Locations and site characteristics

The study was carried out in nearly all eastern German brown-coal districts in Saxony, Saxony-Anhalt and Brandenburg that show spontaneous suc­cession at least in some parts (FLB 2003; Tischew et al. 2004). Forty-four former mines were included in the investigations. Large areas with sponta­neous succession were divided into smaller sites with relatively homoge­neous conditions and considered as sample size. The time since abandon­ment of the investigated mines ranges from 1 to 98 years.

Lignite has been mined in surface mines in eastern Germany since the beginning of the 20th century. Overburden layers 40-120 m thick above the lignite seams had to be excavated and dumped, forming spoil dumps. This process can be compared with the mass turnover of the last glacial epoch (Muller and Eissmann 1991). The groundwater level had to be low­ered significantly. Overburden layers consist of Tertiary and Quarternary substrates that were often mixed during the dumping processes. Quarter­nary substrates are usually more suitable for plant colonization. They are composed of boulder clay, sand and loess. Tertiary substrates consisting of sand, clay and silt often contain fine coal and sulphur. Due to the oxidation of the sulphuric compounds (iron pyrite, marcasite), Tertiary substrates tend to acidify over a long period of time as well as to release high levels of phytotoxic Al3+-ions (Pichtel et al. 1988). Tertiary substrates addition­ally display high bulk density and low macropore volumina. High levels of finely distributed coal-containing admixtures can also influence substrate qualities negatively by their hydrophobic characteristics.

In the investigated mining area, the topsoil, which contained humus, was often not separated when dumped. Dumped substrates, therefore, show mostly low biological activity and low levels of available plant nutri­ents at the beginning of their development. Dumps of the early mining phases (up until the 1920s and 1930s) are exceptional, because they were turned by hand and topsoil containing humus was selectively put on the spoil dumps as the uppermost layer.

The area investigated has a mean annual temperature of 8.0-9.5°C and an annual precipitation of 450-650 mm. Due to industrial mining, vast sur­face mines have been created where isolating effects relating to coloniza­tion processes must be expected. Remnants of natural woodland vegetation (oak-hornbeam forests, beech forests, birch-oak forests, pine-oak forests, floodplain forests) have only rarely remained around spoil dumps.