Soil dumping techniques and afforestation drive ground-dwelling beetle assemblages in a 25-year-old open-cast mining reclamation area

The open-cast lignite mine “Hambach” (NW-Germany) and the accompanying land reclamation replace a large ancient forest. Reclamation of an 1000 ha large overburden dump started 25 years ago and is still in progress. Reclamation methods comprise combinations of different topsoil dumping techniques and reclamation measures such as seeding or afforestation. In accordance with the spatial heterogeneity hypothesis, we hypothesize that beetle assemblages from sites where the topsoil was left structured are more diverse than those from sites with levelled topsoil, and that richly structured mid-successional sites harbour more heterogeneous assemblages than afforested sites. We tested the influence of environmental variables on assemblage characteristics of pitfall-trapped beetles from 17 sites on the overburden dump. The results are based on 35,588 individuals from 339 species. Species assemblages differed significantly between almost all sites. Habitat age did not affect assemblage characteristics, while species richness was significantly influenced by surface structure. Sites that were not levelled and that additionally included pits that slope down for several meters exhibited the highest species richness. Abundance of beetles increased in parallel to the canopy cover, whereas assemblage heterogeneity (MVDISP) decreased. Successional and grassland sites usually exhibited higher diversity (Fisher's α) than afforested sites, but diversity was not significantly predicted by a single environmental factor. Instead, diversity increased along with increasing MVDISP-values. Afforestations speed up the successional process and some euryecious forest species are already well established, but stenecious forest specialists have not yet been found.