HISTORICAL AND CONTEMPORARY MATING PATTERNS IN REMNANT POPULATIONS OF THE FOREST TREE FRAXINUS EXCELSIOR L.

Genetic variation at microsatellite markers was used to quantify genetic structure and mating behavior in a severely fragmented population of the wind‐pollinated, wind‐dispersed temperate tree Fraxinus excelsior in a deforested catchment in Scotland. Remnants maintain high levels of genetic diversity, comparable with those reported for continuous populations in southeastern Europe, and show low interpopulation differentiation (θ= 0.080), indicating that historical gene exchange has not been limited (Nm=3.48). We estimated from seeds collected from all trees producing fruits in three of five remnants that F. excelsior is predominantly outcrossing (tm = 0.971 ± 0.028). Use of a neighborhood model approach to describe the relative contribution of local and long‐distance pollen dispersal indicates that pollen gene flow into each of the three remnants is extensive (46–95%) and pollen dispersal has two components. The first is very localized and restricted to tens of meters around the mother trees. The second is a long distance component with dispersal occurring over several kilometers. Effective dispersal distances, accounting for the distance and directionality to mother trees of sampled pollen donors, average 328 m and are greater than values reported for a continuous population. These results suggest that the opening of the landscape facilitates airborne pollen movement and may alleviate the expected detrimental genetic effects of fragmentation.