Genetic diversity of common reed in the southern Baltic Sea region – Is there an influence of disturbance?

European populations of common reed (Phragmites australis) are considered to build a single metapopulation with an extensive gene pool. Here we characterise the pattern of genetic diversity of 24 common reed populations in northeastern Germany and test how a disturbance by water salinity and mowing influences population genetic and morphological parameters. The investigation of in total 720 samples revealed them as uniformly tetraploid. The four haplotypes found and a set of eight SSR loci combined freely corroborating the hypothesis of a single large metapopulation. A MEMGENE plot based on pairwise genetic and geographic distances explained only 3.27% of genetic variation. Most of the genetic diversity was found within populations implying that gene flow over distances exceeding 10 km seems to be effectively mediated via seeds and pollen. Only in two cases, we encountered spread via rhizomes among different populations. Populations influenced by brackish water salinity show consistently higher values for Nei's unbiased gene diversity index, which we explain by higher disturbance (water salinity, water currents, stronger winds) creating more room for seedling establishment, and/or lower extension of clones. A structural equation model revealed both mowing and water salinity, but not genetic parameters, to have a negative effect on stem width, and put an effect on haplotype composition. Our results point towards a very effective gene flow in this species, a high degree of plasticity, and the importance of disturbance for the accumulation of genetic diversity, perhaps via seedling recruitment. Data Availability Statement: Sequence data that support the findings of this research have been deposited in GenBank with the accession code OK666430. Raw data for all samples, used in this study, and the quality plots of assignment loci as isoloci, have been deposited in Figshare [DOI: 10.6084/m9.figshare.16438668; 10.6084/m9.figshare.16438812].