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Chalara dieback - Managing ash trees and woodland

Practical advice

The recent appearance of the Hymenoscyphus fraxineus fungus in Britain has meant that the future of common ash (Fraxinus excelsior) as a woodland, hedgerow and urban tree species is uncertain. This document provides practical advice on slowing the impact of the disease for anybody with a responsibility for the management of ash, whether in woodlands, parks and gardens, or individual trees. It is based on the expert knowledge of UK researchers and practitioners, and is informed by experience from Europe, where the disease has been established for more than a decade. Evidence on management is not extensive, and action will depend very much on owners’ and managers’ objectives.  The advice given is therefore neither exhaustive nor prescriptive. This Question and Answer document will be revised and republished as our knowledge and understanding of the disease develops.

1. Principles 

The principles underlying this guidance are:

  • maintaining the values and benefits associated with ash woodlands and iconic trees;
  • securing an economic return where timber production is an important objective;
  • reducing the presence and rate of spread of Chalara dieback;
  • maintaining as much genetic diversity in ash trees as possible with the aim of ensuring the presence of ash in the long term; and
  • minimising impacts on associated species and wider biodiversity.


2. Management options

There is currently no cure for Chalara, and no clear method for stopping its spread. Therefore the aim of management, as outlined in the Chalara Control Plan, should be to slow the spread and lessen the impact of the disease.

Before making any changes to existing management regimes, owners and managers should carefully consider their objectives and local circumstances. Any woodland or individual tree can bring a variety of benefits, and be managed for those multiple benefits. The categorisation below is made purely to assist the owner or the manager when thinking about what to do next - in practice a hybrid approach might be appropriate.


3. Managing the risk to the public from dead and dying trees

Public safety is likely to be one the biggest management issues for owners of ash trees in woodlands, parks, roadsides etc as the disease kills or weakens trees over the coming years.

Trees in areas with high levels of public access need to be monitored carefully for risks to public safety, and some felling or pruning of dead or dying trees is advisable if risk assessments show they are a hazard.

Further information about tree safety is available in the publication Common sense risk management of trees


4. Reducing the impact of any future Chalara infection on a timber crop

For uninfected stands, the best way to slow the impact of any future Chalara infection is to promote fast, healthy growth of selected trees. This will not prevent the onset of the disease if spores are present, but will maximise the timber value at the time of felling. Ensure high standards of establishment and silviculture - guidance on this can be found in 'Managing Native Broadleaved Woodland', (£30 from The Stationery Office); or in 'Growing Broadleaves for Timber', (£13.50 from Forestry Commission Publications - quote stock code FCHB009). This should be carried out in combination with:

  • adhering to biosecurity measures to reduce the spread of Chalara; and
  • regular monitoring for signs of Chalara infection, both in mature stands and, particularly, on planting which has occurred in the past five years.


5. Options for Chalara-infected timber crops

a) Younger stands (i.e. up to pole stage)

Younger trees are the most vulnerable to the disease, and are usually rapidly killed. If disease levels are low, selective thinning of diseased and suppressed trees is recommended.

If more than 50 per cent of the mature ash in the stand is infected, the annual rate of spore production will be very high. The economic value and condition of the trees will decline rapidly, and therefore you might wish to take the decision to realise that value. Felling all the ash should be considered to allow regeneration, but you might wish to leave some trees which are close to dying to provide deadwood and biodiversity in the stand. Then:

  • if the stand is a mixture of species, and there are enough trees of other species to form a closed stand within 10 years, it is likely that management objectives can still be achieved without replanting after felling the ash; and
  • if the stand is a mixture and there are NOT enough trees of other species to form a closed stand within 10 years, it is likely that the stand will have to be regenerated after felling by planting alternative species.(See below for guidance.)

If the stand consists of pure ash you will need to consider what alternative species would do well on the site. (See No. 16 below)

b) Older stands

An individual-tree approach is recommended for older stands with infected trees.

Where more than 50 per cent of the crown is infected, and where survival of the tree depends on epicormic shoots, felling should be considered because their economic value is declining, they have become seriously infected, and they will be producing large volumes of spores, which will infect other trees.

Where less than 50 per cent of the crown is infected, trees should be regularly monitored as described above. However, managers should also assess the risk of Armillaria (honey fungus) attack. This is often the ultimate cause of death of ash trees once they are infected with Chalara. Felling should be considered if Armillaria is present on the site and timber production is an important objective.


6. Preserving the environmental benefits of  woodland

A lower level of active management might be your best option where preserving environmental benefits is the key objective. However, you should consider planning to retain ash as a component for as long as possible to provide habitat for those species dependant on ash trees, and allow time for resistant strains to be identified. In general, ash woodlands of high environmental benefit also include a mixture of other tree species which will secure many of the same environmental benefits, albeit with a loss of diversity if ash cannot be retained. However, upland ashwoods could decline in environmental benefits if appropriate interventions are not made. Further guidance will be made available when the impacts on dependant species are better understood and mitigation options have been considered.

