Differentiation of herb layer vascular flora in reclaimed areas depends on the species composition of forest stands

Spoil heaps, resulting from open-pit mining, are deprived of soil cover. A common method of reclamation for such sites is afforestation aimed at creating whole forest ecosystems. Forest stands planted on soilless areas make it possible to determine the effects of different tree taxa on the species composition of flora. The study focused on the vascular species of herb layers in young forest stands formed by Alnus glutinosa, Betula pendula, Pinus sylvestris, Quercus petraea and Robinia pseudoacacia, and mixed stands dominated by Acer pseudoplatanus or Betula pendula. The study included 227 randomly selected plots across the afforested Bełchatów Brown Coal Mine spoil heap. Species composition of the herb layer was composed mainly of synanthropic species (over 70%), and was clearly related to the overstory tree species composition of forest stands. TWINSPAN divided sample plots into two groups that developed under the canopy of stands formed by: (1) Betula pendula, Pinus sylvestris or Quercus petraea, or mixed stands with Betula pendula, or (2) Alnus glutinosa or Robinia pseudoacacia, or mixed stands dominated by Acer pseudoplatanus. The first group was dominated by meadow species (25.8%), while the second had higher richness of ruderal (18.6%) and forest-edge species (14.1%). Plant species associated with fertile and moist forest habitats were more frequent in the second group (Alnus-Robinia), whereas species of acidic and dry forests occurred only in the first group (Betula-Pinus). Non-parametric tests on Ellenberg's indicator values showed that forest stands of Betula-Pinus group created habitat conditions for light-demanding species with low fertility requirements, whereas forest stands of Alnus-Robinia group created habitat conditions for nitrophilous and shade-tolerant plant species. Our data revealed that tree species composition of forest stands is a key factor shaping herb layer conditions and - as a consequence - herb layer species composition despite of initial habitat variability on the spoil heap. These results highlight a key role of forest stand composition in the development of herb layer richness on post-industrial habitats.