Gene flow in a highly dynamic habitat and a single founder event: Proof from a plant population on a relocated river site

Abstract River relocations due to realization of infrastructure projects or flood protection provide a high potential for ecological restorations and the creation of near-natural habitats. Colonization success of new riparian habitats in the dynamic riverine zone is rarely predictable, as the survival of sessile plants is highly dependent on flood events. Relocated rivers offer a unique opportunity to study restoration success, and using genetic information allows tracing to source populations and assessing connectivity. This study focuses on the colonization of a new, 3.4 km long river stretch in the Inn catchment (Switzerland), with an artificial, stable shoreline and a dynamic riverine zone with gravel bars. We assess the colonization success of Myricaria germanica, a flagship species for floodplain pioneer vegetation, 14 years after river relocation. The population with over 600 individuals of which 147 were used for genetic analysis based on 22 microsatellite markers in comparison to 11 potential source populations up- and downstream of the new site. Our results on demography and genetic composition together with information on flood events allow tracing the origin of the subpopulation along the stable shoreline to a single founder event but several founder individuals further upstream. The subpopulation in the dynamic zone is younger and more likely strongly dependent on gene flow from the stable shoreline subpopulation, but allows for rejuvenation at the site. Genetic patterns along the catchment indicate ongoing gene flow, suggesting potential for colonization success for further restorations in the catchment. Our data reveals that near-natural flood dynamics with repeated larger flood events is a key factor for successful colonization of dynamic riparian habitats.