Genetic Divergence in the Natural Regeneration of Black Poplar Along the Vistula River in Poland

Many years of land use transformation within river valleys have drastically changed these ecosystems. Black poplar is a tree species characteristic of riparian habitats, but due to specific environmental requirements, their populations have difficulties with natural regeneration. Here, we genotyped 623 black poplar individuals from four populations located along different sections of the Vistula River. This river, which is the largest in Poland, is characterized by the variable degrees of regulation and transformation of its natural environment. Each black poplar population consisted of a group of mature trees and a group of naturally regenerated trees. Our main goal was to assess the differences between the gene pools of mature trees and natural regenerations. The results showed that all populations were characterized by high and comparable values of the parameters of genetic diversity. Despite this, we distinguished two to three genetic clusters (depending on the method). The genetic differentiation among the natural regeneration groups was almost twice as high as that among the mature tree groups. The gene pools of most natural regenerations diverged from the gene pools of mature tree groups except for one population. This was also the only population where fully generative regeneration occurred, growing in the least transformed middle section of the river. Our research confirms the need to monitor seedlings and saplings along major rivers and to conduct molecular analyses to assess their gene pools. It can be assumed that due to the lack of suitable areas for seed germination, black poplar will reproduce mainly vegetatively, which may be a way to ensure the survival of the species. However, the adaptive potential of the youngest generations is unknown, especially in the face of progressive climate change. We conclude that to preserve black poplar genetic resources, ex situ protection in the form of local clone archives is necessary.