Origin of natural regeneration of black poplar (Populus nigra L.) and co-occurrence with hybrid poplar offspring

• We studied seedlings, juvenile and adult trees of Populus nigra with microsatellites.
• We estimated the proportion of hybrid poplar offspring growing at the same sites.
• We determined clonality of trees and gene flow from outside local stands.
• Competition for regeneration sites is increased with large numbers of hybrids.
• Gene flow among separated stands of P. nigra is maintained.

Black poplar (Populus nigra L.) is a keystone species of riparian ecosystems in large parts of Europe. While adult black poplars were thoroughly surveyed across Europe, there is much less information about the species’ natural regeneration. As the cultivar Populus x canadensis often grows in the surroundings of black poplar stands, introgression is being discussed as threat for black poplar rejuvenation. In the present study, we identified the proportion of black poplar seedlings and juvenile trees at three sites in Switzerland as compared to hybrid poplar offspring growing at the same sites. Taxon identification was based on a combination of a chloroplast and two nuclear markers. Using SSR markers, we estimated the extent of clonality of adult and juvenile trees, estimated genetic diversity of all age classes and determined the proportion of gene flow from outside the local stand. We found different proportions of black poplar regeneration at our three study sites, ranging from no hybrid offspring in Valais, where most adult black poplars relative to adult P. x canadensis occurred, to 90% hybrid seedlings in Thurgau, where the black poplar stand consisted of only four individuals, surrounded by a large number of P. x canadensis. The extent of clonality was generally low, as we identified less than 10% recurrent genotypes among juveniles and adults. We found high numbers of alleles and high heterozygosity in all populations. Substantial gene flow from outside the local stands could be detected in two study sites, indicating that black poplar at least partly overcomes the current degree of fragmentation in Switzerland.