Guidance

Managing woodland SSSIs and ancient woodland with ash dieback (Hymenoscyphus fraxineus)

Updated 24 January 2024

1. Purpose

This document provides management advice to anybody with a responsibility for the management of ash in Sites of Special Scientific Interest (SSSI) woodlands. It also acts as a reference to help guide consistent decisions by government officials who administer SSSI regulations concerning these woods.

This document should be read and taken in its entirety. Individual prescriptions should not be picked out of context. The principles outlined here may also be adopted on other ash woodland, especially ancient woodland, that is managed with biodiversity as a primary objective.

2. Context

2.1 Policy and guidance

This advice is based on the expert knowledge of UK researchers and practitioners and informed by evidence and experience from continental European countries where the disease has been established for over 25 years. This guidance is in line with the government approach to ash dieback set out in the Tree Health Resilience Strategy, published in May 2018. It should be read in conjunction with other ash dieback guidance documents. These guidance notes also cover ash dieback identification, health and safety and managing ash in a range of woodland and non-woodland situations.

2.2 Background

Over half of the woodland and wood-pasture SSSIs (totalling over 51,000ha) in England contain significant amounts of ash. As nationally important areas for the conservation of biological diversity, it is vital that the impact of ash dieback disease on the SSSI ‘features of interest’ is managed to reduce negative effects to biodiversity where possible. Woodland SSSIs are representative of the total amount of woodland biodiversity. As such, it is likely that this advice will be applicable to protecting biodiversity at a range of other sites with ash, especially ancient woods.

The long-term changes resulting from ash dieback are not yet fully understood or realised. Therefore, while various suggestions have been made in this document, it may be worthwhile trying different management strategies, monitoring their effectiveness, and continuing to share practical experience, with appropriate consents in place. Trials, experimental approaches and monitoring have begun in some areas. For example, there is a large-scale project in the Derbyshire Dales, LIFE in the Ravines which is trialling and evidencing different management approaches to managing woods with ash dieback.

However, it is very important that interventions resulting from this advice are tailored to the specific conditions on each site, starting with the SSSI features of interest, and considering:

• the current (and historic) proportion of ash and other trees and shrubs present
• the woodland structure - existing issues and challenges acting on the wood
• its context in the surrounding landscape
• the owner’s objectives
• public access and safety

Long-term planning is essential and consideration of the ash resource should be addressed within your woodland management plan.

2.3 Health and safety implications

Health and Safety Implications on any site need to consider the site users, for example visitors (public access), contractors, deer managers, events, forest schools. Different user groups will need very different approaches, and different parts of the site may have different users. It is useful to zone the woodland according to risk to users while considering abundance of ash across the site. It may be possible to combine essential safety interventions with opportunities for habitat management. Dangerous trees are defined in the Forestry Commission’s guide Tree felling: getting permission under section 2.5.

Diseased trees that are dying are more likely to shed limbs, or lean and collapse, and onset of decline may happen quickly. Where this is likely to pose a safety hazard (for example, adjacent to roads, footpath or in heavily used areas), the first action is to decide if it is possible to redirect people away from the risk.  Where this is not possible, such trees should be made safe (for example, monolithed) or felled (with an appropriate felling licence in place).

There is guidance available on safety measures to undertake during this operation. The Forestry Industry Safety Accord (FISA) has developed ash-specific guidance for forest managers entitled Safety Guidance for Managers – Felling Dead Ash. Further information about tree safety in general, including zoning sites (to ensure that action is proportionate and prioritised) is available in the National Tree Safety Group’s publication Common sense risk management of trees.

2.4 Law and permissions

The legislation that applies to SSSIs and protected species still applies on sites affected by ash dieback. You should refer to  Sites of Special Scientific Interest: managing your land for guidelines on SSSIs. You must comply with regulations and good practice guidance protecting wildlife species and habitats when you’re managing woodland and planning forestry operations. A protected species license may be required, especially where good practice guidance cannot be followed. See Manage and protect woodland wildlife for more information.

Before changing any existing management practises, you will need written consent from Natural England.

If you are planning woodland closure, you should consider:

• how you will minimise risk to human safety
• allowing natural regeneration and natural processes
• your liability regarding trespassers
• whether there are safe areas for deer stalkers undertaking deer management

You should plan for these ahead of closure, for example by providing safe access on rides or safe areas for high seats.

If you are planning a right of way closure, note that this can be done temporarily for a period of up to 6 months through application to the local authority. For permanent or longer term closure, you must seek permission from the Secretary of State. An alternative route makes approval more likely.

The felling of diseased ash within woodland still requires a felling licence from the Forestry Commission unless trees are dead or pose a real danger to public safety. Check with Forestry Commission to avoid potential illegal felling.

3. SSSI woodland and ash

3.1 Designated features on SSSIs

You should start by considering the notified features of interest concerning the biodiversity value of the wood. You should consider whether or not favourable condition is being met in the presence of the disease. This can be found in the citation for any site on the Designated Sites web page. The reason for notification will guide the management of the wood, and what activities will be consented. For example, woods may be notified because:

• they are a good example of ash woodland, or of native woodland
• they contain a rich and diverse ground flora, or for assemblages of species (for example invertebrates, birds, bats)

There may be one interest, or a suite of interests to be considered. Some wooded SSSIs may not include woodland as a feature of interest, for example geological SSSIs.

It is important that even if the wood is notified for ‘ash woodland’, we continue to manage the site as an important ancient woodland. In the event of its losing ash mostly or entirely, Natural England will take a decision on whether it still merits designation as an SSSI. However, specific actions (for example removal of ash) could be directly against the feature of interest.

The citation outlines what particular characters were felt to be important at the time of notification, and these will serve as a starting point to guide management decisions.

