Background and Purpose: Narrow-leaved ash (Fraxinus angustifolia Vahl) is one of the most valuable forest species in Croatia, both economically and ecologically. Although knowledge about genetic variability is important prerequisite for sustainable forest management and conservation, it is scarce in this species. Therefore the aim of this study is to increase knowledge about narrow-leaved ash genetic variability in part of its distribution range in Croatia. Materials and Methods: Seeds were collected from 121 trees originated from 11 Croatian populations, one population in Spain and one population in Ukraine. Seeds were sown separately for each open-pollinated family in plastic containers which were randomly distributed in a nursery located in Zagreb. Plant heights were measured, and flushing was assessed using a six point scale. Statistical analysis was done using SAS software package. GLM procedure was used for analysi's of variance to study the among- and within-population variation. Variance components were calculated by MIXED procedure. Turkey-Kramer test was conducted to assess differences beween studied populations. Results and Conclusions: There were statistically significant differences between populations as well as between families within population, except for the first growing season height, indicating between-population genetic differentiation for studied traits. Populations from Metkovic and Stara Gradiska showed the greatest differentiation. There were no differentiation between populations originating from Posavina (the Sava liver valley), except for the population from Stara Gradiska. This can be explained by continuous distribution and gene flow in that part of the species distribution. In contrast, populations that originated from submediteranean area showed differentiation probably caused by genetic drift. Genetic drift could also be an explanation for differentiation of the population from Stara Gradiska, although another possibility for its genetic differentiation could be selection against early bud burst due to frequent spring frosts in the area. There was no correlation between phenology and growth traits in nursery conditions.
