Enderle, R., Bußkamp, J. and Metzler, B. 2017. Growth Performance of Dense Natural Regeneration of Fraxinus excelsior under Attack of the Ash Dieback Agent Hymenoscyphus fraxineus. Baltic Forestry 23(1): 218-228.

   Ash dieback, caused by the ascomycete Hymenoscyphus fraxineus, is a tree disease, which currently devastates European ash populations. Only a very small fraction of ash individuals exhibits a high degree of quantitative genetic resistance and is likely to survive the disease. We investigated the growth performance of differentially diseased saplings in order to assess the impact of ash dieback on individual competitiveness in dense natural ash regeneration. The research took place on three sites in south-western Germany. From summer 2013 to winter 2014 / 2015, 20.4% of the monitored ash saplings died. In general, shorter trees were more severely diseased. There were no differences in shoot length between healthy or moderately infected trees, whereas shoot length was significantly reduced in trees with more than 50% of symptomatic shoots (p ≤ 0.006). These highly impacted trees significantly lost tree height to the disease, whereas only marginal height reduction could be detected for lesser affected trees. Our results indicate that trees resistant enough to maintain at least 50% of their crowns are generally able to survive competition in dense regeneration. Thus, promotion of natural ash regeneration could be an effective measure complementary to breeding for resistance to preserve ash as a tree species in the forests.

Keywords: Ash dieback, Fraxinus excelsior, Hymenoscyphus fraxineus, Competition, Resistance, Tree diseases.