3.2 Ash dieback and ash mortality

The level and rate of tree mortality will vary across different sites and can be influenced by a wide range of factors. Primarily, a tree’s level of susceptibility to ash dieback (Hymenoscyphus fraxineus) is determined by its genetic makeup (genotype) in combination with the environment it is growing in. Trees that are highly susceptible to the disease can die very quickly and even large trees can succumb in a few years. Other trees can resist the disease for longer periods of time and some trees, with high levels of resistance may appear largely unaffected. Trees are likely to be more susceptible if they are suffering environmental stresses, for example from water logging or over-crowding. However, you should retain seed-bearing trees, especially if they have healthy crowns (where safe to do so).

Secondary infections, such as those caused by honey fungus (Armillaria sp), can significantly increase mortality rates. Younger trees (5cm to 25cm diameter at breast height (dbh)) are much more susceptible than older trees, although mature trees can succumb surprisingly rapidly. Where young trees are surviving among trees with heavy mortality, they should be retained. In terms of environmental conditions, wetter sites have been observed to have higher mortality, although in the UK ash growing on chalk (very dry) has also shown high mortality. There is current research underway on modelling environmental risks.

We consider individual trees with more than 75% of their crowns healthy as having a good level of disease resistance where they are within a known area of infection and surrounding trees are more severely affected. It can take several years following the arrival of ash dieback at a site to identify the healthier trees. Tolerant and resistant trees can still produce good annual growth increment. Trees with more than 50% of the crown affected will show little or no annual growth increment and are likely to die. Resistance to the disease is highly heritable and will be passed onto new generations of trees. However, from a woodland management perspective, it does not matter whether the tree is “tolerant” or “resistant”, it matters how healthy it is and its ability to survive.

Within Europe, to date no trees within infection zones have been found to be completely free of the disease, but recent research has shown that some degree of resistance may be possible (for example Methingeram and others 2022). The resistance is polygenic (affected by multiple genes) so you can expect a spectrum of resistance and tolerance levels. Of a sample of 140 trees tested from the UK, 1 in 6 showed resistance as a heritable trait genetically. However, the geographical distribution of this is still being researched, and this may not play out under natural infection conditions. Other studies have shown that ash trees which come into leaf early and shed leaves early are more likely to be resistant to the disease.

At best, the conclusion from studies in continental Europe estimate 2% to 5% of the ash population will show a useful degree of genetic resistance, whilst Kjær and others (2011) believe that under current infection pressure, only 1% have the potential of producing tolerant offspring.  Even then they will be expected to have up to 10% of their crowns damaged by ash dieback. Current research from European sites indicates they have suffered up to 85% mortality, and that natural ash woods showed lower mortality than plantations (Coker and others 2018). However, the disease continues to affect these sites, and this is not likely to be the final outcome.

3.3 Genetic diversity: a natural tool

Conserving the genetic diversity within our tree species and the processes that determine it are important for sustainable forest management and increasing the resilience of Britain’s forests and woodlands. The genetic diversity within a tree species at any one time is the result of many dynamic processes. It provides the source for future adapted trees and woodlands (Hubert & Cottrell, 2014).  Therefore, encouraging and securing natural regeneration from seed is a key approach to ash woodland management.

Trees are naturally good at defending themselves from the impacts of pests and diseases. Most natural populations that reproduce by seed can promote genetic diversity within populations in response to change (see figure 1). Every time trees produce seeds, this results in a cohort of seeds that either successfully germinates, or fails to do so (this is the first selection). As the seedlings grow, pressures continue to act upon them resulting in some of them surviving and others dying. In addition, every seed is likely to come from 2 different parent trees (the pollen can be from nearby, but may be from trees miles away) and every seed takes different genes from each parent tree. This means there is a huge amount of possible variation.

Each year, they are subjected to different pressures that will lead to a different selection, for example a hot dry summer, a colder winter or a wet spell. These selection pressures do not happen in nursery conditions. Different selection pressures may happen in nurseries that could be actively counter-adaptive for wild populations. This is why natural regeneration is so powerful.  Keeping as much natural regeneration as possible in the wood can give the individual trees the best possible chance to respond to any new threat. If the pressure is too great, the cohort will not survive. That does not mean that another cohort with new recruitment will also not survive, because there will be new combinations of genes, acting under different conditions. The best possible chance of genetic selection working happen when:

• there is a large parent population
• there is high genetic variation in the population
• seedlings are produced in large amounts
• there is high tolerance to the pest or pathogen
• they are growing in the presence of the pathogen or threat

In the case of ash dieback, there is large population with high genetic variation, and they seed prolifically. There is not a high tolerance to the pathogen, but because the pathogen acts so fast on young trees, it’s easy to detect and promote surviving ash within a population early on. 

Figure 1: Illustration of how the genetic variation to a pathogen leads to selection for more resistance to that pathogen in the second generation

Image shows parent population with a number of trees contributing relatively more seedlings in presence of the pathogen, leading to offspring population that has a higher overall resistance to pathogen.

Even trees that are not healthy can confer useful genes for both ash dieback (because resistance is thought to be controlled by many different genes) and for other threats to future generations. While it is common practice among foresters to remove trees with poor form and those which are more susceptible to this disease, leaving as many trees standing as possible allows more of the genetic material to pass onto the next generation. This retains genetic diversity which could be useful for other threats in the future.

3.4 How will the designated features be affected?

Many SSSI woodlands containing ash are designated for their W8 or W9 ash woodland vegetation community (NVC), often on ancient woodland sites, or found in mixtures with oak in W10. They often have outstanding ground flora, for example rarer plants species like the helleborines and rich lichen assemblages. They can have a high diversity of native tree and shrub species associated with these communities. They may also be designated for invertebrate assemblages including moths, butterflies and saproxylic (deadwood) invertebrates, woodland birds, molluscs and bats.

