Growth and wood anatomical adjustments of Fraxinus excelsior to the infestation of the invasive fungus Hymenoscyphus fraxineus

Since the 1990s the invasive fungus Hymenoscyphus fraxineus has led to severe crown dieback and high mortality rates in Fraxinus excelsior in Europe. In addition to a strong genetic control of tolerance to the fungus, previous studies have found high landscape variability in the severity of dieback symptoms. However, apart from heat and humidity-related climate conditions favoring fungal development the mechanistic understanding of why smaller or slower growing trees are more susceptible to dieback remains less well understood.

Here, we analyzed three stands in Switzerland with a unique setting of eight years of intra-annual diameter growth and annual crown health assessments, together with ring-width and quantitative wood anatomical measurements preceding the monitoring, to investigate if wood anatomical adjustments can help better explaining the size-related dieback phenomenon.

We found that slower growing trees or trees with smaller crowns already before the arrival of the fungus were more susceptible to dieback and mortality. We show that defoliation directly reduces growth as well as maximum earlywood vessel size, and that the positive relationship between vessel size and growth rate causes a positive feedback amplifying crown dieback. Because leaf necrosis happens during late summer when ring formation has already finished, photosynthesis is heavily reduced during a time when non-structural carbohydrates (NSCs, sugars and starch) are stored. Thus, we hypothesize that a lack of NSCs (mainly sugars) leads to lower turgor pressure and smaller earlywood vessels in the next year impeding efficient water transport and photosynthesis, and is responsible why smaller and slower growing trees show stronger symptoms of dieback and higher mortality rates.