Small-scale spatial genetic structure of Populus nigra 2013 seedlings on a new sandy-gravelly bar appeared after fluvial maintenance operations in the Loire River (France)

Black poplar is a key species of the Loire riverine ecosystem, and in addition to its important genetic intra and interpopulation diversity, represents one biological marker of the sediment morphodynamics and fluvial metamorphosis of the river. This species is threatened by human impacts, notably because of fluvial management works. The main objective of this work is to study small-scale spatial genetic structure of Populus nigra 2013 seedlings on a new sandy-gravelly bar appeared after fluvial maintenance operations in the Loire River (France). Our study focuses in a mosaic of islands (13 hectares) constituting the Conservation Unit Mareau/St-Mesmin of the national programme of conservation of genetic resources of P. nigra. In september 2012, the vegetation (black poplar) of the central island was uprooted and the bar level lowered in order to maintain the flow capacity of the river. During 2013 spring and summer, the fresh sediments of the new island has been regularly surveyed to identify and geocode successive regeneration zones. In September 2013, 31 seedlings of 50 different zones were collected for genetic analysis using 11 microsatellites DNA markers (total 1550 seedlings). Genetic diversity was expressed in terms of : percentage of polymorphic loci, observed and effective number of alleles, observed and expected heterozygosity and genetic differentiation (Fst). Although significantly different from zero, Fst within the 50 regeneration zones is very low and level of genetic diversity is comparable to putative parental populations (mature trees on the river banks). The low level of structure was further investigated using Structure software. The results will be presented and discussed in terms of microtopography, water level fluctuation, phenology of seed dehiscence and sediment texture. Such research work will lead to recommendations for future management of the Loire river, in terms of genetic diversity of Populus nigra populations.