Recent research has shown that ash has unique ecological properties and functions in these woodlands. It is at one end of a spectrum for many qualities such high nutrients in senescent leaves and low lignin, meaning it can rapidly cycle soil nutrients, its bark pH, and the dappled light beneath its canopy in high summer. Other tree species do not do this, and the ground flora communities are likely to change because of its loss of ash. Careful use of mixtures of other native tree species to cover as many ash-related traits as possible could be one potential mitigation for species. Across the UK, at least 955 species make use of ash trees as a habitat, although on any one site only a proportion of these will be present. Some of these species are either wholly or partially dependent on ash. These species are vulnerable and likely to decline if suitable alternative habitat is not provided when ash dies. This has been addressed by the Natural England Commissioned Report NECR 151. There are also 15 case studies linked to this report that cover a range of sites across the UK.

3.5 Condition assessment

Many woods will maintain their woodland interest if we can succeed in diversifying the native tree and shrub species presenter if we encourage or select disease-tolerant ash.

If a site has ash dieback, it will not automatically go into unfavourable condition, but it will go onto our threats register. Natural England standard target for tree disease is: “No rapid loss of native species due to unnatural factors (greater than 10% in a 5 year period)”. Exceeding this will trigger a site to be switched into unfavourable condition. However, if the land owner is already in discussion with Natural England about diversifying the tree and shrub species, and taking steps as set out in this guidance, the site condition may be classed as ‘unfavourable recovering’.

An SSSI woodland with ash in it is likely to have ash dieback and its features are at risk. A Condition Threat and appropriate Action should be recorded. The Action may include establishment of a management plan to diversify and reduce other stress factors (see 3.4).

Where ash dieback is present and the loss of native species is less than 10% over a 5-year period, then there is no impact on the condition. We should still try to manage the ash dieback and record this on an appropriate Condition Threat Action.

If the loss of native species is more than 10% over a 5-year period, then the condition is ‘unfavourable’:

• if the appropriate management is in place to manage a shift in component species to re-establish the woodland structure, then (where this is the only factor affecting condition) it is ‘unfavourable recovering’ -  a Condition Threat Action must be recorded to reflect the continuing need to manage the site
• if appropriate management is not in place, the impact of the disease is continuing to increase or there are other factors affecting condition, then an ‘unfavourable declining’ category is likely to be most appropriate

In the future, if there is a permanent adverse effect that fundamentally alters the notified features of specific sites, then advice from Natural England will need to be sought as to the way forward. 

3.6 Existing challenges and wider resilience

Often the ash dieback disease is affecting woods which have existing issues and challenges, such as:

• a reduced range of tree and shrub species
• tree regeneration being unsuccessful, usually through either lack of light (for example no open space) or grazing or browsing by deer and other animals
• a lack of structural diversity across the wood in terms of tree size classes, shrub layer, open space and dead wood
• damage to trees and regeneration from grey squirrels or other tree pests and diseases, or other negative influences like invasive non-native species and climate change impacts

Addressing these now is important so we have more options for addressing ash dieback as the disease progresses. Action to address these issues will also help to ensure the wood is resilient to other pests and diseases and future climate change.

4. Management of ash woodlands

This section covers an overview encapsulating key principles concerning management of ash woodlands, and then discusses various management options which could potentially be applied to existing ash woods to modify them and mitigate against the impacts of ash dieback in the future.

Landowners and managers who have ash in their woods should immediately consider reviewing and regularly updating their woodland management plan (or creating one if they do not already have one). Woodland management plans should be compliant with the UK Forestry Standard. Before making any changes to existing management regimes, owners and managers should carefully consider their objectives and local circumstances, including the current structure and composition of the woodland. Consents to change management on SSSI woodland will be required from Natural England. Managing ancient and native woodland in England (2010) and the woodland wildlife toolkit can provide useful information for managing woods for biodiversity.

You should regularly monitor for ash dieback to assess the health of ash trees as the disease progresses (we suggest annually between late July and early August). You can use the canopy assessment in the Tree Council’s Ash Dieback Toolkit. Repeating the same survey will help show how quickly the disease is progressing. Other symptoms to monitor include identifying phenotypic traits like early leaf budding and senescence, and presence of basal lesions. However, you should avoid making premature conclusions regarding levels of disease tolerance, as the health of individual trees can vary from year to year.  However, if at a population level the ash woodland is decline, then you should develop and, where appropriate, implement active management options. You may also wish to strategically monitor the population of ash across the site, in order to prioritise management, as detailed by Alsop and Goldberg (2018).

Identifying and felling diseased trees pre-emptively will not prevent the disease spreading because it is already widespread and is wind-dispersed by spores which can travel long distances. The best strategy for the long-term viability of ash is to retain canopy trees for as long as possible, and promote and protect natural regeneration. It is expected that most new saplings will die, but any that remain will be very important. Ash is highly palatable to both deer (until they reach pole stage) and pheasants (at the seedling stage) and natural regeneration should be protected. Where pheasant release pens are consented, you should carefully consider their location.

Identifying and retaining seed bearing trees is very important. Seed bearing trees sometimes have less straight, clean stems than non-seed trees, and may be selected out for this reason. It is important to retain them to promote natural regeneration.

Retention of ash trees is important because they are part of the feature of interest. There can be a conflict because managers may perceive the long-term difficulty of accessing and managing a woodland with dying ash trees in it. However, you can achieve this by retaining a proportion of ash across the site, or to committing a section of the woodland to minimum intervention, as set out in the UK Forest Standard (UKFS) Guideline 8 ‘Biodiversity’, and Guideline 9 ‘Ecological processes’, table 6.1.1. If necessary, this area could be larger on an SSSI but would need to be assessed on merit.

