Marçais, B., Husson, C., Caël, O., Dowkiw, A., Saintonge, F.-X., Delahaye, L., Collet, C. and Chandelier, A. 2017. Estimation of Ash Mortality Induced by Hymenoscyphus fraxineus in France and Belgium. Baltic Forestry 23(1): 159-167.

   Ash dieback induced by Hymenoscyphus fraxineus has emerged as one of the most serious health problem for European forests in the last ten years. However, precise estimation of the mortality induced by the pathogen is still scarce and this hampers management of affected stands. In this work, we used data of several surveys done since 2010 in France and Belgium to estimate the mortality rate associated with ash decline depending on the time of the pathogen presence in the area; for that a 2 steps procedure was used. First, we did an estimation of the frequency and severity of collar lesions associated with H. fraxineus depending on the length of the pathogen presence and for 2 trees size classes (lower or higher than 25 cm dbh). Then the annual mortality rate was estimated depending on collar lesion severity, dbh class (lower or higher than 25 cm) and time since pathogen presence. The global mortality induced by H. fraxineus was computed from those 2 types of data by a bootstrap approach. Additionally one survey observing young stands was used from which mortality was computed directly. We find that if mortality is drastic in very young ash stand affected by H. fraxineus (less than 5 cm dbh), with annual mortality reaching 35% 5-6 years after arrival of the pathogen in the stand, it is much more moderate for trees with dbh above 25 cm, with annual mortality reaching 3.2% after 8-9 years of pathogen presence. Annual mortality rates are intermediate for trees in the 5-25 cm dbh class and reached 10-11% after 8-9 years of pathogen presence.

Keywords: Ash dieback, Fraxinus, mortality, invasive pathogen.