-based indicator values. These AFS were introduced under eight tree species that have created specific levels of light and soil acidity within 30 years after plantation on farmland. Acidifying tree species severely limited the survival of all introduced AFS except Hyacinthoides non-scripta. We observed the highest survival of AFS in moderately shaded stands and the best growth of AFS in stands with a low light level. Temporary canopy gaps promoted the survival of AFS, but also provoked dominance by light-demanding competitors before canopy closed again. The two hemicryptophyte AFS, Primula elatior and Oxalis acetosella, increased by means of sexual reproduction, while the geophytes (Anemone nemorosa, Mercurialis perennis, Pteridium aquilinum, and Hyacinthoides non-scripta), mainly expanded vegetatively. As a conclusion, we advise managers that aim to restore a species-rich forest herb layer in post-agricultural forest, not to promote strongly acidifying tree species. The regulation of the level and/or frequency of stress, soil acidity and shade in this case, can be used as a management tool to mitigate the competitive exclusion promoted by P eutrophication.