In recent years, the emerging infectious pathogen, Hymenoscyphus
fraxineus, has caused severe ash dieback with high mortality throughout European forests. Common ash (Fraxinus excelsior L.) is a keystone species that supports many
associated species in forest ecosystems. It also produces valuable timber and
commonly planted in European forests. The present thesis studies the genetic
interaction between planted and native ash forests by measuring pollen and seed
dispersals across forests and landscapes. Further, the thesis studies to what
extent ash dieback triggers natural selection in favor of healthy trees based
on the variation in reproductive energy and success. The research is based on a
combination of field observations and DNA analyses using population genetics
approaches.