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ASH DIEBACK

Executive Summary

Ash dieback, now widespread across Scotland, is a highly destructive disease of ash trees which causes canopy decline, stem defects and significant tree mortality.

However, the hidden impacts on wood structure of an infected tree are the key to the rate of decline and the risks that the tree poses. As the disease spreads within the tree, it will be deprived of water and nutrition. Branches will then become brittle, die and fall.

The Health and Safety Executive (HSE) has acknowledged that tree works have one of the highest accident incident rates. Anyone planning, managing, or undertaking work on diseased ash trees should understand that they pose increased risk.

As we learn more about the impacts of ash dieback on our treescapes, it is important to share knowledge that can improve our collective response. In anticipation of a greater number of ash trees requiring removal across Scotland over the coming decade, this ‘Ash Works’ guidance has been created to assist arborists, contractors, forest and woodland managers, local authorities and other agencies having to deal with infected ash trees.

It draws upon experience from professionals already working on these trees and outlines how some tree work methods have been adapted to reduce risk to the people working on them. It is not designed to provide a definitive method on how to deal with the felling of infected ash trees and should be used to complement existing risk assessments and health and safety practices.

Dying ash trees can present health and safety risks for both the road users and those managing the infected trees.

Specific risks associated with ash dieback include:

• Infected ash trees are reported to be increasingly brittle and behaving differently to healthy trees, and even trees with early-stage infections are increasingly unpredictable.

• As ash dieback progresses within the tree, working on diseased ash becomes increasingly hazardous.

• As the canopy condition declines, diminished hinge integrity in infected trees is also making them less predictable to fell safely and increases the risk of potentially fatal incidents without additional precautions.

• As the canopy declines, climbing trees with rope and harness cannot be recommended due to the increased likelihood of anchor point failure.

• Vibrations created when working under infected ash can lead to deadwood falling from canopy.

• Typical working methods e.g. the use of wedges, may now increase risk of limb and stem failure.

As a result of these risks, increased use of mechanisation is strongly recommended during the felling operation to reduce the likelihood of harm to operators. Work on an infected tree is much safer when undertaken by machinery with Falling Object Protection Structures including cranes, grapple saws, harvesting heads and Mobile Elevated Work Platforms that can protect the operator.

Chainsaw operations and tree climbing with ropes and harness, should be kept to a minimum and only carried out by the most competent, skilled and well-equipped operators. This guidance outlines changes to the conventional layout and organisation of the work area which are also becoming necessary to protect all parties from the risk of falling objects, including increased working distances, felling in the direction of the tree’s lean, and larger escape routes.

It is important to note that these points are for guidance purposes only and must not be relied upon as each tree, and the risks it poses, must be professionally assessed on site.

Introduction

Ash dieback and its implications

Ash dieback is a highly destructive disease of ash trees caused by a fungus called Hymenoscyphus fraxineus (previously called Chalara fraxinea). It causes leaf loss, canopy decline and embrittlement of the wood in the trunk and branches, increasing the hazards posed by the declining tree. It can kill young and coppiced ash trees quite quickly e.g. within a season. Older trees appear to resist it for some time, until prolonged exposure or a secondary infection caused by another pest or pathogen, such as Inonotus hispidus, attacks them in their weakened state eventually causing them to succumb. The disease was first officially recorded in the UK in 2012 and is now widespread across Scotland, England, and Wales. The effects of H. fraxineus infection can be seen in an ash tree’s crown condition and in stem defects. However, crown condition alone cannot be taken as a measure of the hidden impacts on wood structure. As the condition of ash trees visibly declines, this is leading to an increase in tree management work required on ash across the UK. Understanding how the disease physically affects trees is vital, for risk assessment and management – helping to protect both the individuals working on the trees and the wider public.

As we learn more about the progression of ash dieback within a tree, it is important to share knowledge that can improve our collective response and safeguard tree workers. It is, therefore, up to all of us to play our part in protecting ash where possible and to undertake tree work as safely and sensitively as possible. In the preparation of this guidance note, interviews were conducted with a range of contractors who are dealing daily with ash dieback infected trees. They shared practical considerations for others having to work with these increasingly dangerous and unpredictable trees.

This guidance outlines some of the typical tree work methods used in areas affected by ash dieback. It is important to note that these points are for guidance purposes only and must not be relied upon as each tree, and the risks it poses, must be professionally assessed on site.

Monitoring the progression of ash dieback is vital to help protect those working on the trees and the wider public.

Signs and indicators of ash dieback Section 1

An ash tree infected with ash dieback becomes ‘stressed’. Its outer crown starts to die back. It will lose some leaves, and other symptoms can sometimes include diamond shaped lesions around small branches and twigs and / or dark patches called ‘basal lesions’ at the base of its trunk. Infected ash can display some or all of these symptoms to varying degrees. More details and images can be found in Appendix 1

Ash is a ripewood species and has fewer preserving chemicals laid down in the woody tissue than heartwood species and is therefore less able to resist decay fungi. As the disease spreads, the tree will be deprived of water and nutrition. Branches will then become brittle, die and fall. Eventually the disease may kill the tree or make it susceptible to other pathogenic organisms like the shaggy bracket fungus, Inonotus hispidus, that can lead to trunk / stem failure. As a consequence, a proportion of these infected and weakened trees will begin to pose safety risks, especially if they are next to high footfall areas such as a busy road, public pathways, schools, power lines, railways or areas frequently used by the public.