While retaining a significant amount of ash, it may also be desirable to diversify the structure and composition of the wood. This will be advantageous in woods with a very high dominance of ash. In these circumstances, you should promote other species already on site by encouraging natural regeneration and consider whether any native species are missing from the wood that previously occurred there (an understanding of the woodland National Vegetation Classification (NVC type) and associated sub-community can inform decision making). If there are species dependent upon ash in the area which can also utilise other trees or shrubs, you could introduce these. Other relevant considerations would be to look at the reasons for notification. If there are other interest features also present on the site, you can promote and manage for these. Mature ash with epicormic growth from the main stem, or broomstick growth in a secondary inner crown, indicates a highly-stressed tree which is likely to die faster. If you are reducing the ash canopy, these would be the best ones to remove.

Thinning or harvesting mature ash as part of a normal programme of silvicultural management of the wood could continue in line with the advice in Table 1 below. If infection is not apparent in mature stands where thinning operations are required, you should retain ash trees with the biggest crowns or those which are prime (biggest and healthiest) amongst their cohorts (Skovsgaard and others 2017). Once stands become infected, continue to retain and promote those trees with highest levels of tolerance as they have the best chance of survival and reproduction.

You should identify and retain seed trees should be identified and retained, regardless of their condition, where it is safe to do so. You should consider what will regenerate beneath dying or felled trees, and whether planting is necessary and appropriate (see below).

You should consider the environmental variables when planning the future management of ash woodlands. There may be some instances where it is not appropriate to undertake tree removal, but underplanting would be beneficial to maintain a canopy into the future, such as calcareous slopes. This may be a preferable option for non-commercial woodland.

The following are a variety of options that may be suitable on different sites. These are potential, future management suggestions you could apply across all or part of the woods. More than one option could be applied to different parts of the same wood. The advantages and suitability are outlined below.

In all cases, proposed changes to the existing management plan on SSSIs should be consented by Natural England, and any felling carried out with a felling licence from the Forestry Commission. Additionally, surveys for protected species should also be undertaken. If the site is a Special Area for Conservation, a Habitat Regulations Assessment will be required. Any alteration to the management should be recorded in the site or woodland management plan to inform any mid-term review and eventual revision at the expiry of the plan.

Herbivore impact

A focus on addressing the existing factors affecting the condition of a woodland is vital to the conservation of the ash resource, and the key factor in influencing natural regeneration of ash is in understanding and managing deer impact. This is critical when considering any of the alternative woodland management approaches suggested here and impacts upon both the natural regeneration of ash and other tree species, and upon the success of planting.

Likewise, the effect of grazing pressure by other herbivores, both wild (for example rabbits, hares and squirrels) and domestic (for example sheep and cattle) must be considered in tandem with the existing level of deer impact. ‘If deer impact is significant, it is almost certain this impact will need to be reduced before (or alongside) the introduction of livestock grazing, even where that introduction has clear conservation benefits to the structure or function of the woodland ecosystem. Advice on deer management and tree protection can be provided by Natural England Advisers and Forestry Commission Deer Officers.

4.1 Continuous Cover Forestry

Continuous Cover Forestry (CCF) is appropriate for woods with a good mix of species.

It is a resilient management form which gives a heterogeneous age structure. Management of browsing animals is essential as CCF often results in wider spacing of trees with open space necessary for recruitment through natural regeneration of future generations. Continual management allows for reactive management and stem selection. Biodiversity responds very well to this management.

You should consider that it takes a long time to convert from a high forest or coppice woodland into a continuous cover stand. However, costs can be recouped from selling product from CCF.

4.2 Stand diversification

Stand diversification is appropriate for areas over 1 hectare with a single age structure of predominantly ash, where the ash is healthy or showing minor signs of ash dieback. Diversification within a SSSI will be looking at other native or site-accepted species, for example Alsop and others 2016.

You should retain healthy looking trees and seed bearing trees, natural regeneration and all native trees that are not ash

You should consider restructuring by taking out small coupes of ash (0.25ha to 0.74ha) in patches through the wood. Replant with other native broadleaved trees and shrubs appropriate to the site. These can be tagged with GPS coordinates for ease of management. Encourage natural regeneration of any species already present. Deer management is essential, and deer fencing may be desirable.

4.3 Thinning operations

Normal, planned forestry operations should proceed as planned. Thinning only allows for the felling of up to 30% of the crown area or stem volume and should be undertaken evenly across an area. There are no restocking conditions attached to a thinning-only felling licence. This is different from a ‘regenerative thinning operation’. A regeneration felling licence can vary in percentage, for example from 20% to 75% or even higher in some cases, and comes with restocking conditions.  On SSSIs, you should carefully consider the appropriateness of regenerative thinning, with monitoring undertaken ahead of consent, and careful reviewing of the volume proposed for the thinning operation. Some considerations to note:

For mixed broadleaves including a component of ash

Some studies have shown that ash trees present at a low proportion of mixed stands are no less affected by the disease than whole stands of ash. However, anecdotally they seem less affected and from the overall perspective of the wood, the impact of their demise will be lower in mixed stands. They may show symptoms more slowly, owing to physically fewer spores landing on them but have an equal chance of succumbing to the disease as ash in other situations.

In this situation, when undertaking a thinning operation, remove ash if over 50% of the crown is dead, otherwise ensure retention of ash and continue operation as normal.

Ash regeneration may occur better in mixed stands because of the variable shade. We advise that you monitor the woods and look for where ash regeneration is successful and protect it from browsing. Deer management will be essential and protection with small enclosures may be beneficial.

For pure or almost pure ash stands

Although ash does not necessarily respond well to thinning when it is already mature and dense, sites with high air or soil humidity will lead to increased spore production from the Hymenoscyphus fraxinea fungus and thinning may mitigate somewhat against this.

