Gene flow and reproductive success in ash (Fraxinus excelsior L.) in the face of ash dieback: restoration and conservation

Ash dieback decreases individual reproductive success of ash trees leaving healthy ash overrepresented as seed and pollen parents for next-generation seedlings. Substantial gene flow over hundreds of meters combined with superior fertility of healthy trees creates optimism for the species’ future in European forests. Hymenoscyphus fraxineus is causing high mortality in European ash (Fraxinus excelsior L.). Due to inheritable resistance to the pathogen, natural selection is likely to act in favour of improved resistance in ash forests following natural regeneration. Still, the frequency of healthy trees is low, and the effect of natural selection will depend on survival, reproductive success and the dispersal capacity of healthy trees under natural conditions. We aim to test whether healthy trees contribute more to the next generation and to infer their potential for dispersing progenies across the forested landscape. Using parentage modelling, we estimate mating parameters and dispersal distances of seeds and pollen and compare realised reproductive success of healthy trees to that of unhealthy ones. Healthy trees are overrepresented as the parents of randomly sampled seeds and seedlings in the forest, although that is more pronounced on the female side. We observe long dispersal events and estimate the mean seed and pollen dispersal distances as 67 m and 347 m, respectively. Variation in reproductive success results in selection in favour of lowered susceptibility to ash dieback. The large dispersal capacity decreases the risk of genetic bottlenecks and inbreeding and allows resistant trees to disperse their genes into the neighbourhoods of substantial sizes.