Interplay between tree genetic variation, plant community composition and environment in forest communities dominated by black alder (Alnus glutinosa (L.) Gaertn.)

Studies in community genetics have often revealed a relationship between genetic diversity of the focal species and species diversity of the associated biotic communities. This relationship was studied in forest communities dominated by black alder (Alnus glutinosa Gaertn.), one of the few tree species tolerating an anoxic environment of waterlogged soils. It is a dominant species of tree overstory in two types of communities: alder carrs, forest swamps with stagnating water, and riparian forests occurring along smaller waterflows, periodically flooded with a considerable water level fluctuation during the vegetation period. Plant community composition and genetic variation of alder populations were studied in 218 black alder communities of both types distributed along a broad latitudinal transect from the Pannonian lowland to the Western Carpathians (Hungary, Slovakia, Poland). Species diversity was significantly higher in riparian stands than in alder carrs, while no difference was observed in the genetic diversity. The analysis of population structure revealed differentiation between Pannonian and Carpathian populations, which may be attributed to different migration pathways during the Holocene. No correlation was observed between genetic diversity of alder and species diversity of the associated vascular plant communities. On the other hand, using the ddRAD-sequencing approach applied to 96 trees, we identified 19 single-nucleotide polymorphisms significantly associated with climatic and soil variables. However, the hypothesized bioindication function of the plant community composition on the genetic variation of black alder as a focal species was not confirmed.