VISUAL ASSESSMENT OF ASH DIEBACK

There is now a widely used method for assessing the health of ash trees affected by ash dieback, which involves the visual assessment of the canopy. Looking at the health of a tree’s crown is a quick and useful gauge of its overall health; however, crown health is not the only symptom of ash dieback. The canopy assessment system also deliberately does not directly link to specific management decisions, but rather helps local authorities and land-managers to identify trees that need to be inspected by an expert. When repeated over time it also helps track any declines in the tree’s health.

The condition of the canopy provide a useful gauge of an ash tree’s health.

Case study 1: Ash Health Assessment System

West Lothian and other Scottish Local Authorities have adopted a system to score the decline of an ash tree’s canopy (developed by Suffolk County Council). The canopies of the ash trees are scored, assessing the percentage of the crown that remains. Using this four-category framework allows a tree to be assigned to a health category which informs subsequent potential action. The four categories are:

Class 1: 100% - 76% remaining canopy

Class 2: 75% - 51% remaining canopy

Class 3: 50% - 26% remaining canopy

Class 4: 25% - 0% remaining canopy

Class 1

Class 3

Class 2

Class 4

All images © Gary Battell

UNDERSTANDING HOW A TREE RESPONDS TO ASH DIEBACK

A tree’s decline is not linear or necessarily predictable, which makes decision making and prioritisation difficult. As a result of reduced water conductivity and saturation in the sapwood and ripewood, infected ash trees experience structural changes to the wood:

• Embrittlement: Branches and even large stems dry out and become brittle and more prone to snapping. In some extreme cases, standing dead ash can appear like airdried timber that has been stored for a few years – which effectively it is.

• As the wood of the trunk gets drier, it becomes stiffer: This means the tree does not move in response to wind or other forces and is therefore more prone to structural failure. (Water allows the wood to remain flexible by allowing the wood fibres to move more flexibly).

• Increased decay: In addition to embrittlement, the dysfunctional wood is more susceptible to decay from saprophytic fungi. This can be obvious where the sap and ripewood are exposed. Sometimes this is indicated by seasonally visible fruiting bodies of decay fungi.

Infected ash wood can therefore result in:

• Unpredictable failure: Unhealthy wood creates a tree that is less likely to tolerate being pulled or shocked by a physical force, leading to an increased risk of unpredictable failure.

• Branch failure: Affected branches are susceptible to snapping with the loss of flexibility in the wood causing the more brittle wood to fall if shaken by wind or movements created by felling, particularly with machinery (such as tree shears) or the use of wedges.

• Stem failure: Advanced stages of dieback can result in the stem becoming more likely to fail, especially where there are structural weaknesses such as old pruning wounds or historic limb failures. These areas of the stem will be less tolerant of shaking or sharp movements.

• Root plate failure: Decay in the structural roots becomes more likely in the presence of Armillaria (honey fungus) leading to complete failure of the tree.

The combination of all these factors results in ash trees becoming unpredictable and riskier to manage as the disease spreads within the tree.

Ash has a tendency to snap in its healthy state, leaving detached or broken limbs. These natural tendencies to snap have been exacerbated by ash dieback.

For more details on the ‘Signs and indicators of ash dieback’ – see Appendix 1.

As dieback progresses, there is a greater likelihood of stems splitting due to embrittlement.

Snapped ash stem due to embrittlement of the wood.

Ash dieback increases decay in the wood of infected trees.

The particular hazards posed by ash dieback Section 2

The hazards of being struck by trees or parts of the tree have always been a factor in tree management work, but ash dieback increases the likelihood of these events happening, and therefore increases the risk.

It is well known that serious accidents and fatalities are more frequent when working on trees than in other activities, as explained on the Health and Safety Executive’s (HSE) website:

‘Statistics show that the vast majority of fatal and major injuries in tree work are associated with chainsaw operations, being struck by a tree/tree branch or a fall. To put these figures into a wider context, tree work has a major injury incidence rate higher than that of the construction industry’. (HSE Website)

The HSE has also documented some of the risks posed by works on ash:

A 48-year-old man was found trapped beneath a felled tree

An assistant warden at a reserve was leading a party of volunteers felling an ash tree overhanging a farmer’s field. The tree it split, striking him on the back of his neck and side. He suffered fractures to ankle, collarbone and rib but died later from a pulmonary embolism. Some species of trees, such as ash, are more liable to split when being felled using conventional felling cuts – particularly if these trees are overhanging. The risk of splitting can be minimized by the use of advanced felling cuts. Those carrying out this type of work should be properly trained and competent in these techniques.

