Distance-dependent mating but considerable pollen immigration in an isolated Quercus rubra planting in Germany

Gene flow affects the genetic diversity and structure of tree species and can be influenced by stress related to changing climatic conditions. The study of tree species planted in locations outside their natural range, such as arboreta or botanical gardens, allows us to analyse the effect of severe fragmentation on patterns and distances of gene flow. Paternity analysis based on microsatellite marker genotyping was used to analyse how fragmentation affects gene flow among individuals of Quercus rubra L. distributed in a small isolated group of trees (15 trees) planted in the arboretum on the North Campus of the University of Göttingen. For paternity analysis, 365 seedlings from four seed parents were selected and genotyped using 16 microsatellites. The analysis revealed the majority of pollen (84.89%) originated from trees within the site and identified three large full-sib families consisting of 145, 63 and 51 full-sibs. The average pollen dispersal distance for the four seed parents ranged from 17.3 to 103.6 meters. We observed substantial genetic differentiation among effective pollen clouds of the four seed parents (G’’_ST = 0.407) as a result of cross pollination between neighboring trees. No self-fertilization was observed. Gene dispersal via pollen followed the expected distance-dependent pattern, and we observed a significant influx of external pollen (15.11%, ranging from 8.64 to 26.26% for individual seed parents) from a diverse set of donors (30). Long-distance pollen dispersal could explain the presence of significant genetic variation even in isolated natural Q. rubra populations.