Investigations were carried out on 104 sample sites for the classification and description of chronologically and spatially differentiated successional series. Abundance and species-cover percentage of vascular plant species in all vegetation layers were estimated (modified Braun-Blanquet scale, Wilmanns 1998). In order to assess stand structure, all trees of 1 cm or greater diameter were measured in a plot of 400-2,000 m2. Further investigations were carried out with respect to selected site factors, e. g. soil acidity, soil texture, nutrient availability, cation-exchange capacity, coal content, humus and fly-ash coating (for detailed methods, see Tischew et al. 2004). The classification of seral successional stages in space and time was carried out using hierarchical and non-hierarchical cluster analysis as well as multivariate ordination methods. The following bioindicative parameters were included for every plot: (1) percentage of vegetation layers, (2) weighted percentage of vegetation units (e. g. dry psammophytic grasslands, mesophilic grasslands, woodlands), (3) stand density per hectare of pioneer, intermediate and climax tree species and (4) stand basal area of all trees per hectare.
Special aspects of colonization processes were analyzed exclusively in brown-coal districts north and south of Halle: five northern mining areas from the region around Bitterfeld and five southern mining areas from
Geiseltal and its surroundings. Complete species composition was identified on sample sites (each about 2 km2) by several mappings from 1998— 2002. The investigated sample sites were characterized by open-land and woodland stages (age: 2-55 years). About 1,100 releves were included, which were conducted during several research projects. To determine the regional species pool, we used the dataset of the flora mapping of the states Saxony-Anhalt and Saxony, which was done in grid cells (5.5 x 5.5 = 30.25 km2).
Statistical tests and cluster analysis were carried out using SPSS 10.0 and PCORD 4.0. Multivariate ordination methods were carried out using CANOCO 4.5 (ter Braak and Smilauer 1998).