A 64 year-old self-employed contractor died after being struck on the back of the neck by a tree branch

He was contracted to remove two ash trees, close to a building. Working with another qualified tree surgeon he attached a rope to the tree and to a tractor. The plan was to fell the tree using the rope to control the direction of fall. He made two cuts at the base of the tree and signalled for the tractor driver to move forward which he did. The driver noticed contractor lying on ground. It is believed he was struck by an unseen branch, hung up between the two trees. Hung-up trees introduce a much greater level of risk to any felling operation.

As the HSE has acknowledged the risks of tree work, and the prevalence of ash dieback is likely to increase the risks in an already risky activity, any ash tree work should factor in the increased risks to the worker. This is usually achieved through a survey (see section 4), to determine whether action should be undertaken, leading to an assessment of the scale and methodology of the works required.

HEALTH AND SAFETY

Safety is everyone’s responsibility. There are specific regulations that need to be complied with when managing trees or machinery including (not a comprehensive list):

• Health and Safety at Work Act 1974

• Management of Health and Safety at Work Regulations 1999

• Provision and Use Work Equipment Regulations 1998 (PUWER)

• Working at Height Regulations 2005

• Other regulations such as Workplace (Health, Safety and Welfare) Regulations 1992.

For guidance on how these can be managed relating to trees – see the National Tree Safety Group guidance: ‘Common sense risk management of trees’ 2024.

HAZARD ASSESSMENT BASED ON DISEASE STAGE

Each of the four stages of ash dieback progression presents differing levels of risk and urgency of response. However, care must be taken with assessing the stages of the disease. Remember that this four-stage assessment is a useful but simplified tool for assessing the tree’s health, but does not directly link to specific management decisions, as there may be other structural or health issues to consider e.g. secondary infection. The Devon Ash Dieback Resilience Forum developed the first matrix to deal with the management options for diseased ash trees in high-risk areas. This model incorporates the four Ash Tree Health Classes and Devon’s proposed management reactions to each. It has subsequently been adapted by others and is presented here as the basis of a model that could be adapted locally. It describes not only the increased risks posed by diseased ash, but also the suggested management and works actions. You can view the matrix in Appendix 2

Common sense risk management of trees

It is recommended that landowners, organisations and companies develop their own strategies for ash dieback management which include the acknowledgement that risk levels change through the four stages of decline. For example, Forestry and Land Scotland introduced this system, including a fifth category ‘0’ for healthy trees. More detail is available on p32 of the Scottish Ash Dieback Toolkit (1)

Ash dieback poses a risk not just to tree workers, but also to visitors to forests and woodlands, road users and those neighbouring ash trees. The Tree Council has created ‘Ash Dieback Disease: A Guide for Tree Owners’ (2) to help with identification of diseased trees, and advise tree owners on their responsibilities and reasonable steps to take to manage risk.

Guidance is also given by Scottish Forestry on managing ash dieback in Scotland – see ‘The management of individual ash trees affected by ash dieback (Hymenoscyphus fraxineus) in Scotland’ (3) and ‘The management of woodlands affected by ash dieback (Hymenoscyphus fraxineus) in Scotland’ (4) for further details.

Planning work on trees with ash dieback Section 3

Each ash tree is different, and professional expertise with infected trees is needed to make management decisions when dealing with these unpredictable and dangerous trees. When planning any work, involve the right people with the right competence at the earliest opportunity and build a safe plan from the start. The Forest Industry Safety Accord (FISA) have shared a number of ash dieback safety items from Euroforest which can be found on their website. FISA has also published its own Technical Note on Planning Chainsaw Operations

The practical guidance presented in this document has been developed through interviews with a range of experienced contractors. It outlines some of the considerations and typical tree work methods used in areas affected by ash dieback.

As an ash tree declines, due to ash dieback or other factors, it is likely to become increasingly unsafe to climb due to the higher risk of anchor point failure (the point where climbing equipment attaches to the tree). Attempts to fell from ground level will also become more hazardous as the disease increases the risk of failure of the felling hinges and increases the likelihood of dropping brittle limbs that may result in serious injuries for the chainsaw operator.

Work on an infected tree is much safer when undertaken by machinery with a Falling Object Protection Structure (FOPS) that can protect the operator. The use of machinery for all ash dieback works should therefore be considered as the default, to reduce the risk of harm.

All infected ash trees are different and will need professional assessment before management decisions are made.

FACTORS THAT NEED TO BE CONSIDERED BY PEOPLE EMBARKING ON ASH DIEBACK WORK INCLUDE (ALTHOUGH NOT A DEFINITIVE LIST):

• Is felling really needed? Consider whether an infected tree can be safely left in situ – see the National Tree Safety Group guidance. Declining ash trees that may eventually die can also continue to contribute ecological benefits if kept in the landscape for as long as possible. Therefore, only fell trees where safe to do so. Consider keeping ash trees in the landscape.