If the stand is young enough (less than 25 years), a ‘pre-thin cleaning’ may give dominant trees an advantage of reducing competition. Older stands may be halo-thinned around a highly tolerant tree, or around other tree species to promote generation of them. That may also be managed by creating canopy gaps to allow more regeneration and structural diversity. Select for healthy trees which can be identified at the pole stage, as they will be exhibiting visible signs of the disease, most notably on the stem discoloration.

As important as thinning are the potential risks of not thinning. Young, pure, unthinned stands represent a very high-risk category (Skovsgaard and others  2010, Schumacher 2011, Bakys and others 2013, Havrdová and others 2017, Marçais and others 2017, Alsop and Goldberg 2018, Forestry Commission 2018b), particularly in a high humidity environment and where they may already be under some stress due to sub-optimum site conditions. Continental European experience would suggest that total stand collapse due to H. fraxineus is only associated with dense, pole stage stands, a result of high spore loads and small crowns (Bladon and others 2016). Furthermore, ash responds poorly to late thinning so timely interventions are critical to stand health and vigour (Hawe and Short 2016).

Felling a large proportion of mature, diseased ash in the same stand, whilst retaining a few tolerant trees can have the effect of suddenly raising the water table, especially in lowland woods. This stresses the remaining trees and making them more vulnerable to infection by honey fungus. It is preferable to retain more mature trees as a structural component of the woodland. Retention of tolerant trees where close to other trees may be more successful.

Where thinning has not taken place in the early stages, it is better to look at options such as stand diversification (see 5.2) to diversify the stand. However, instead of thinning operations, it may be appropriate to target trees for removal that have epicormic growth or have greater than 50% crown mortality. Trees with basal lesions and armillaria may also be targeted for removal, with the exception of seed-bearing trees which should be retained unless they are considered unsafe.

Where you need machinery to fell or remove trees in an SSSI, you must ensure that activity is low impact in areas of rich ground flora and take all possible precautions to minimise ground flora disturbance and soil compaction (for example brash mats). You may need to consider greater flexibility for timing of operations to enable them to be undertaken at a less damaging time. Consent from Natural England is required for these activities.

4.4 Coppicing, overstood coppice, and singled trees

Unless ash coppice stools harbour unusually good levels of disease tolerance, they can be particularly susceptible, especially recently cut ash coppice re-growth. Evidence from Suffolk has shown that coppicing ash trees will result in the death of 80% of the stand within 3 to 4 years (Fuller 2016). However, you may decide to continue coppicing if the wood is currently in active coppice management and the feature of interest would be disrupted by stopping because it benefits from the coppicing regime (for example, ground flora or dormice). This would be when the benefits of the temporary open ground is more important than the ash stools themselves.

Appropriate actions will be guided by the SSSI citation and management plan objectives. You will need to promote regeneration, or potentially replant with other coppice species in the event of the death of the majority of the ash stools. This should occur as part of the ongoing coppice cycle, in small coups (rather than across the whole site) so that the conversion would be gradual throughout the coppice cycle. Continuing with a regular cutting cycle will also allow any resistant ash stools to be identified more quickly and will maintain the coppice conditions which are potentially supporting other woodland wildlife (such as ground flora and butterflies). It will cause disruption to the product in the following cycle, as the gapping up will take longer to re-establish.

Old overstood ash coppice

Some old overstood ash coppice are big stools that have veteran features. Due to their age and size, it is unlikely they will regenerate successfully when coppiced.

Retaining them can be beneficial. The mature stools will produce seeds and overstood coppice creates habitats for bats. However, in certain situations, felling the trees might also be a viable option, especially where they are taking up space in a wood which is resulting in too much shade. It may be preferable for the management of the wood to manage them. Bringing them back into coppice rotation (see above) would be a positive action.

You could coppice some of the areas to allow natural regeneration to develop before the trees collapse and die, whilst retaining other areas (with minimum intervention). You should try to retain coppice stools that are highly tolerant. If highly tolerant trees are coppiced, then potentially they should regenerate successfully. Deer management will be critical to the success of coppicing.

Singled coppice

In many woods that have previously been coppiced, the ash has been singled. This is where a coppiced, multi-stemmed tree is reduced to a single, dominant stem. These have visible boles at the base. They may be more susceptible to diseases and in some cases may be weaker and liable to collapse. They are likely to have veteran features and rot holes. These may be good areas to retain as minimum intervention but could potentially fail at the base. You could consider reintroducing them into a coppice cycle.

Hazel coppice with ash standards

Where hazel is not being cut regularly, and becomes overstood, it may act as a canopy in its own right, offering some protection to the wood and retaining the woodland habitat. However, it forms a very dense shade and can prevent ash regeneration. You should cut hazel coppice at the same time as ash management and promote suitable habitat for ash regeneration. Deer management will be essential.

The ash dieback fungus re-infects the trees each year. In some situations, a dense understorey may act as a physical barrier to the fungal spores reaching the canopy after they are released from the fruiting bodies on the fallen leaf stems. However, high humidity at the leaf litter layer favours fruiting body production and spore production. Spore density is highest near the ground (0m to 5m). However, the fungus can also infect through the roots of ash.

4.5 Pollarding and veteran trees

Ash pollards are highly vulnerable to ash dieback. They are a small subset of the population and the likelihood of retaining them indefinitely is low. However, their retention for as long as possible is a priority as they have high biological and cultural interest.

If they are healthy, old or veteran ash pollards in a regular pollarding cycle should continue to be pollarded. It is possible these trees have a better chance of survival because of the compartmentalisation within the stems (Bengtsson 2015). If the cutting of pollarded trees is not required imminently, then to avoid unnecessary works, you should monitor the health of the trees as the disease progresses.

