Natural hybridization between Populus nigra L. and P. x canadensis Moench. Hybrid offspring competes for niches along the Rhine river in the Netherlands

Black poplar (Populus nigra L.) is a major species for European riparian forests but its abundance has decreased over the decades due to human influences. For restoration of floodplain woodlands, the remaining black poplar stands may act as source population. A potential problem is that P. nigra and Populus deltoides have contributed to many interspecific hybrids, which have been planted in large numbers. As these Populus x canadensis clones have the possibility to intercross with wild P. nigra trees, their offspring could establish themselves along European rivers. In this study, we have sampled 44 poplar seedlings and young trees that occurred spontaneously along the Rhine river and its tributaries in the Netherlands. Along these rivers, only a few native P. nigra L. populations exist in combination with many planted cultivated P. x canadensis trees. By comparison to reference material from P. nigra, P. deltoides and P. x canadensis, species-specific AFLP bands and microsatellite alleles indicated that nearly half of the sampled trees were not pure P. nigra but progeny of natural hybridisation that had colonised the Rhine river banks. The posterior probability method as implemented in NewHybrids using microsatellite data was the superior method in establishing the most likely parentage. The results of this study indicate that offspring of hybrid cultivated poplars compete for the same ecological niche as native black poplars

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 F st 0.042–0.135 based on AFLP, 0.002–0.037 based on microsatellites). Overall population differentiation among rivers was considerable (F st 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). It is concluded that the restoration of the natural habitat and the re-creation of the natural dynamics of the floodplain, in combination with sufficiently sized and spaced natural populations as seed sources, are the most important measures for sufficient natural regeneration and conservation of this species in the future

European primary forest database v2.0

Primary forests, defined here as forests where the signs of human impacts, if any, are strongly blurred due to decades without forest management, are scarce in Europe and continue to disappear. Despite these losses, we know little about where these forests occur. Here, we present a comprehensive geodatabase and map of Europe’s known primary forests. Our geodatabase harmonizes 48 different, mostly field-based datasets of primary forests, and contains 18,411 individual patches (41.1 Mha) spread across 33 countries. When available, we provide information on each patch (name, location, naturalness, extent and dominant tree species) and the surrounding landscape (biogeographical regions, protection status, potential natural vegetation, current forest extent). Using Landsat satellite-image time series (1985–2018) we checked each patch for possible disturbance events since primary forests were identified, resulting in 94% of patches free of significant disturbances in the last 30 years. Although knowledge gaps remain, ours is the most comprehensive dataset on primary forests in Europe, and will be useful for ecological studies, and conservation planning to safeguard these unique forests