The vegetation development is slowed by heaping blast-furnace slag. This material offers an unfavourable growing medium with extremely alkaline conditions which have a toxic effect on plants, especially on deep-rooting species. These extreme habitat conditions lead to a low rate of succession. First mosses, e. g. Ceratodon purpureus and Bryum argenteum, colonise the slag substrate (Rebele and Dettmar 1996). Gradually the pH value is decreased through eluviation under the influence of rain. The substrate will then be populated by ruderal species like Chenopodium botrys and Arenaria serpyllifolia (Punz 1989).
Acceleration of vegetation development
For the acceleration of vegetation development, seeds of Senecio species (S. viscosus, S. vernalis), Reseda luteola and Inula graveolens are brought into the mine spoil ground of the former sinking pond.
This process improves the succession conditions and offers a natural source of seeds and other diaspores for further succession. The yellowflowering colour of these species corresponds to the typical colour of pioneer vegetation on industrial sites (Dettmar 1999). When viewed from the near distance or by zooming, the various textures, flower shapes and habits of the plants are visible. Areas of accelerated natural vegetation development are marked by coal heaps.
The applied restoration treatment, investigated by M. Jochimsen, involves the application of small amounts of sand and fertilizer as required to increase the plant coverage (Jochimsen 1987). At first the seeds are sown in limited linear areas organised in a rational arrangement. Gradually the contours change into amorphous structures as the plant cover increases until the original sowing area cannot be defined anymore.
Natural succession of pioneer vegetation
Fig. 9. Areas with deceleration (left) and acceleration (center) of vegetation development and natural succession of pioneer vegetation (right)
Finally the pioneer species are replaced by the communities of secondary succession. A herbaceous vegetation with a reduced number of species results, e. g. Hypericum perforatum and other perennials (Jochimsen 1991). Based on vegetation mapping of similar sites in the Saarland, the following species are also to be expected: Conyza canadensis, Epilobium lanceola – tum, Saxifraga tridactylites and others (Maas 2002). Areas of natural pioneer vegetation are marked by coke heaps.
Fig. 10. Restoration treatments: sowing Fig. 11. Expansion of plant cover