You should not re-pollard previously neglected pollards (or veteran coppice stools) unless necessary, as this will place too much stress on the tree where it is also under infection pressure. If the tree is unsafe, you should consider diverting footpaths in the first instance and only felling the tree as a last option. If the tree is susceptible to ash dieback, the ‘pathway’ for the disease to the main stem is shortened when the tree has been cut. This means the new shoots are worst affected, making it difficult for the tree to recover from the intervention.

By not undertaking pollarding, some veteran ash might undergo very severe mechanical failure.  However, ash sometimes survives after such a failure, although this will be tempered if new growth is affected by the disease.

Where possible, you should try to ensure a sustainable population of future ash veterans by retaining and pollarding nearby ash trees (favouring healthier trees rather than those with moderate to severe signs of ash dieback) and protecting ash regeneration.

4.6 Dead wood

Ash dieback, especially if accompanied by honey fungus, can lead to a sudden influx of dead wood. There is no reason why a proportion of this cannot be used or marketed as firewood (with consent). However, diseased ash firewood becomes powdery quickly.

Dead wood is an important resource in woodland ecology. Because of our exploitation of them, woods in this country have a lower dead wood resource than in a natural forest. It would benefit the biodiversity of the wood if a proportion of deadwood were retained (both standing, where safe to do so, and fallen). The minimum proposed by UKFS is 20m3 per hectare over the rotation of the crop and the minimum in UKWAS is at least 3 standing or fallen decaying trees per ha. More is desirable on most SSSIs (around 100m3 per ha). If it is likely to prevent woodland management, it can be moved, but still retained.

Note that bats, which are protected by law, are likely to quickly find and utilise dead and dying trees. Full surveys should always be undertaken when carrying out management works in accordance with UKFS.

4.7 Minimum intervention – ensure browse control/deer management strategy

Minimum intervention enables naturalistic woodland structures to form. The concept is that natural processes (for example flooding or strong winds) that result sudden change are accepted, and allowed to recover naturally. It is only suitable at a whole site level on larger sites. However, partial minimum intervention in areas of more mature forest can also be beneficial for a range of species and allow natural dead wood levels to build up.

It may be appropriate to zone minimum intervention zones in those areas where mechanised harvesting would prove difficult and environmentally damaging for example steep slopes on ravines and in damp woodland adjoining riparian corridors. Deer management is still practised in minimum intervention areas because otherwise deer quickly learn to lie up in these areas. If an invasive plant or shrub species were to establish at some point in the future management intervention may be necessary.

Minimum intervention zones are appropriate for diverse species with rich shrub layer and mixed age woodland or areas where it is desirable to retain a proportion of the wood for example, where management may be damaging. You should manage rides and pathways to ensure they are safe to use and provide open space. 

The potential outcomes include:

• collapse of ash population
• natural regeneration of ash and other species
• high amount of deadwood is retained

4.8 Underplanting

Where canopy decline provides light conditions, you should consider underplanting dying stands of ash with native species suited to the site. This can be difficult in practice as once the canopy of ash starts to collapse, you cannot access the new trees. It may be better to create small areas of open space and allow them to infill, or plant. You could also trial underplanting in discrete groups or patched. 

4.9 Clearfell ash

It is rarely suitable to suggest clearfell on an SSSI, and you should try other options first. However, if the majority of the trees have crowns which are over 50% dead, you will need to consider removing them all at the same time. On lowland sites, where the trees are still alive, clearfelling could draw up the water table and harm ground flora and new trees. This is dependent on the aspect and occurs less on sloping sites.

Clearfell should only be considered as a last resort. You should assess the canopy towards the end of summer but ahead of senescence (August to September). Where there is regrowth of understory such as hazel, you should try to retain and protect it. If there are groups or individual ash that are less affected, you should assess whether they are wind-firm and retain them, especially if they are seed trees.

Where trees are surviving with heavy impact, you should retain at least 20% of surviving trees, favouring areas where other trees or shrubs are present to diminish the impact of a rise in the water table.

4.10 Replacement trees

Tree and shrub species should be established by natural regeneration where possible, for example by creating space for new regeneration around existing specimens of the desired trees and shrub species. Ash regeneration could be an important part of regeneration and you should try and protect it.  Deer management is especially critical for the success of natural regeneration.

To identify which alternative tree species could be encouraged to support the wildlife supported by ash, you should refer to the reports and databases on Natural England Commission Report 151.  You should check the National Biodiversity Network (NBN) and use any other necessary survey data to find out what wildlife species are present on your site. You should also check their level of association with ash on the associated spreadsheet of ash-associated biodiversity linked to the report, and the suitability of alternative species for wildlife and the site conditions. The Woodland Wildlife Toolkit can also help. You can also use the Ecological Site Classification tool. The Ecological Site Classification tool was developed to advise on species suitability for productive forestry purposes and also on the suitability of a site for developing specific NVC native woodland communities over time. However, the range of native tree species is limited and does not include some common native tree species that might be appropriate, for example field maple and hazel.

We recommend encouraging tree and shrub species which support ash wildlife, focusing especially on any species historically lost from the site through previous management. Where this information is not available, consider promoting other native trees associated with ash habitat at your site using the appropriate National Vegetation Classification community as a guide. You should consider the composition in nearby semi-natural woodlands on similar soil types. These may include other main canopy trees like oak and beech. You could also encourage less frequent species such as:

• birch
• rowan
• whitebeam
• aspen
• hornbeam
• willow
• alder
• lime
• yew
• holly
• field maple
• hazel
• wych elm
• cherry

And other key nectar bearing species such as:

• hawthorn
• blackthorn
• crab apple
• dogwood
• rose

There may be a case for introducing tree species which are at the limits of their current natural range at the site in question where it would be expected that these species might arrive naturally over the next few decades (for example beech).

