Can Danish expertise help save the UK’s ash tree population?

Symptons of ash die back.

By Anders Lorenzen

UK nature lovers and conservationists were devastated when the news broke that the UK’s ash trees are greatly at risk from a virile fungus that has so far killed 90 % of Ash trees in Denmark.

But what hope is there, if any, of salvaging this iconic and ancient tree species? I caught up with two Danish experts on the issue, both of whom are currently engaged in helping the Danish government find a solution to the problem. Here is what they told me:

The ash dieback (Chalara raxinea) is caused by an infectious pathogen (Hymenoscyphus pseudoalbidus) attacking the ash and scientists believe that it enter the trees through it’s leaves, and apart from the leaves also affects the bark and shoots. Young trees might be killed immediately when it enters but older trees are more resistant.

It’s unclear where the disease first originated, but it was first properly spotted in Poland in 1992, followed by a recurrence ten years later in Denmark in 2002. It is feared that the disease could eventually wipe out the entire European ash population.

Despite having experienced such a drastic ash dieback, Denmark is actually being used as a model for UK authorities, who are watching to see whether the disease can be beaten. According to Biologist Ditte Christina Olrik from the Danish Ministry of Environment, there is a glimmer of hope; a small fraction of trees (2.5 %) seem to remain healthy when seed implantation experiments are carried out in heavily infected areas where the majority of the trees are suffer from the fungus. This resilience is genetic, which means that healthy parent trees will produce healthy seeds which can be collected and propagated.

This autumn, Naturstyrelsen* and Copenhagen University visited a series of state and private owned forests in Denmark, to find and mark up healthy ash trees;  they have so far succeeded in finding more than 100 healthy trees across the country which appear to be immune from the dieback. Next steps will be a collection of graft cuttings, propagation, resistance tests and then plantation of the healthy trees in a seed orchard. This is a long term project and we are looking at 15 years, so roughly in 2030 before the first healthy seedlings can be harvested from graft cuttings, which themselves will only be ready to planted out in 2015.

According to the Danish experts, it is very likely that the same resistance will also be found in UK trees, and by applying the same tactics as Denmark has, there will thankfully also be some hope for saving the UK’s ash tree population in the longer term.

* Naturstyrelsen is an arm of the Danish Environment Ministry.

Thanks to Iben Magrethe Thomsen, Senior Adviser in the Forest and Countryside Department at Copenhagen University and Ditte Christina Olrik, Biologist at Naturstyrelsen (an arm of the Danish Environment Ministry) for contributing to this article with their research.

Sub edited by Kirstie Wielandt

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