During the recent decades, the ascomycete Hymenoscyphus fraxineus has been spreading across Europe causing dieback of European ash (Fraxinus excelsior) and threatening the existence of the species. Still, several studies have suggested positive effect of stand diversity on ash health condition. The aim of this study was to evaluate the effect of stand composition and structure on the health condition of ash in young stands in Latvia. Among the tested stand properties, number of shrub species and tree height were the main factors affecting ash health condition, yet their effect differed regionally. In the eastern part of Latvia, richness of shrub species in forest stands showed positive relationship with ash health, while the taller trees displayed worsening health condition. In the western part of Latvia, the relationships were inverse. Such differences in the biotic relationships might be related to the differences in climate, hence spread and development of the pathogen, and/or genetically determined susceptibility of different populations of ash.

Keywords: Fraxinus excelsior; Hymenoscyphus fraxineus, species diversity; dieback; mixed stands; populations.