• What are the legal considerations? Works may be required to discharge the tree owner’s duty of care, but any works must be carried out in accordance with relevant health and safety obligations, wildlife legislation and felling permissions. Seek guidance from a suitably qualified person e.g. an arboriculturist or forester before scheduling work. Also consult the HSE guidance note ‘GS6 – Avoiding danger from overhead powerlines’, and underground cables, plus other relevant guidance.

• What is the tree’s condition? There is now a widely used method for assessing the health of ash trees affected by ash dieback, which involves the visual assessment of the canopy. Use this canopy assessment as a basic indicator of likely risk, although other factors, infections and site conditions should be used to develop a risk assessment.

• Are road closures or temporary stoppages necessary to allow the safest felling method? Traffic Management (traffic lights and lane closures) may not be sufficient to manage risk to the public from debris as the more brittle nature of some ash trees causes the debris to spread further. Explore the possibilities for road closures to allow safer working by collaborating with your local highways authority.

• What are the constraints to managing the risks to the public, other land users and infrastructure? Local authorities and national park authorities have powers to temporarily suspend core paths and land from access rights for a period of up to six days under section 11 of the Land Reform (Scotland) Act 2003. These closures are called Exemption Orders. These may need to be granted in advance; however, Scotland operates a responsible right of access on foot, on cycle or on horseback across all other land. Advice can be found in the Scottish Outdoor Access Code.

This infected ash tree would be classified as a Category 4

Temporary path closures may be necessary where the risks posed by infected trees are high.

• What other risks might be posed by all aspects of the work and how can they be mitigated? Other risks could include chainsaw use or working at height. Consider ways to eliminate or substitute these as part of the risk assessment. This will involve planning the controls, an example of which is set out in a ‘Guidance Note’ shared by Euroforest (see graphic 1 below).

• Can the tree be safely left standing –removing all risk to an operator?

• Fell ash before deadwood forms

• Consider the landowner liabilities and whether the public risk of a decaying tree can be managed

• Can the tree be felled mechanically –harvester or tree shears – operator in FOPS cab

• This may require additional costs or planning for elements such as traffic management and machinery movements

• Can the risk of falling dead wood be removed for example by dismantling the crown using tree shears or harvester

• The use of felling aid tools such as jacks and mechanical wedges may reduce vibration and shocks in the felling operation – reducing risk of falling dead wood –availability of remote controlled mechanical wedges will allow work from outside of the immediate risk zone

• Provide additional training or use only the most experienced and competent operators

• Ensure all aid tools are available and fit for purpose

• Program works for periods where any deadwood will be more clearly visible – in the case of ash dieback this may be when trees are in leaf and dead wood is clearly visible

• Ensure operators have effective PPE

• European operators have been experimenting with a “Body Armour” solution - this is currently not available commercially but may provide future options...

• What are the concerns of the tree workers? They will be at the greatest risk, so their advice on how to create safe sites and work methods must be listened to. Involve competent operators in planning from the earliest opportunity, to ensure work is planned safely, risk is properly assessed and adequate resources are available.

• Is mechanised felling possible? This should be prioritised as the first management option (if safe operation and access allows).

Mechanical felling can offer greater protection to the tree worker with greater control over the tree and additional protection e.g. being in a ‘Falling Object Protective Structures’ (FOPS) rated cab. Use the following in line with the available guidance:

› Crane – AA technical guide 4: use of mobile cranes in tree work

› Grapplesaw – FISA Safety Guide 608 Tree Shear & Grapple Saw

› Harvesting head – FISA Safety Guide 603 Harvesting Head

› Mobile Elevated Work Platform – AA Safety Guide 5: Use of Mobile Elevating Work Platforms in Tree Work

MEWPs, cranes and FOPS-rated machinery with grapple saws and tree shears are being more frequently used to manage infected ash.

• Are you up to date with industry standards to establish safe systems of work for tree work operations at height? Have regard to the AA Industry Code of Practice ‘Tree Work at Height’ if felling individual trees as an arborist, whilst foresters can find current industry guidance on the FISA website – Euroforest - Safety Guidance - Planning - felling dead ash.

• Have you allowed safe enough working distances? When felled, infected ash trees are likely to create large amounts of flying debris on impact with other crowns and the ground, due to the brittle nature of the wood. If the falling tree strikes another infected ash tree, this will increase the risks of falling debris as the second crown may also fracture. Therefore, safe working distances should be greater than two times, and even up to three times, the height of the tree to be felled.

• Is the canopy of the tree to be felled connected to others? Infected ash trees with interconnected canopies are a risk due to embrittlement. If a branch holds on to an adjacent canopy, it will pull on both trees, which is a tension load. This can cause the failure of either or both trees, which could cause them to fall unexpectedly.

• Which contractor to appoint? Given the risks involved in working on diseased ash, this is not work suited to newly qualified or inexperienced chainsaw operators.