Taking no action will:

  • eventually reduce the proportion of ash in the woodland;
  • increase the amount of deadwood (standing and on the ground);
  • allow resistant trees to be identified; and
  • lead to increased spore production from the woodland

A more proactive management approach will:

  • help to reduce spore levels and the rate of onward infection;
  • open up the woodland to allow natural regeneration to take place;
  • let more light into the stand; and
  • encourage regeneration and structural diversity.

For very small, isolated woodlands, the management of leaf litter during the autumn and winter will help to reduce re-infection levels the following year. However, this can be very time consuming and expensive on anything other than a small scale, and does not cure the disease, but it might buy some time.


7. Priority or protected species on site

If there are rare, threatened or protected species with a particular requirement for ash to be present, specific advice might be needed for that site to maintain them. Current advice recommends a presumption against felling mature ash trees, with which some European Protected Species are associated. However, impact on protected species should be considered in all forest operations. 

Protected species guidance.


8. Protecting valuable veteran ash trees, pollarded ash, or ash trees in an urban setting

Currently the only effective option to reduce spread of the disease is to remove all ash leaf litter from around the trees in the autumn and winter to reduce the local source of spores the following summer. There is some evidence from continental Europe that leaf removal, possibly coupled with the lower humidity levels in parkland and urban tree environments, can significantly reduce and slow the impact of Chalara.

Safe disposal of the leaves on site can be achieved by burning, burial or composting, although in some areas and circumstances disposal might need to be undertaken by a local authority.

Managing ash leaf litter

Urban and veteran ash trees should be surveyed to establish the level of infection present, and the disease status of the tree should, ideally, be assessed by a professional before agreeing any work programme.

Where no infection is present or suspected, any routine, planned work on ash trees should continue. The timber and brushwood can be removed, chipped and processed as usual. Best practice, however, would include burning, burying or composting the foliage or twigs on site where possible. The ecological benefits of leaving deadwood on site should be noted (UK Forestry Standard, Forests and Biodiversity Guidelines)

For pollarded trees the current recommendations from the Swedish authorities, where there is a long history of pollarding, is to avoid all restoration cutting of old pollarded ash trees for the time being if there is not an acute risk that they will fall apart. However, for both healthy and infected ash trees which have been pollarded regularly, pollarding should continue until such time as we know more. If possible avoid pollarding all trees in the same year, but spread the pollarding out over several years. It is very important to revisit these trees and assess the impact of ash dieback.


9. Infected urban or veteran trees

There should not be a presumption to fell these trees. Veteran trees in particular can provide many important environmental and social benefits, even when dead. Any work on a tree should be undertaken after a risk assessment, which should consider age, condition, the number of other trees in the locality and their species, the potential risk of further infection, and the danger to the public. The cost of taking or not taking action is also likely to be a factor in any final decision.

Where infected trees are pruned or felled and there is a Statutory Plant Health Notice on the site, ash logs or firewood may only be moved off the site with authority from the Forestry Commission. Twigs and leaves should remain on site, and be burned, buried or composted if practicable. Strict bio-security measures must be employed.


10. Infected trees in hedgerows and rural situations

Hedgerow and rural trees often make significant visual contributions to the landscape, and many also provide environmental benefits. Public safety and cost are likely to be key considerations in managing these trees, and expert advice is advisable.

Management options will depend on the trees’ situations and locations, and the environmental and social benefits they bring. Managers are therefore advised to consider the questions and answers above on preserving environmental benefits and managing veteran and urban/garden trees for guidance.

To ensure continuity of ash standard trees in hedgerows, planting of replacement trees of an alternative species will ensure environmental benefits of hedgerow trees will be preserved.


11. Trimming ash hedges

There is no specific advice about this. However, the guidance for ash trees in parks and gardens above is equally relevant. Safe disposal of the leaves on site can be achieved by burning, burial or composting, although in some areas and circumstances disposal might need to be undertaken by a local authority.

Managing ash leaf litter

However, we recognise that this is unlikely to be an option for hedges in the countryside, particularly those where the hedge is cut using a tractor with a mechanical flail. At present there is no practical control strategy in these instances. If a hedge requires cutting, it is probably better to do so rather than neglect it. It is best to avoid cutting during the summer months and during the bird-breeding season, when the spore production from dead leaves on the ground is highest, and disturbance might increase dispersal. Further advice on hedgerow management can be found at the Natural England website.


12. Increasing resilience of woodland to chalara

The single best strategy is to increase the genetic and age diversity of your woodland. Developing stands of mixed species should make your woodland less vulnerable to disease, and adopting a continuous-cover approach, where practicable, is one way to promote higher levels of species and age diversity.


13.  Encouraging ash regeneration

Planting ash is currently not possible because of the restrictions on moving planting material, but it would not be recommended even if these restrictions were lifted.

Natural regeneration is the preferred method of replacing ash stands. Tolerance of Chalara dieback of ash is likely to be highly heritable, so natural regeneration from tolerant trees is the preferred option for replacing the species in areas which retain sexually mature trees (more than 30 or 40 years old). 

