Ash dieback: why trees will last much longer than people in Britain think

THE British Isles have the highest percentage of ash in their woodlands, compared to the rest of Europe. I can understand why foresters worry about the invasive fungus Hymenoscyphus fraxineus, which causes (novel) ash dieback. Nor am I surprised amenity tree owners like municipalities, the National Trust and, apparently, the Ministry of Defence are girding themselves for battle (Forestry Journal, March 2021).

In Denmark, we also saw the ash dieback panic spread from forestry to tree owners and managers in urban areas and the open landscape. But it was never true that the majority of ash trees were dead or dying, as stated in the FJ March issue (page 32). I really wish people would stop perpetuating this falsehood.

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In addition, we now know the situation is less dire than first assumed. We do observe a progressive decline in health, and damaged ash stands, and we do observe substantially reduced growth rates in affected trees. But our studies have also shown you can find a few healthy ash in most stands. More importantly, most ash trees with moderate crown symptoms stay alive for years, even in forest situations with lots of honey fungus attacks (see Figure 1).

Figure 1: Survival among 299 ash trees (age 18 to 100) monitored from 2010 to 2019. In total, 30 per cent of the trees died over 10 years, but all six plots could still be considered ash stands. At the beginning of the study period 43 per cent of the trees had mild or no crown symptoms, 41 per cent had moderate to obvious crown symptoms, and 15 per cent had severe crown symptoms. Of the latter, less than one third were alive after nine years, but 60 per cent of the obviously affected trees and 90 per cent of the healthy looking trees were still alive. Only 10 per cent of the trees did not have signs of basal stem necroses caused by either Hymenoscyphus fraxineus or Armillaria (honey fungus).

I have worked with ash dieback for almost 20 years and have tried to share my knowledge both nationally and internationally, and in particular with British colleagues since 2015. I have given talks to foresters and amenity tree managers several times, and the message is always the same: Do not panic! But be aware of where and how to deal with sick ash trees.

The problem is always how to get the balance right between the two extremes of ignoring the issue and felling every ash in sight. At the moment, I mainly hear stories of widespread felling, from people in Britain who object. There can be two explanations for this, and they are not mutually exclusive: the visible mortality, combined with the expectation of future mortality.

Figure 2A: Ash roadside trees in Denmark in 2019. The tree on the left is healthy and has never shown any symptoms since monitoring started in 2007. The two ash trees to the right have occasionally exhibited severe symptoms, but in 2017–19 their crowns were badly defoliated for more than one year in a row. In 2017, necroses were also observed at the base of stems. Both sick trees fell over onto the bicycle path in 2020, but luckily no one was hurt.

In our experience, the most damage is seen about 10 years after the disease arrives, which is where Britain is right now. The most susceptible trees are dying, and symptoms are visible in many ash trees. This can lead to the mistaken assumption that you might as well remove all ash with dieback, rather than having to come back a couple of years later to fell more dead trees. Instead, trees with slight and moderate symptoms should be considered as potential survivors for years to come.

In my opinion, there are two main reasons to fell ash trees:

• Harvesting valuable ash in production forests before the timber loses value.
• Preventing accidents when dying ash are a hazard to surroundings.

Figure 2B: Basal necrosis on the central tree in October 2017. In the cross-section honey fungus decay is visible behind the dead bark in March 2020.

The latter issue is likely to cause difficulty and controversy. In some cases, managers are unfairly accused of overreacting, when they are simply doing their duty in removing unstable trees. In other cases, ash trees are felled prematurely or unnecessarily, which is both a waste of money and destructive for the ecosystem. Ask yourself whether it’s not worth keeping an eye on ash trees for a couple of decades and gradually removing dying individuals, to avoid losing thousands of trees without good reason.

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The decision to remove ash trees along roads, train lines, public footpaths and similar places with significant public use should be based on rational observation of important symptoms: severe crown dieback and signs of decay in roots and stem base. In fact, the fungus should simply be considered another stress factor in amenity trees, and not a death warrant like Dutch elm disease. In most cases these simple guidelines for ash tree removal should suffice:

• Trees with over 50 per cent defoliation in at least two subsequent years.
• Trees with severe crown symptoms combined with extensive dead bark at stem base.

Figure 3: Even if there are symptoms of ash dieback on roadside trees, as in the left picture, there is no reason to remove the trees. Removing dead branches will improve both safety and aesthetics. The pollarded ash trees in the right picture are healthy and valuable.

The worse defoliation/dieback and the more bark damage visible at the base of the tree, the faster the trees will deteriorate and become hazards. Roadside ash trees should be removed after a maximum of three years of severe crown symptoms (usually more than 75  per cent defoliation, see Figure 2).

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Dying trees are a danger not only to their surroundings, but also to the people felling the trees. I applaud the decision of the contractor (KWD) in the MOD case to use machinery to deal with sick ash trees. But I still wonder how badly affected those trees really were.

Figure 4: Severely affected ash trees along a busy bicycle path should be dealt with, when symptoms persist for more than one year. Note how epicormic shoots are standing upright along the lower branch, a typical sign of a distressed tree.

I strongly urge amenity tree owners and managers to document the disease development with crown photos each year, especially if you expect to be confronted with questions from the public about the need for felling. Likewise, tree lovers who fear unwarranted ash massacres should takes pictures of trees they want to protect, in order to document their health status and survival potential. Facts, not fear, should fuel the decision to fell or not to fell.

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