Aims of this dissertation were to determine genetic variation of narrow-leaved ash populations across divergent habitats in Croatia and Europe, to explore the influence of ecological factors and climate changes on genetic variation, and to examine the degree and mechanisms of hybridization with common ash. In Croatia, significantly lower genetic diversity and higher differentiation was revealed in the Mediterranean region when compared to the Continental region. In addition, genetic variation was significantly correlated with the environmental variation. Results suggest that environmental heterogeneity may promote genetic and ecological divergence of populations, that known populations in Croatia may represent two divergent ecotypes and confirm an important role of ecological factors in shaping genetic variation. A bidirectional cline of genetic diversity was revealed across Europe: it was declining significantly from West to East and from North to South. Climate warming may enable northwards expansion of F. angustifolia and its hybrids. Consequently, ongoing climate changes may negatively affect the overall genetic diversity and possibly adaptive potential of this species. Results indicate that refugia from climate change are potentially located in the northerly parts of the current distribution, where core high-diversity populations occur and suitable habitat is predicted to remain stable under future climate. Hybrid populations between F. angustifolia and F. excelsior are mostly found in areas where the niches of the two species are predicted to overlap. Number of days of frost in January, summer precipitation and summer temperature can potentially limit the extent of hybrids.