In addition to formal, recognised industry standard training, seek evidence of their:

› experience of felling large broadleaved trees

› awareness of ash dieback

› previous experience of dealing with trees with ash dieback

› attitude to risk and their understanding of the particular risks whilst working on ash

› understanding of reasonable mitigation methods

• Have you built a safety-first culture into your operational plans? Time and money constraints must not put the safest solution for workers out of reach; safely felling diseased ash will require greater competence and more robust logistical planning such as road diversions/closures etc., which will invariably cost more. Workers will be undertaking a high-risk task, and completion of work may take longer. Make sure resource and time do not compromise safety of operators, people, nearby property or equipment.

Crown connection should be a primary consideration as a risk factor.

Working on trees with ash dieback Section 4

Professional competence is needed to make effective management decisions when dealing with infected ash trees. In preparation of this guidance note, discussions were conducted with a range of contractors who are dealing daily with ash dieback infected trees. They shared practical considerations for others having to work with these unpredictable and dangerous trees.

Mechanisation is becoming more common to manage infected ash trees.

Note: In the case of forestry machinery, the ‘risk zone’ will be displayed on the boom.

WORKING METHODS TO CONSIDER:

Those working on ash trees need to carefully manage the increased risks posed by ash dieback.

Experienced practitioners have suggested the following useful changes in working methods:

 A fully mechanised operation to manage workers’ exposure to risk. This also reduces the strain and physical stresses on workers. Fell or dismantle by machine as the priority.

 Plan for traffic management and/or safety zones. These could be vehicles and/or people, bikes, horses etc.

 The wider use of cranes by contractors especially on larger individual ash with good access.

 Anchor points should try to use downward and inward forces as opposed to outward shearing forces.

 Fell with the lean first. Avoid wedges as the shock through the stem may dislodge limbs. Winch and pull lines are no guarantee if the hinge fails (stem will pivot off the stump if it wants to go in another direction, i.e. with the lean).

 Be aware that hinge wood is now unpredictable. Trees with a lean or weighting against the direction of felling will also put more pressure on a now weaker hinge, increasing the likelihood of failure; plan where possible to fell with the lean, not against it.

 Avoid the use of bore cuts on the centre hinge of diseased trees as this may reduce control of the tree.

 Spread the load from climbing or winch ropes over multiple stems when anchoring, to ensure that the load is distributed and reduced on any individual anchor point.

 Escape routes should be planned, prepared and maintained throughout tree works, to provide sufficient movement away from beneath the crown of the tree being felled, or any adjacent tree where there is a risk of limbs being pulled free. FISA 302 states to ‘move at least 3 metres into the escape route’, but in the case of diseased ash this should be greater. Where an escape route cannot be made and maintained, the felling method must be re-assessed.

 Radios (hand or headsets) may be useful to aid communication between workers on site. Clear communication protocols must be agreed before work starts, with all workers on site understanding them.

 Working method should take account of falling distances, which may be up to three times tree height and the effect of debris (shattered branch parts) on the groundworker, plus other site workers and public.

 Crown connection should be a primary consideration as a risk factor.

WHEN APPROACHING AN ASH TREE, MAKE A ROBUST ASSESSMENT OF THE RISK:

Be prepared to admit when you don’t have the necessary competence, resources, or time to fell any trees safely, or when the tree is in too poor a condition to safely hand-fell. At this point, the planning and risk assessment must be revisited with the Landowner, Agent, Works Manager or Contractor etc.

To help identify indicators of ash dieback and therefore potential structural weaknesses, you should use a principle of ‘Look Up; Look Down’ and be on the look out for the following:

Look up

• Leaf loss: Stage 1, 2, 3, or 4?

• Broken or decayed limbs?

• If not in leaf, sparse crown?

• Limb locked into other trees?

• Are neighbouring tree canopies in similar condition?

• Evidence of failure/decay in surrounding trees?

• Consider climbing plants, such as clematis, which are tenacious and strong enough to turn or hold trees during felling

Look down

• Basal lesions? (found on particularly wet ground)

• Fungal fruiting bodies?

• Flaking bark? (if loose, peel off and inspect timber)

• Evidence of previous fallen limbs/debris/wind damage?

• Any evidence of breaks or stress fractures in stem?

• Be aware that ivy or evergreen climbers can hide visible defects

• Check root plates for any evidence of cracking or lifted soil, which may indicate movement, a failed buttress, or decay in the junction of root plate and stem base

WHEN PREPARING THE TREE:

• Be aware that working on or under ash trees causes vibrations that can lead to deadwood falling from the canopy. Hammering wedges, moving machinery, or attaching strops or cables may be enough to dislodge overhead hazards, so a robust pre-felling assessment must be made beforehand (‘Look up, look down’).

• Understand that connected canopies are a risk with ash dieback due to embrittlement. If a branch holds on an adjacent canopy, it will pull on both trees, which can cause the failure of either or both trees, which could cause them to fall unexpectedly.

• Clear tree marking and mapping helps to communicate correctly which trees need work and any variation in removal method. This information must be passed to workers undertaking any tree felling in advance. A pre-commencement meeting before work starts is essential to communicate any specific hazards, controls, or methods to workers before undertaking any felling.

