The impact of ash dieback on veteran trees in southwestern Sweden

Abstract

Ash dieback (Hymenoscyphus fraxineus) is a fungal disease which affects ash throughout Sweden. Monitoring to study of the impact of ash dieback on veteran trees was undertaken in southwest Sweden in 2009, 2011, 2013, 2015, and 2020. The study found that 94.5% of the ash trees observed were affected by ash dieback disease in 2020 compared with 62% in 2009. 70 of the studied ash trees have died (21%) since the monitoring began. In 2009 there was no relationship between girth and ash dieback, but in 2020 the correlation between girth and the impact of ash dieback was statistically significant. In 2020, also for the first time during monitoring, the ash trees in the shade were significantly more affected by ash dieback, compared with trees standing in the open. This difference was not detected in 2013 or 2015. The effect of ash dieback on pollarded trees has varied between the years studied, but in 2020 there is no longer any significant difference between the pollarded and the non-pollarded ash trees. There was however a significant difference in the mortality rates between the groups of trees, with ash trees pollarded in more recent times having the highest mortality. Therefore, the recommendation in relation to veteran trees with ash dieback is that all pruning on veteran ash trees should be avoided. Pollarding should only be done on ash pollards that are in a regular cutting cycle and are not showing any symptoms of ash dieback. If possible, clear around old ash trees if they are in shaded conditions. Given that there are relatively few studies on the impact of ash dieback on veteran ash trees, the results of this study should also be relevant outside of Sweden and for the management of ash trees in non-woodland situations. Key words: Ash dieback, pollards, veteran trees, Hymenoscyphus fraxineus, ash

Neighbours matter and the weak succumb: Ash dieback infection is more severe in ash trees with fewer conspecific neighbours and lower prior growth rate

“…This provides robust evidence to answer yes to the second question: trees in a more vigorous condition had greater resistance to the disease. This result is compatible with earlier studies based on less precise evidence that showed a negative relationship between the severity of ADB and tree 'vitality' in Scandinavia (Bengtsson et al, 2021;Timmermann et al, 2017), and the conclusion of Skovsgaard et al (2017) that vigorous trees can better compensate for the effects of H. fraxineus. Previous studies have also shown that tree height can be a significantly strong predictor of lower rates of ADB defoliation (Erfmeier et al, 2019), as well as individual tree vigour (Dobbertin, 2005), and our result that lower ADB severity is significantly associated with a larger diameter at breast height matches the findings of previous studies (e.g.…”