Priority ash felling, Exmoor National Park

Client: Exmoor National Park Authority
Service: Ash dieback risk management, inspection and selective felling

Background: ash dieback on Exmoor

Ash dieback is a serious fungal disease of ash trees, caused by Hymenoscyphus fraxineus(formerly known as Chalara). It leads to progressive leaf loss, dead branches in the crown and, eventually, the decline and death of the tree. The disease is now widespread across the UK and is expected to kill a large proportion of our native ash, changing the landscape and creating significant safety risks where infected trees stand near roads, paths and property.

Ash is also an important ecological species, supporting hundreds of other organisms including lichens, bryophytes, invertebrates and fungi. Many lichens are particularly dependent on ash bark, and the loss of ash through ash dieback poses a serious threat to nationally rare and scarce lichen species.

Within Exmoor National Park, ash trees are closely interwoven with historic stone walls, veteran box shrubs, old hedgebanks and traditional field boundaries, contributing to both landscape character and biodiversity. Exmoor National Park Authority appointed Devon Tree Services to deliver a structured programme of inspection and felling that would manage safety risks from ash dieback while retaining as many high‑value and ecologically important ash trees as possible.

Our brief and objectives

Our brief was to develop and implement a proportionate ash dieback management strategy on National Park Authority land. The key objectives were to:

Address high‑risk ash trees affected by ash dieback along key access routes, boundaries and near infrastructure within the National Park.
Manage risk to the public, National Park staff, historic structures and utilities, particularly along priority footpaths, near buildings, car parks, stone walls and beneath overhead services.
Protect the safety of our staff, recognising that dead and severely affected ash (health classes 3–4) can become unpredictably brittle and are often unsafe to climb, requiring careful choice of work methods and access systems.
Retain high‑value ash and associated habitat features wherever reasonably safe, including mature and veteran trees with significant amenity value, trees supporting rare or notable lichens, veteran box shrubs, and ash that were structurally integrated with walls and other heritage features.
Align the programme with evolving national guidance for protected landscapes, favouring inspection‑led management and the retention of tolerant ash where possible.

Inspection, triage and constraints

Initial inspection of ash tree stock

We began with a systematic walk‑over inspection of ash trees across the Authority’s land involved in the project. Each tree or group of trees was assessed for:

Overall condition and visible symptoms of ash dieback, including leaf loss, crown dieback, lesions, deadwood and structural integrity.
Proximity to targets such as public and permissive footpaths, estate and access tracks, buildings, car parks, stone walls, veteran box shrubs and overhead utilities.
Ecological and heritage constraints, including veteran trees and shrubs, lichen‑rich trunks and branches, and old walls or banks that required protection. Local records and national guidance on ash‑dependent lichens informed these assessments.
Access considerations for safe work, including machinery access, ground conditions, the presence of fragile structures, and the ability to establish safe working positions and escape routes.

This baseline survey allowed us to map high‑risk zones and sensitive features, providing the foundation for a proportionate and defensible management approach in line with recognised tree risk management principles.

Triage system: condition, target, access and constraints

To prioritise work across a large number of ash trees and multiple sites, we developed a triage system based on four main factors:

Tree condition – the extent of ash dieback symptoms and structural defects such as cracks, decay and major deadwood.
Target – the likelihood and potential consequences of failure, with higher priority given to trees near busy public footpaths, roads, car parks, buildings, overhead utilities, stone walls and key National Park infrastructure.
Access and operability – how safely and practically work could be undertaken, including whether safe climbing was possible, whether MEWPs or other machinery could be used, and how to prevent damage to walls, banks and veteran shrubs.
Ecological and heritage constraints – presence of veteran trees and shrubs, known or potential rare lichens, and other high‑value habitat features, where retention and minimal intervention were preferred wherever risks could be managed.

Using this triage framework, we were able to:

Identify urgent felling and dismantling works where serious structural defects combined with high‑value public or infrastructure targets.
Allocate more detailed inspections to key trees and sensitive locations.
Flag trees and features for retention and monitoring, especially where ecological or heritage value was high.

Staff safety and work methods

The safety of our climbers and ground staff was a central consideration throughout the project. Trees significantly affected by ash dieback can become brittle and may fail suddenly, sometimes before they appear completely dead, making health class 3–4 ash particularly hazardous to climb.

In response, we:

Minimised climbing in class 3–4 ash, avoiding rope access where the degree of dieback, decay or structural weakness made climbing unsafe, in accordance with emerging industry guidance.
Favoured mechanical and platform‑based solutions (e.g. MEWPs or suitable machinery) for higher‑risk trees, especially those presenting significant hazards to public paths, roads, utilities or structures.
Used rigging and sectional dismantling where fine control was required to protect walls, veteran shrubs and other targets beneath the tree.
Implemented robust site‑specific and dynamic risk assessments, reviewing them continually as work progressed and as additional information about tree condition became available.