• Ensure the layout and organisation of the work area protects all parties from the risk of falling objects. FISA have published guidance here on:

a. ‘Increased public activity’

b. ‘Managing Public Safety on Harvesting Sites’

c. Guidance for Forestry Works Managers - Planning Chainsaw Operations - Guidance document 002

• Reduce residual risks following felling operations by developing a brash plan. This should include how brash will be removed from or left on site safely, without affecting future use by the public, other forest and woodland users, subsequent operations (forest establishment/planting), and the environment. FISA have published a safety bulletin from Forestry and Land Scotland about the ‘safe management of timber stacks’

Increased brittleness of branches can result in significant quantities of brash which may need to be managed.

WHEN FELLING THE TREE:

• Consider not climbing whenever possible but if climbing is necessary - understand any selection of load bearing anchor point should be carried out in accordance with current good practice.

› Follow the AA Industry Code of Practice ‘Tree Work at Height’ (ICoP).

› The Arboricultural Association Technical Guide 1: Tree Climbing and Aerial Rescue provides essential advice.

› If anchor point strength is reduced – use larger and lower forks.

• Understand that climbers need to constantly assess the suitability of any chosen anchor point.

› Remember that the strength of wood may be reduced.

› Understand that ash wood is better to load in compression, rather than tension. Therefore, any load needs to be pulling down towards the centre of the tree, in line with the fibre direction. Any forces pulling forks/branch unions outward could lead to anchor point failure.

• Remember that there are increased risks of branch failure, due to shock movement of the crown or branches during felling.

• Remember anchor points are increasingly likely to fail under tension. Ensure lines are passed over large forks, and branches, to produce compressive forces.

• Always check for evidence of recent felling or tree works in the vicinity and check the crowns of adjacent trees in case these have suffered damage, which could result in potential hazards e.g. falling branches or tree failure.

• Be aware there are increased risks of hinge failure due to decay or embrittlement as a result of a lack of tensile strength in the wood, or presence of secondary pathogenic fungi.

• Be aware that changes in ash may result in potentially fatal ‘barber-chair’ incidents. This happens when the tree is being felled, as the tree worker is in the process of a backcut, when a tree unexpectedly and rapidly splits upwards along the length of the trunk, from the back of the hinge. This results in the unpredictable failure of the remainder of the tree. To reduce risks, different tree felling methods are needed.

• Look up. Always check other trees for connections or ‘reliance’ on the tree being felled.

• If using mechanically-assisted felling be aware that diseased ash may be safest felled in the direction of the lean. If using a winch line, consider:

› Minimum shock loading – are wedges still reasonable to use?

› Reduced hinge strength under tension due to shorter wood fibres. This tension/pulling force can be created by lifting a hinge with a wedge leading to hinge failure – compressive/ downward/pushing forces applied by rope/line from a winch is preferable.

› Control of felling direction is better using lines and ropes with a winch.

• Increase working distances – between workers and for operatives on winches or other machines by increasing safety zones.

• Ensure clear and well-maintained appropriate escape routes – the majority of serious injuries in tree felling are as a result of being struck by an object, and this will include material falling from the crown of diseased trees. Tree workers need a safe, controlled environment to work in, and the use of assisted felling methods decreases risk as workers are not close to the falling/moving tree. Experience from professionals working with infected ash trees is now suggesting that escape routes should generally be greater than those recommended in the FISA 302 guidance to provide the best chance of avoiding any unexpected falling hazards from the tree.

• Limb shatter when felled trees hit the ground – healthy ash trees are prone to shatter when felled – this creates a risk of ejecting material in all directions. Diseased ash trees, and particularly those with significant embrittlement, decay or rot in the limbs or stem, may shatter more readily when felled, so additional precautions should be taken when felling.

Limb shatter debris on a railway line

Conclusions

Work on infected ash trees is hazardous and unpredictable, requiring adapted working practices, including mechanical and mechanically-assisted felling. The primary consideration is to ensure that all works are carried out safely for operators and bystanders. This requires knowledge of ash, ash dieback and its likely impacts on a tree.

This guidance has been created to show some of the additional risks of working with infected trees, in anticipation of a greater number of ash requiring removal across Scotland over the coming decade.

However, it is important to note that not all infected ash trees will need to be removed or require work. Retaining infected trees, where safe to do so, eliminates the risk to tree workers of operating on unpredictable trees.

HOW CAN WE PROTECT THE FUTURE OF ASH TREES?

In addition to protecting workers, retaining declining and dead ash trees allows them to contribute ecological benefits and habitat. A small proportion of ash trees that have ‘genetic tolerance’ to ash dieback need to survive and reproduce, to create the next generation of tolerant trees.

It is therefore critically important that we keep ash trees in the landscape, wherever safe and possible to do so. This will enhance the chances of producing more tolerant trees, whilst also protecting the wildlife they currently support.

