Floodplain willows in fragmented river landscapes: Understanding spatio-temporal genetic patterns as a basis for restoration plantings

In Central Europe, floodplain fragmentation by dykes and river regulation by dams has led to a fundamental loss of riparian softwood forests and dramatically restricted natural regeneration of the Salicaceae species involved. As a consequence, reforestation is one of the widely-used measures to maintain this valuable vegetation type. Knowledge about the genetic population structure of existing stands of willows and poplars is necessary to preserve their genetic potential in the long term. In this study, we investigated clonal and genetic diversity patterns of Salix viminalis for 23 stands, taking into account young and old age classes and locations in front of the dykes in the active floodplain and behind the dykes in the inactive floodplain along the Elbe River, Germany. Microsatellites were used to assess effects of stand and floodplain fragmentation and to evaluate the extent of clonal structures. We detected considerable numbers of multi-ramet-clones almost exclusively in the active floodplain with old stands exhibiting the largest number of these clones. Individual clones mostly covered distances of less than 10 m, but single long distance dispersal events of vegetative propagules were also detected. Genetic diversity patterns revealed one continuous population although a certain small-scale spatial genetic structure was identified. Our study suggests that riparian softwood forest species have efficient dispersal strategies which allow the population to survive altered floodplain conditions from a genetic point of view. Management advice is given for selecting genetically suitable planting material for restoration purposes.