Guidance on the successful use of natural regeneration can be found in  'Managing Native Broadleaved Woodland', (£30 from The Stationery Office) or in 'Growing Broadleaves for Timber' (£13.50 from Forestry Commission Publications - quote stock code FCHB009.)


14. Regeneration from coppice

Regenerating a stand using coppice shoots from felled, infected trees is not recommended. Chalara can be isolated from roots, and it is thought to be highly systemic, so coppice regrowth from the infected trees is also likely to be infected.

If the aim is to reduce infection pressure on a site, or allow space for natural regeneration of ash or planted trees of other species to establish, it is recommended that coppice stumps are killed using approved herbicides. However, in ancient woodlands or woodlands where coppice is an important cultural factor, only do this if coppice stools are infected with Hynenoscyphus fraxineus . For such woodlands, postpone coppicing ash until more information is available – newly planted trees and freshly cut coppice are particularly vulnerable to infection. 


15. Options if there are no tolerant mature ash trees on site

If there are no apparently tolerant mature ash trees left on a mixed-species site, and regeneration has failed, and if there are enough trees of other species to form a closed stand within 10 years, it is likely that your management objectives can still be achieved without carrying out further regeneration.

In other cases the stand should be regenerated by planting alternative species, until Chalara-tolerant strains of ash can be made available.


16. Alternative species for planting

The choice of species must always be guided by management objectives,  site conditions and designation status of the site. For example, on brown earth soils the range of alternative broadleaved species is wide and includes aspen, beech, birch, cherry, field maple, hornbeam, oak, lime, rowan, sweet chestnut and sycamore.

On other sites the choice is much more restricted. For example:

  • on ground-water gleys - alder, aspen, willow and oak are possible alternatives;
  • on rendzinas - beech, birch, field maple, whitebeam, hawthorn, holly and the wayfaring tree could be considered; and
  • if the site is designated for conservation purposes (for example, as an SSSI, SAC or SPA), or is an ancient woodland, advice should be sought from conservation agency advisers.


17. Re-using tree shelters if they have had diseased ash plants

Even if Chalara spores are present they will not pose a risk for any non-ash saplings being grown in the shelters, so re-use on the same site will pose no problems. There is a small risk of transferring the disease to another site, either by moving leaves or spores with the shelters or stakes. If you must use the shelters or stakes on another site, then you should ensure that any leaf litter is fully removed, and a period of at least a month elapsed before moving them. This will ensure that any H. fraxineus spores will have died.



18. Further guidance on species choice

Detailed guidance on species choice in native broadleaved woodland can be found in 'Managing Native Broadleaved Woodland'.

Ecological Site Classification (ESC) can also be used to investigate the suitability of species to a site, and will help managers consider the options with regard to longer-term climate change. See our online Decision Support Service.

Non-native species can also be considered for sites with few constraints, using our guidance

It is important to note that some alternative species, such as beech, sycamore and Norway maple, are very susceptible to bark stripping by grey squirrels.


19. Deer control

Deer should always be controlled where establishment by planting or natural regeneration is an aim. This is particularly important when regenerating ash stands infected with Chalara, because it is likely that deer will tend to target healthier, tolerant ash seedlings, rather than susceptible, diseased and dying plants.


20. Logs and firewood

Uninfected sites

Ash wood may continue to be moved from uninfected and infected sites within Great Britain.

However, we strongly recommend the simple precaution of brushing leaf and shoot material from logs, firewood and vehicles before they leave the site, as a precaution against the possibility that the disease might be present, but not obviously apparent, and could be spread unintentionally with logs and firewood.

Infected sites

See the current guidance, Effects of legislation on the timber and firewood trades.


21. Breeding Chalara-tolerant ash

Research in mainland Europe shows that some ash trees show signs of tolerance of Chalara dieback of ash, although so far no individual trees have been found to be totally resistant. However, a few in each population consistently show low levels of infection, and these could be used to create a Chalara-tolerant breeding population for restocking infected areas in the future.

In 2016 an ash tree in a woodland in Norfolk was found in good health, despite being surrounded by heavily infected ash trees, and British scientists have identified the genetic markers for tolerance, raising hopes that it will be possible to breed tolerant ash for the future.

This tolerance is heritable (i.e. it can be passed on between generations), and appears to be due to a suite of genes rather than a single gene. This is important, because the disease tolerance is less likely to break down due to genetic change in the H. fraxineus fungus if the observed tolerance is based on a combination of several genes in ash. It should therefore be possible to breed some degree of tolerance into ash populations, but it will take several years before this will provide planting stock for the market.

Woodland owners in particular can help by:

  • not felling any mature ash trees unless necessary for public safety or timber production reasons; and
  • monitoring their ash trees’ health over the coming years, and reporting to the Forestry Commission or the Living Ash Project any which appear to be little affected by the disease.


22. Chemical control methods

There are currently no fungicides or other chemicals approved for use in UK forests for controlling Hymenoscyphus fraxineus. We are working with manufacturers of some products to identify whether any of them might have value in some situations.

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Last updated: 9th January 2018