Acknowledgements and disclaimer

This Supplementary Guidance Note for Ash Works in Scotland was commissioned by Scottish Forestry, and produced by The Tree Council, on behalf of the Ash Dieback Safety Group, a subcommittee of the Ash Dieback Risk Group (Scotland) which includes the following organisations:

• Aberdeenshire Council

• Angus Council

• City of Edinburgh Council

• Community Woodlands Association

• Confor

• East Lothian Council

• Forest Research

• Forestry and Land Scotland

• Forestry Contracting Association UK

• Highland Council

• Lantra

• Loch Lomond and the Trossachs National Park

• National Farmers Union Scotland

• National Trust Scotland

• NatureScot

• Network Rail Scotland

• Royal Botanic Gardens Edinburgh

• SASA

• SCOTS Roads Group

• Scottish Government Improvement Services

• Scottish Land and Estates

• Scottish Power Energy Networks

• Scottish & Southern Electricity

• Scottish Tree Officers Group

• Scottish Woodlands

• South Ayrshire Council

• South Lanarkshire Council

• The Arboricultural Association Scotland

• Transport Scotland

• West Lothian Council

• Woodland Trust

The Tree Council and Scottish Forestry would like to thank all those who have contributed information to help with the production of this guidance note which includes the members of the Ash Dieback Safety Subgroup:

• The Arboricultural Association Scotland

• Euroforest

• Forest Research

• Forestry and Land Scotland

• Forestry Contracting Association UK

• Health & Safety Executive

PHOTO CREDITS AND COPYRIGHTS

• Scottish Forestry

• Scottish Woodlands

• Tilhill Forestry

• FISA Chainsaw Safety subgroup

Ian Turner, The Tree Council – Cover photo, pages 6, 10, 14, 15, 17, 21, 31, 32

Jon Stokes, The Tree Council – pages 4, 8, 17, 19, 22, 24, 30, 31, 32

DISCLAIMER

The content of this document is provided for informational purposes only and should not be construed as legal guidance on any subject matter. Tree safety is a continually evolving matter and the authors and contributors do not accept any responsibility or liability for any loss, damage, injury or fatality incurred as a result of relying on its contents.

APPENDIX 1: SIGNS AND INDICATORS OF ASH DIEBACK

WHAT ARE THE SIGNS OF ASH DIEBACK’S PROGRESS IN A TREE?

One widely stated indication that ash dieback has moved into the crown of a tree is the presence of diamond shaped lesions around small branches and twigs, but this can often be hard to see.

There following are therefore a range of more useful indicative symptoms and features of ash dieback to look for in an ash tree:

CHECK THE CROWN FOR:

• Wilted leaves: Evidence of wilt can be due to the girdling of the stem by the fungus, causing wilting of the leaves and drying out of the wood.

• Blackened leaves: Visible evidence of infection as dead leaves are often retained within the crown.

• Erratic canopy shape: Ash dieback causes a proliferation of bunches of leaves, creating a ‘Pom-pom’ effect.

• Deadwood in the canopy: Common feature of ash dieback affected trees, with deadwood of varying sizes. This increases the risk of branches falling unpredictably, mainly due to movement by wind, tree work or forestry operations.

• Ash trees impacted by ash dieback appear to be susceptible to secondary pathogens e.g. Inonotus hispidus (see right) The presence of this fungus indicates that the tree is in serious decline, increasing the risks the tree may pose.

• Decay Fungi such as Pereniporia fraxinea.

CHECK THE TRUNK AND STEM FOR:

• Basal lesions: A ‘bruised-looking’ area and/or lifted, cracked areas of bark at the base of the stem (between the root plate and stem). These basal lesions can be hidden by moss, so particular care needs to be taken during inspection. These lesions appear to be created by ash dieback.

• Other infections and disorders: Other disfiguring pathogens also create lesions and cankers on ash stems, and it is therefore quite possible to confuse other pathogens with ash dieback. One of the most significant is bacterial canker of ash (Pseudomonas syringae subsp. Savastanoi pv. fraxini) which creates black wartlike knots or holes, but this is unrelated to ash dieback.

CHECK THE GROUND FOR:

• Excessive quantities of small deadwood on the ground: Look at the areas under the canopy of a tree, where large quantities of deadwood, particularly small twigs, can indicate the condition of the tree, and importantly the amount of dieback.

• Secondary pathogens e.g. honey fungus – Armillaria: The most well-known and pathogenic of the fungi that impact ash.

• Movement around visible roots: Visible gaps between root and soil can indicate the failure of structural roots and root plate lifting, which can create risks from falling trees.

Lesions on ash stems caused by bacterial canker of ash.

Basal lesion at the base of an ash stem.

WHAT ARE THE CHARACTERISTICS OF ASH WOOD?

As the living part of bark known as sapwood matures, it creates a growth ring, which forms the wood of the tree. Tree species vary in their wood characteristics, as each species has a different growth strategy for survival. For example, elm and oak are ‘heartwood’ species, whereas ash is a ‘ripewood’ species (see below). This means that these trees all react differently when being pruned, having limbs removed or being felled.

