Spread of Hymenoscyphus fraxineus in Latvia: Analysis based on Dynamics of Young Ash Stands

In Latvia, during the last 15 years (2000–2015), the area of common ash Fraxinus excelsior forest stands has decreased by 40.6%. The dieback was predominantly caused by the fungal pathogen Hymenoscyphus fraxineus. Mostly young stands (up to 40 years old) were affected, accounting for 77.3% of the area of dieback. In this paper, we analysed the dynamics of young ash stand area within nature regions in Latvia to attempt to determine patterns of spread and the major migration routes of H. fraxineus. As suggested by the available data, the invasion of the fungal pathogen began in the southern part of Latvia, and then gradually dispersed across the country. The largest decline of young ash stands, during the period 2000–2010, occurred in lowlands. According to our estimate, the average rate of dispersal in Latvia was 40 km per year. At the scale of Latvia, the dispersal routes of fungal pathogen H. fraxineus largely coincide with the major migration corridors of biota and are related to macro-relief forms and their configuration.