Fine-scale gene flow and genetic structure in a relic Ulmus laevis population at its northern range

Ulmus laevis is mainly distributed in Central and Eastern Europe. The present study took place in Southern Denmark, which together with southern Sweden and Finland, is the Northern range of the species distribution area. The study site (Krenkerup Haveskov) is the only presumed natural continuous population of U. laevis in Denmark. The forest is a part of a formerly larger carr forest. Scattered trees (U. laevis) are found in nearby woods. With seven polymorphic microsatellite loci, we revealed a fairly low genetic diversity in the parental generation with 2-7 alleles per locus and average gene diversity (H (e)) = 0.5. There were no signs of a recent population decrease in U. laevis from Denmark. In contrast, the only known larger population of U. laevis from the Netherlands showed significant genetic signals of a recent bottleneck. The outcrossing rate was not significantly different from 1, indicating absence of self-pollination. Gene flow was found between the continuous population and trees in the nearby woods. We found significant spatial genetic structure which may be due to short dispersal distances of the winged fruits of U. laevis. Due to the low genetic diversity, the strong spatial genetic structure and the outcrossing nature of the species, it may be especially vulnerable to size reductions.