Natural hybridization and potential seed set of sympatric Populus nigra and Populus × canadensis along the river IJzer in Flanders (Belgium)

Background and aims – In Europe, native Populus nigra generally grows sympatric with related exotic cultivated poplars. Hybridization followed by introgression and genetic swamping is often mentioned as a potential threat for the conservation of P. nigra. This potential threat is supposed to be higher when the cultivated poplars are reproductively more effective compared to P. nigra. We investigated hybridization events of P. nigra females with cultivated P. × canadensis and P. nigra 'Italica' along the IJzer river in Flanders (Belgium). We also compared the reproductive potential of P. nigra and P. × canadensis. Methods – After first determining the genotypic diversity within P. nigra, we looked for diagnostic microsatellite alleles of P. deltoides in the offspring of open-pollinated P. nigra. We also studied the possible paternity by the cultivar P. nigra 'Italica'. The reproductive potential of P. × canadensis and P. nigra was compared in terms of potential seed set. Key results – This study provides evidence for natural hybridization between P. nigra and P. × canadensis in West Flanders. In 58% of the seedlings from the open pollinated P. nigra trees, at least one diagnostic allele of P. deltoides was detected. P. nigra 'Italica' was likely the father of only one seedling. The exotic P. × canadensis showed a significantly higher potential for seed set compared to native P. nigra. The genotypic diversity of P. nigra was extremely low with only two (female) genets among 209 black poplars. Conclusions – Natural hybridization with P. × canadensis occurs and may pose a significant threat to the conservation of P. nigra in West Flanders. To reduce the risk of introgression and genetic swamping of P. nigra by P. × canadensis, we strongly advise reforestation using multiple genotypes of P. nigra from neighbouring regions taking into account balanced sex-ratios.