Gene flow and mating patterns in individuals of wych elm (Ulmus glabra) in forest and open land after the influence of Dutch elm disease

Dutch elm disease has severely reduced the number of large trees of U. glabra in Denmark. Consequently, the distance between large trees has increased and the overall density of the species has decreased. Patches of small trees with stem diameters up to 10 cm are, however, relatively frequent. With four microsatellites we studied potential differences in genetic diversity, mating patterns and pollen flow in trees of U. glabra that occur either in a continuous forest (Suserup Forest) or isolated in the open land. We found no indications of selfing in forest or open land but indications of biparental inbreeding in offspring of isolated trees. Estimates of effective pollen donors (N _ep) and minimum number of pollen donors (N _p) were alike in forest and open land (N _ep of 31 and 34 and N _p of 4 to >6 and 3 to >6, respectively). The number of alleles was also very similar. With indirect methods we found that average pollen dispersal was 104 m under forest conditions. The average distance between the isolated trees and their potential pollen donors was further, thus suggesting that effective pollen in the open land on average moves further than in a dense forest. Finally, 28% of small trees (diameters up to 10 cm) produced fruits. Reproduction at a young age may be a key stone in the survival of U. glabra as the vectors of the disease prefer older trees.