Ash Dieback – the threat to our non-woodland trees

Logo: The Tree Council

Jon Stokes, The Tree Council

For over 40 years The Tree Council has campaigned for non-woodland trees, and particularly hedge trees, with the help of its 8,000 Tree Wardens and more than 150 member organisations.

Trees are vulnerable to many different threats which is why we need an ongoing national strategic approach to monitoring, caring and safeguarding trees if we are to continue to benefit from them. One of these threats is the arrival of Ash Dieback which has increased the risks to the UK’s non-woodland treescape in a way not seen since Dutch Elm Disease in the 1960’s and 1970’s.

Ash landscapes - The Cotswolds (Tree Council)
Ash landscapes - The Cotswolds (Tree Council)

Ash Dieback is caused by a fungus called Hymenoscyphus fraxineus [synonym – Hymenoscyphus pseudoalbidus (anamorph - Chalara fraxinea)] that arrived from Asia into Europe during the 1990’s. This invasive fungus causes the death or dieback of Fraxinus excelsior trees and other Fraxinus species. For simplicity, this article will refer to Ash Dieback.

Ash Dieback was first recognized in the UK in 2012.

The late Professor Oliver Rackham, in his book ‘The Ash Tree’ (Footnote 1) states, ‘There are nearly as many ash trees in Britain as there are people’, to then reflect, ‘…but what does that statement mean? Like most statistics, it is hedged about with problems of definition (how big does a little ash tree have to get before it is counted?)’.

Having reviewed the available data, The Tree Council suggests that there are 27.2 – 60 million ash trees in non‐woodland situations (greater than 4cm diameter at breast height DBH) compared to 125.9 million in British woodlands with an area larger than 0.5 ha, with potentially 2 billion saplings and seedlings in woodlands and non‐woodland situations in the UK.

Ash Dieback affecting young hedgerow tree (Tree Council)
Ash Dieback affecting young hedgerow tree (Tree Council)

Research suggests that in hedges, large trees matter for their structural presence, microclimate, shelter and shade, as well as individual species providing a specific food source. It is important simply that a mature tree should exist (except for some lichen communities where ash is the favoured host). As ash is the most common hedgerow tree, many of which are mature, wide‐scale loss of ash will severely impact the ecological value of the UK’s landscape.

The potential loss of so many trees in a wide range of habitats means landscape changes at both the macro and micro level. From individual gardens or streetscapes, to the loss of swathes of hedgerow trees or small copses, the impact of the disease will be visible. Non‐woodland ash, as hedgerow trees or parkland, are referenced as the defining feature of the landscape in 40% of the 159 local Natural Character Area descriptions of England (Footnote 2).

In 2014 The Tree Council was asked by Defra to ‘investigate non‐woodland ash numbers and the potential impacts that may occur as a result of the spread of Chalara / Ash Dieback’. We have been working closely with Fera (the Food and Environment Research Agency) during this work.

Over the last few years we have organised over 30 specific non‐woodland ash meetings attended by over 300 specialist tree professionals. We have also tapped into our parish and community group volunteers by organising another 30 plus events for Tree Wardens.

Ash trees shedding branches and increasing health and  safety risks (Tree Council)
Ash trees shedding branches and increasing health and safety risks (Tree Council)

In addition, key staff from our 150+ Tree Council member organisations have been involved in our work. Finally, we have been working closely with many local authorities across the country developing good practice and case studies.

Our research suggests that the decline or die back of large numbers of non-woodland ash trees poses a number of risks particularly health and safety issues arising from trees by road, rail and in public spaces. In addition, the risk of flooding is increased through changes to the landscape, alongside the loss of biodiversity.

Back in 2015, we recommended the development of ‘Local Action Plans’ as a joined up nationwide response in preparing for the impacts of Ash Dieback (Footnote 3). Now we are working on a ‘Toolkit’ to support the development of these Local Action Plans. The Toolkit will be available from The Tree Council in the early part of 2018 and available on our website for all to use.

But there are many other threats to Britain’s non-woodland trees, besides Ash Dieback. Some of these we have lived with for decades such as Dutch elm disease which still threatens those elms that become large enough for the Scolytus Elm Bark beetles to colonise the tree and carry the fungus to a new host. Other, newer, threats such as Asian Longhorn Beetle (Footnote 4) or Sweet Chestnut Blight (Footnote 5) need control measures and vigilance to ensure that they are contained, neutralised and do not spread.

However, there is a long list of pests and diseases that are currently not found in the UK and which we need to ensure do not reach our shores. For example, the bacteria Xylella fastidiosa which damages and kills a wide range of trees and shrubs. For details on all tree pests - see the UK’s Plant Health Risk Register at      

Signs of ash dieback in the twigs (Tree Council)
Signs of ash dieback in the twigs (Tree Council)

All sectors and individuals with a common interest in trees need to work together strategically, to ensure new pests and diseases do not take a foot hold and devastate our non-woodland tree stock any further.

The Tree Council is a unique nationwide umbrella organisation that brings together professional tree specialists through our member networks, and volunteers, through our Tree Warden networks. During our annual Tree Care Campaign, we encourage the need for better care for all trees. We actively support the work of our members, including our Tree Warden Networks, by raising awareness of pests and disease. Working with these and in consultation with others, we are now close to supporting the development of Local Action Plans through completing a Toolkit to evaluate, monitor and manage the response to Ash Dieback. The value of our parish and community volunteers cannot be overestimated. For example, it was a Norfolk Tree Warden who first spotted and reported Ash Dieback in the wild in 2012.

It is increasingly important that everyone involved with trees is well informed of the symptoms of tree diseases and is constantly vigilant, reporting anything suspicious to the Forestry Commission using their Tree Alert webpage (  

The future for Britain’s non-woodland trees depends on this generation valuing them for their contribution to our environment, to our wellbeing and to the continuing landscape of our country. But this future can only be secured through a fundamental re-evaluation of all the practices and policies used to create and manage a landscape with non-woodland trees in it. Through working with others and bringing together knowledge and insights across sectors, The Tree Council is playing a crucial role to ensure we act now.


1) The Ash Tree, by  Professor Oliver Rackham (Paperback Monographs 2014)

2) National Character Area Profiles: data for local decision making, part of Biodiversity and ecosystems and Landscape, by Natural England (30 September, 2014)

3) The Tree Council ‘Chalara and Non-Woodland Trees’ Report (Forestry Commission, 2015). Available:

4) Asian Longhorn Beetle is a serious tree pest which when found in Kent recently resulted in the removal of 2,166 trees to stop its spread.

5) Sweet Chestnut Blight is a fungal disease that can kill the tree and was recently discovered in London, Kent and Devon.

First published in CJS Focus on Forestry & Arboriculture in association with Ancient Tree Forum on 20 November 2017

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