Abstract
Since the 1990s the invasive fungus Hymenoscyphus fraxineus has been threatening European ash (Fraxinus excelsior), a tree species with high ecological and economic importance. This pathogen is causing severe crown dieback, leading to high mortality rates across Europe and is present in Switzerland since 2008. In this study, we provide a comprehensive overview of the temporal evolution of crown damage and mortality rates in Switzerland over the 2009–2019 period. Harnessing the power of the annualized design of the Swiss national forest inventory (NFI), we show that annual mortality rates (AMRs) of ash increased significantly since the arrival of the fungus, with stronger effects in small trees [<26 cm diameter at breast height (DBH)]. Mortality modeling confirmed a size and growth-rate dependent mortality probability (MP). It also revealed that stands with higher humidity – either through higher mean annual precipitation or more humid soil conditions – showed also increased MP. Decreasing host abundance with increasing elevation was also associated with lower MP. Special ash surveys performed over the last 10 years still show a large percentage of ash trees with very low defoliation. This gives hope to finding possible tolerant or resistant trees for (inter-)national breeding programs. In the mean-time our results reinforce previously published management guidelines to promote not only healthy big trees, but also healthy and fast-growing young trees in more open stands for long-term conservation of ash in Europe.
Highlights
Invasive species represent an increasing challenge for forest and tree health globally (Holmes et al, 2009; Moser et al, 2009; Pyšek and Richardson, 2010; Trumbore et al, 2015)
We provided a comprehensive overview of crown damage and mortality rates of ash trees in Switzerland over the past 11 years
For the first time we provide a national-wide and geographically unbiased assessment of mortality rates before and after the arrival of H. fraxineus harnessing the power of the annualized design of the Swiss national forest inventory (NFI)
Summary
Invasive species represent an increasing challenge for forest and tree health globally (Holmes et al, 2009; Moser et al, 2009; Pyšek and Richardson, 2010; Trumbore et al, 2015). Some invasive pathogens can lead to near extinction of tree species and threaten whole ecosystems and their functioning (Pautasso et al, 2015). A prominent example is Ash dieback (ADB) in Europe. ADB is caused by the ascomycete Hymenoscyphus fraxineus (Baral et al, 2014) native to Eastern Asia (NE China, Korea, and Japan). Since the disease has spread fast (around 75 km/year) and is threatening European ash (Fraxinus excelsior) populations throughout Europe (Gross et al, 2014). The wind-dispersed pathogen spreads through ascospores, formed on old rachises of ash in the litter, that infect leaves in summer and cause necrotic lesions that lead to early leaf wilting (Gross et al, 2014). The fungus may further spread to the xylem through the petiole– shoot junction leading to dieback of shoots and twigs (Hanácková et al, 2017). Repeated annual infections disrupt nutrient and water transport and lead to gradual crown dieback and the tree’s death
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