In heartwood species, the wood cells contain waste products and preservative chemicals (terpenes and tannins). These substances are toxic to decay fungi and work together with the continued water saturation of the wood, to exclude air. This combination prevents the growth of decay fungi, reducing decay. The majority of our ancient and veteran trees in the UK are from heartwood species.

However other tree species have ripewood such as ash, willow, alder and planes. These trees differ in their biology, as fewer preserving chemicals are laid down in the woody tissue, resulting in ripewood having less ability to resist decay fungi. As a ripewood species, an affected ash tree therefore decays more rapidly whilst standing. It becomes brittle and sheds branches, and the tree becomes more likely to fail, which increases its risk of causing injury.

HOW DOES ASH DIEBACK IMPACT THE ASH WOOD?

Ash has a vascular system with large earlywood vessels (produced in the spring) and smaller latewood vessels (produced later in the summer). This gives ash an advantage in environments with wetter spring weather and drier summers.

Tree ring studies of ash dieback affected trees have shown a significant decline in the size of these earlywood vessels, leading to less efficient water transportation. Trees badly affected by H. fraxineus also have fewer sugars moving around the tree, both because there are fewer leaves to produce the sugars and also because there is less water moving around the tree, taking the sugars with it.

The knock-on effect of these reductions in water and sugar movements is that there is less water saturating the wood in the stem and branches, causing the tree to be more vulnerable to attack from secondary pathogens. In addition, as the disease spreads, the fungus produces chemicals that are toxic to the tree. These move through the branches, killing the bark and damaging other vessels (phloem) below the dead bark.

Research in Switzerland has shown that this loss of vessel size and loss of leaves creates a feedback loop within the tree. This increases the impacts of the disease, as less water means fewer leaves, meaning less sugar, meaning fewer leaves, meaning less water etc. This accelerates the decline of the tree, but in unpredictable ways.

Ash wood – showing close up of ‘growth rings’ (earlywood - left and latewood - right)

APPENDIX 2: DEVON RESILIENCE FORUM – ASH DIEBACK RISK MATRIX

Model for the Management of Ash Trees in High Risk Areas* affected by Ash Dieback

* - High Risk Areas would include highways (motorways, A/B/C roads), railways, urban areas, areas around occupied buildings (especially schools, hospitals, shops, offices & housing etc) & Infrastructure (powerlines, gas lines & telecoms)

Please note this table uses the % of tree canopy loss as the indicator of the ash tree health

% of Tree

(** - the assessed likelihoods are based on current knowledge)

Current Routine Inspection Process may suffice

Ideally increase monitoring to at least annual inspection of trees or areas

Intense Monitoring inspection of trees/areas likely to be required

Action Unlikely to be required

Action Maybe Required

Action Probably Needed

Action almost certainly required

Assess Tree/Site Specific risks noting that other tree defects maybe present & create additional risks

Very High Risk as Tree Structural Failure becoming imminent

APPENDIX 3: REFERENCES

• Diminished vessel diameter as a possible factor in the decline of European ash (Fraxinus excelsior L.) Mirela Tulik, Katarzyna Marciszewska, Jacek Adamczyk

• Amplifying feedback loop between growth and wood anatomical characteristics of Fraxinus excelsior explains size-related susceptibility to ash dieback. Stefan Klesse, Georg von Arx, Martin M. Gossner, Christian Hug, Andreas Rigling and Valentin Queloz

• Spread and severity of ash dieback in Switzerland – tree characteristics and landscape features explain varying mortality probability. Stefan Klesse, Meinrad Abegg, Sven E. Hopf, Martin M. Gossner, Andreas Rigling and Valentin Queloz

• Hymenoscyphus pseudoalbidus, the causal agent of European ash dieback. Molecular Plant Pathol 15,5–21. Gross, A., Zaffarano, P.L., Duo, A. and Grünig,C.R. 2012

• Arboricultural Association Industry Code of Practice ‘Tree Work at Height’ 2nd edition

• Arboricultural Association Technical Guide 1: Tree Climbing and Aerial Rescue – an update (Last Updated: 27/01/2020)

ASH DIEBACK PRACTICAL GUIDANCE PLEASE VISIT:

• Arboricultural Association – Ash Dieback Guidance for Tree Owners, Managers, Contractors and Consultants

• FISA guidance – Essential information on the felling of diseased ash

• FISA / Euroforest – Ash Dieback safety guidance items

• Forestry Contracting Association – Felling Diseased Ash guidance

• Scottish Forestry guidance – The management of individual ash trees affected by ash dieback (Hymenoscyphus fraxineus) in Scotland

• Scottish Forestry guidance - The management of woodlands affected by ash dieback (Hymenoscyphus fraxineus) in Scotland

• The Devon Ash Dieback Resilience Forum – Replacing ash: appropriate tree selection

• The Tree Council guidance – Ash Dieback: A Guide for Tree Owners in Scotland

• The Tree Council guidance – Ash Dieback: Action Plan Toolkit for Scotland