Abstract
The genetic structure of remaining black poplar ( Populus nigra ) trees on the banks of the Dutch Rhine branches was investigated using the AFLP technique. In total, 143 trees, including one P. deltoides and some P. x euramericana , were analysed using six AFLP primer combinations which generated 319 polymorphic bands. The AFLP patterns showed that some of the trees sampled as P. nigra were clearly different. These deviating patterns were also observed for the P. deltoides tree and all trees already identified as hybrid P. x euramericana . Hybrids between the two species are morphologically sometimes difficult to distinguish from the species itself. Two important possible source populations for recolonization of the riverbanks of the river Rhine, consisting of mature flowering P. nigra trees, appeared to consist of only a few genotypes each. In contrast, young black poplar trees growing alone or in small groups downstream of the possible source populations appeared to be predominantly generatively derived because no clones of mature trees were found among them. Therefore vegetative propagation seems a very local strategy whereas colonization of new areas appears to occur through generative propagation. Whether the genetic diversity within these black poplars is sufficient for recolonization of river banks and survival of the metapopulation is a question for further research.