The key function of SSSIs is to protect the native biodiversity (including genetic diversity) across the country and to use these sites as a natural resource for future environmental change. If stands or woods are dominated by ash, or have very little or no seed source of the desired species, then planting could be considered. Planting stock should ideally be grown from trees already in the SSSI or from sites nearby. This will help retain genetic adaptation to the site present in these species. On SSSIs, the assumption is that 95% site native species is maintained to prioritise the special interest of the site, unless a different planting mixture percentage is consented. Typically, this would be to support specific species.

We recommend using local provenance to aid natural selection processes, instead of the use of provenances of native species from 2 to 5 degrees south of the planting site as a component of the planting mix.  The use of planting stock acquired from continental Europe carries a significant biosecurity risk so we recommend only using plants certified as Plant Healthy.

Introducing non-native (to the UK) tree and shrub species to an SSSI is not usually acceptable or necessary for the management of the site and its features.

Many SSSIs with ash also have a proportion of sycamore. As a non-native tree, sycamore (especially the seed bearing trees) has been cut out of many SSSI and other nature conservation sites in the past, in an attempt to control its spread. This is because its shady canopy can limit the rich ground flora associated with ash. However, research has shown that sycamore has some similarities to ash:

• nearly half of the species associated with ash can also use sycamore
• it has similar bark pH which is important for some lichens

In European ash forests sycamore is a native component, and it has now naturalised itself into many UK woodlands. As a veteran tree, sycamore can provide an excellent habitat for bats in its flaky bark, and heart rot qualities similar to ash (white rot).

It is likely that where sycamore is present with ash, and the ash dies, that sycamore will fill the gaps if left undeterred. We need to consider on a site basis how appropriate this is, and whether it is better to have natural regeneration of sycamore or introduced planted stock of other species like oak, lime or beech, which may have similar shady canopies to sycamore. Sycamore is more vulnerable to squirrel damage than many native species and squirrel management should be considered if timber production in an objective. In the east and east midlands, it is prone to sooty bark disease which also carries human respiratory problems. It would usually be appropriate to enhance with additional tree and shrub species where they are not present, to prevent dominance by a single species.

Where sycamore is not already present on an SSSI it should not be introduced. Where it is present and the impact of sycamore on ground flora is a concern we recommend manipulating stand structure to allow more light to reach to forest floor, and maintaining the total amount of sycamore in the canopy at a low proportion, ideally below 15% canopy cover.

5. Species alternatives

You must start by examining the site and determine what species are threatened by the loss of ash.

Natural England Commissioned Report (NECR) 151 examines against 6 different groups:

• birds
• bryophytes
• fungi
• lichens
• invertebrates
• mammals

If diversification is one of your objectives, you may use, in small quantities, for enrichment planting:

• site-native species for which less information on ash dependent species is available (for example, native lime)
• any site native species unexamined in the NECR 151 report, but suitable to the site (for example identified by the ESC tool)

The knowledge in these reports, while very thorough, was not exhaustive. Diversifying woody species is likely to give rise to more overall diversity, even if it is not supporting the species that currently utilise ash.

You should only select non-native species known to support ash-associated biodiversity if the relevant ash-dependent species are known to be present on the site, and if there are no native tree and shrub species that also support them. If the species supported can be found on native trees or shrubs, they will be the preferred species on SSSI woodlands. Similarly, if the species they support are not found in the SSSI, there would be no reason to promote their use.

There are 955 ash-associated species. Sessile and pedunculate oak (combined as one assessment) are known to support 67% of ash-associated species (640 of the 955 species). Beech, elms, hazel and birch are all known to support more than 400 ash-associated species. Elms and rowan were found to support over 300.

Crab apple, holly, large-leaved lime, and blackthorn are all known to support over 100 ash-associated species. Wild service tree is known to support less than 10 ash-associated species from available data, but data quality is low with data on association unknown for most species.

There has been specific research into lichens associated with ash, published by Natural England. Read the guidance where lichens are important on the site.

Association within species groups

Oak species are known to support the greatest number of ash-associated birds, invertebrates, lichens and mammals. Hazel is known to support the greatest number of ash-associated bryophytes (55 of the 58 ash associated bryophytes). Elm species are known to support the greatest number of ash associated fungi, with oak in second place for bryophytes and third for fungi. Beech is known to support the second greatest number of ash-associated fungi. Rowan is known to be good for ash-associated birds. Elm species are known to support a good number of ash-associated fungi and invertebrates.

Oak species, birch species, rowan, beech, aspen, elder, alder, hornbeam, small-leaved lime and elm species all support more than 50% of ash-associated birds.

Hazel, oak, goat willow, grey willow, aspen, field maple, alder, beech, hornbeam, hawthorn, small-leaved lime, elm, elder, crab apple all support more than 50% of ash-associated bryophytes.

Oak species, elm species, beech, hazel, birch species, rowan, alder and aspen are all known to support more than 50% of ash-associated lichens (Figure 3.7). Sycamore was the only non-native tree species known to support more than 50% of the ash-associated lichens. Further information in the lichens report (Sanderson and Lamercroft 2022)

These examples, more detail of which can be found in the NECR 151 report, illustrate why knowing which species are important on the site will help guide species choice.

