Nine years of vegetation development in a postmining site: effects of spontaneous and assisted site recovery.
Highly disturbed areas such as surface-mined land provide a great challenge for ecological restoration. The goal is to identify appropriate restoration approaches in a continuum between technical reclamation and spontaneous succession. In particular, on slopes endangered by erosion, appropriate methods are needed that quickly establish vegetation cover but also take into account the natural potentials of the site. In the mined area Rossbach (Saxony-Anhalt, Germany), we evaluated the effects of spontaneous succession and assisted site recovery (species introduction through hay transfer and sowing) during a 9-year experiment. We asked how rates and pathways of vegetation development differ between treatments and whether species composition converges over time owing to species exchange. The application of green hay as well as the sowing of regional seed mixtures clearly accelerated vegetation development and led to the rapid establishment of species-rich grasslands. Hay transfer was most successful owing to the high amount of transferable target species. Moreover, both treatments facilitated the establishment of cryptogams and provided effective erosion control. Also, hay transfer and sowing clearly affected the pathway of succession. Calamagrostis epigejos migrated from nearby source populations and became increasingly dominant at sites with spontaneous succession. In contrast, the species-rich grasslands established after hay transfer and sowing were highly resistant to invasion of Calamagrostis and other ruderals. Species exchange between treatments led to increasing similarity in vegetation composition over time. Nine years after implementation of the experiment, we did not find any significant differences between treatments in terms of total vegetation cover, species richness and the number of target species. However, the dominance ratio between target and nontarget species differed significantly. Species introduction through hay transfer and sowing led to a permanently higher abundance of grassland species and a lower coverage of ruderals compared with spontaneously developed sites. Hence, our results highlight the importance of initial floristic composition and the order of species arrivals for long-term vegetation development. Synthesis and applications. Hay transfer and sowing of regional seed mixtures are appropriate restoration tools to achieve rapid revegetation when no potential seed sources of target species are available nearby or there are undesirable species that need to be suppressed. Our results show that introduced grassland species are able to grow under postmining site conditions and can migrate into adjacent spontaneously developing sites. A combination of spontaneous and assisted site recovery can promote the development of species-rich grasslands in postmining landscapes.