This approach reduced exposure of our staff to the most dangerous trees, while still enabling the National Park Authority’s safety and management objectives to be achieved.

Health classes and how they guided decisions

To bring consistency to decision‑making, we categorised ash trees into health classes based on remaining foliage (canopy) density, informed by ash dieback guidance.

Health class 1: approximately 100%–75% foliage density – trees with full or near‑full, healthy‑looking crowns and only minor or early signs of disease.
Health class 2: approximately 75%–50% foliage density – trees showing noticeable thinning of the crown but still retaining a substantial live canopy.
Health class 3: approximately 50%–25% foliage density – trees clearly in decline, with significant dieback and consolidating deadwood.
Health class 4: approximately 25%–0% foliage density – trees in severe decline or effectively dead, often with extensive dead crown and a high likelihood of structural weakness.

In practice, we applied the following principles:

Classes 1–2
- Generally retained and placed on a monitoring regime, particularly where they contributed to landscape character or hosted lichen interest.
- Localised works (e.g. removal of defective limbs) carried out where specific defects or high‑value targets justified intervention.
Classes 3–4
- Prioritised for intervention where they were close to high‑risk targets such as roads, main footpaths, car parks, buildings, stone walls, veteran shrubs and utilities.
- Work methods chosen with staff safety as a primary concern, with climbing used only where residual tree condition allowed, and preference given to mechanical felling and platform‑based operations.

At the beginning of the contract, this framework led to a relatively high volume of removals in areas where many trees fell into health classes 3–4 near sensitive public and infrastructure targets. As guidance evolved, our focus shifted to selective removal of class 4 and the poorest class 3 trees, with a stronger emphasis on retaining and monitoring class 1–2 trees and better‑conditioned class 3 trees wherever risk could be managed.

Retaining key, veteran and lichen‑rich trees

From the outset, Exmoor National Park Authority was keen to retain key trees and veteran features that were central to both landscape character and biodiversity. We therefore identified:

Mature and veteran ash trees with significant amenity and structural presence.
Trees known or suspected to support notable or rare lichen assemblages, based on site observations and regional and national lichen information.
Veteran box shrubs, old hedgebanks and stone walls closely associated with particular ash trees, creating composite habitats typical of the National Park landscape.

Research and conservation guidance highlight that ash supports a very large number of lichen species in the UK, including many that are nationally rare or threatened, and that ash dieback could put some of these at risk of local or national extinction. Veteran trees and associated deadwood are also recognised as high‑value ecological resources, especially in protected landscapes.

Informed by this, we:

Marked key and lichen‑rich trees as priority retention where it was safe to do so, using localised hazard reduction rather than felling wherever practicable.
Adjusted rigging, anchor points and felling directions to avoid damage to trunks and limbs with significant lichen growth.
Protected walls, banks and veteran shrubs during operations with careful machine routing and ground protection.
Integrated these ecological and heritage constraints into our triage and work programming, so they were considered alongside safety and access from the outset.

Responding to evolving guidance

During the life of the contract, technical and policy guidance on ash dieback management continued to develop, with a clear shift towards inspection‑led management, proportionality and retention of ash where reasonably safe, particularly those which appear more tolerant.

We responded by:

Revisiting our triage criteria to give greater weight to detailed inspections, especially around high‑use public areas and high‑value trees.
Tightening our thresholds so that large‑scale pre‑emptive felling was replaced by more selective removal focused on the poorest condition trees (health class 4 and high‑risk class 3) in sensitive locations.
Ensuring that ecological constraints – particularly veteran trees, lichen‑rich stems and heritage structures – were explicitly recognised in our recommendations and method statements, in line with best practice for national parks and other protected landscapes.

Outcomes for Exmoor National Park Authority

By combining structured inspection, a clear health‑class framework and strong emphasis on staff safety and ecological value, Exmoor National Park Authority has:

Significantly reduced risk from ash dieback‑affected trees along priority paths, roads, utilities and around buildings and structures on Authority‑managed land.
Protected the safety of tree workers, with high‑risk ash managed through careful planning, alternative access and machinery rather than routine climbing in class 3–4 trees.
Safeguarded heritage and habitats, including stone walls, veteran box shrubs, veteran ash trees and lichen‑rich stems, by integrating these constraints into all stages of planning and execution.
Retained valuable and potentially tolerant ash, supporting long‑term landscape character and biodiversity within the National Park in line with current best practice and national guidance.
Established a repeatable inspection and triage model that can be rolled out across other National Park Authority holdings and shared with partners as ash dieback continues to develop