Structure of the genetic diversity in Black poplar (Populus nigra L.) populations across European river systems: consequences for conservation and restoration

Black poplar (Populus nigra L.) is a keystone species for riparian ecosystems in Europe. We analysed the structure of genetic diversity of 17 populations from 11 river valleys that are part of seven catchment systems (Danube, Ebro, Elbe, Po, Rhine, Rhone, and Usk) in Europe, in relation to geography and river management. In total, 1069 trees were genotyped using AFLP and microsatellite markers. The trees had an observed heterozygosity of 0.74 (range 0.59¿0.82 across microsatellite loci). The majority (72.6¿90.8%, depending on the marker system) of the genetic variation was present within populations. Most pairs of populations along a river were relatively similar (pairwise Fst 0.042¿0.135 based on AFLP, 0.002¿0.037 based on microsatellites). Overall population differentiation among rivers was considerable (Fst among populations was 0.268 based on AFLP, and 0.081 based on microsatellites). An analysis using the program Structure indicated that all populations recruited plants from several clusters. Geographically close populations tended to draw from the same Structure clusters, including populations from adjacent catchments. The Danube and Inn populations in Austria were genetically more similar to the Vltava population (Elbe catchment) in Czech Republic than the geographically more distant populations along the Tisa and Prut rivers of the Danube catchment in Ukraine. This indicates that gene flow and dispersal takes place across fairly large distances and between river catchments. Consistent with this result, a principal coordinate analysis of genetic distances among individual trees based on AFLP bands showed large overlap of populations, although the French and Spanish samples formed distinct clusters, and the samples from the Ticino (Italy) were at an intermediate position. The extent of clonal duplication was highest along regulated rivers, with e.g., 41% clonal duplication along the Rhine in The Netherlands (up to 32 trees for one genet). The Usk contained a man-made population (two genotypes along the entire river, one genet present as 70 trees out of 72 trees sampled). No clonal duplication was found along dynamic rivers, such as the Ebro (Spain), the Drome (France), and the Tisa and Prut (Ukraine).