6. Decision tree for SSSI management

Step 1: determine the feature of interest

The first step is understanding what is being protected. (See section 3 for more detail)

Step 2: assessing existing composition and threats

• assess existing composition of wood (species, distribution, soil variability, rides and paths)
• undertake health and safety assessment
• retain all (or as close as possible) ash trees with crown more than 50% healthy (where undertaking thinning, assess tree crown health first)
• retaining healthy seed trees (at least 30) for as long as possible
• encourage and protect natural regeneration (consider several small, for example 10m by 10m fenced enclosures around denser patches of ash regeneration or close to healthy seed trees)
• actively manage deer

Step 3: assessing future composition

• understanding the current composition. Evaluate if there are any dominant species (including ash)
• promote other native species already on site
• consider whether there are any native tree or shrub species that are absent when you would expect them to be present
• retain naturalised species present on site (for example, sycamore - if they are present, retain, but set an upper limit for example  30%). Do not plant naturalised species that may regenerate freely

Step 4: introducing new (or presumed missing or lost) species

• natural regeneration is the preferred establishment
• if this fails, plant (or direct seed) native, local provenance preferred
• if introducing native (or non native) species, the rationale would usually be to support an existing, threatened species already on site. Do not introduce naturalised species that are absent from site unless specifically for supporting a species on site and there are no other native alternatives that the species would use
• if stock is unavailable, determine if it is necessary to plant this season.  If it is, then look for nearest provenance zone moving south. Stock should conform to the Plant Health Management Standard (Plant Healthy or Ready to Plant certified)

Step 5: management choices

• continuous cover forestry is a positive management choice allowing flexibility and formation of a good structure. In climate change terms, it also preserves carbon in the soil
• coup selection can work well. We encourage leaving seed bearing ash and a proportion of ash, where canopy health is more than 50%
• you should retain seed-bearing trees where crowns are relatively healthy
• you should retain areas of ash in ‘minimum intervention reserves’ to maintain genetic diversity for as long as possible and retain increased dead wood for biodiversity
• under a ‘normal thinning’ operation you should monitor and retain healthiest ash
• traditional management, for example coppice with standards, can give an opportunity to promote ash regeneration as part of the recruitment of standards (most will be unlikely to survive, so high frequency of ash regen is positive). You should take advantage of deer-protected enclosures
• overstood hazel understory ideally should be brought back into active management or cut during a thinning operation. The hazel canopy is likely to be too shady and prevent regeneration
• we do not advise minimum intervention across whole sites or clearfell

7. References

Alsop, J (2015) Woodland and tree management in the wake of Ash Dieback (Hymenoscyphus fraxineus): Experience from Continental Europe www.wcmt.org.uk.

Alsop, J. & Goldberg, E (2018) A methodology for risk assessing the likely loss of ash as a woodland component, and the associated impacts on canopy cover, ecosystem integrity and continuity of woodland conditions, as a result of ash dieback (Hymenoscyphus fraxineus). Quarterly Journal of Forestry January 2018 Vol 112 No.1.

Bengtsson, V, and Stenström, A. (2017) Ash Dieback – a continuing threat to veteran ash trees? In R. Vasaitis & R. Enderle (eds), Dieback of European Ash (Fraxinus spp.): Consequences and Guidelines for Sustainable Management, 262 – 272.

Coker, T.L.R, Rozsypálek, J, Edwards, A., Harwood, T.P., Butfoy, L., and Buggs, R.J.A (2018) Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic. Plants, People, Planet: doi.org.

Kjær, E.D., Mckinney, L.V., Nielsen, L.R., Hansen, L. N. & Hansen, J.K. 2011. Adaptive potential of ash (Fraxinus excelsior) populations against the novel emerging pathogen Hymenoscyphus pseudoalbidus. Evolutionary Applications, 5, 3: 219-228.

Metheringham, C, Plumb,W.J, Stocks, J.J., Kelly, L.J. Gorriz, M.N., Moat, J., Buggs, R.J.A., Nichols, R.A. (2022) Rapid polygenic adaptation in a wild population of ash trees under a novel fungal epidemic. bioRxiv. https://www.biorxiv.org.

Mitchell, R.J., Bailey, S., Beaton, J.K., Bellamy, P.E., Brooker, R.W., Broome, A., Chetcuti, J., Eaton, S., Ellis, C.J., Farren, J., Gimona, A., Goldberg, E., Hall, J., Harmer, R., Hester, A.J., Hewison, R.L., Hodgetts, N.G., Hooper, R.J., Howe, L., Iason, G.R., Kerr, G., Littlewood, N.A., Morgan, V., Newey, S., Potts, J.M., Pozsgai, G., Ray, D., Sim, D.A., Stockan, J.A., Taylor, A.F.S. & Woodward, S. 2014(a). The potential ecological impact of ash dieback in the UK. JNCC Report No. 483.

Mitchell, R.J., Broome, A., Harmer, R., Beaton, J.K., Bellamy, P.E., Brooker, R.W., Ray, D., Ellis, C.J., Hester, A.J., Hodgetts, N.G., Iason, G.R., Littlewood, N.A., Mackinnon, M. Pakeman, R., Pozsgai, G., Ramsey, S., Riach, D., Stockan, J.A., Taylor, A.F.S. & Woodward, S. 2014(B). Assessing and addressing the impacts of ash dieback on UK woodlands and trees of conservation importance (Phase 2). Natural England Commissioned Reports, Number 151..

Fuller, R.J. 2016. Management responses to ash dieback (Hymenoscyphus fraxineus) in woodland: implications for woodland structure and resources for biodiversity. Research Report no. 685. British Trust for Ornithology ISBN: 978-1-908581-80-8 50pp.

Sanderson, N & Lamercroft, D (2022) Impacts of Ash Dieback Hymenoscyphus fraxineus (Chalara) on priority lichens and potential mitigation options (NECR428). naturalengland.org.uk.

8. Further reading

Find a specific tree pest or disease
Tree Health Resilience Strategy
LIFE in the Ravines: a project managing ash
Tree felling: getting permission
Apply online for a felling licence
The UK Forestry Standard
Sites of Special Scientific Interest: guidelines on managing your land
Find out about the citation and features of interest on designated sites
Manage and protect woodland wildlife
Tree Council’s Ash Dieback Toolkit
Managing ancient and native woodland in England
Woodland Wildlife Toolkit
Common sense risk management of trees - Forest Research