Genetic Scorecard Indicator - European Ash

High likelihood of diversity loss. The general persistence of the species across its range is threatened by huge predicted losses from wherever ash dieback has established, with mortality ranging from ~70-85% depending on stand-type (DEFRA, 2019). Highest population losses from ash dieback are predicted for young trees, those in very moist soils and in woodlands which are ash-dominated. Impact is correlated with time since first arrival, but populations across the UK are now experiencing losses; ash dieback is now widespread in Scotland, with confirmed infections in over 30% of hectads (Anon, 2024).

Population definitions:

Populations are defined based on management units. This species is widely and continuously distributed across UK in large numbers (Stroh et al., 2023; NBN, 2025). The four Great British Regions of Provenance (Herbert et al., 1999) were selected here to represent the major population groupings / management units (treating Northern Ireland as an additional region) as they broadly reflect the environmental variation across which the species is distributed and are likely to capture major patterns of genetic structure. 

Ne500: The proportion of populations that have an effective population size of more than 500 (Demographic - using the ratio Nc 5000 = Ne 500)

• Proportion of populations with Ne > 500 in Scotland = 2/2
• Proportion of populations with Ne > 500 in UK = 5/5

PM: Proportion of populations that existed in 2000 that still exist in 2025.

• Proportion of populations maintained in Scotland = 2/2
• Proportion of populations maintained in UK = 5/5 , estimated 2025

Substantial populations remain, but the estimated loss since the start of the Ash dieback epidemic is around one third of the population.

Functional variation

High likelihood of loss of functional variation, as locally adapted populations may encounter catastrophic losses. In all studied stands, only 1-5% of trees are suggested to be tolerant to ash dieback, but with a measurable genetic component, suggesting the potential for increasing tolerance by breeding and through natural selection in situ (Cavers & Cottrell, 2015; DEFRA, 2019). As the impact is massive and acts especially severely against early life stages, selection pressure will be high and the rate of adaptation could be rapid - it may already be underway (Metheringham et al., 2025).  

Divergent lineages

Limited divergence from European populations precludes loss of major divergent lineages, but there is potential for loss of locally adapted populations. 

Hybridisation/Introgression

No major hybridisation issue threatening genetic diversity. In some parts of Europe where ranges overlap, F. excelsior can hybridize with F. angustifolia and other congeners.

Low turnover - constraints on adaptive opportunities

High mortality and severe pressures are predicted to limit regeneration and adaptive change. Risks due to combined stresses from soil conditions, pest and pathogen pressures, and competition from faster-growing species all result in higher susceptibility to tree- and stand-scale losses, which limit healthy population turnover and adaptive evolution. Deer browsing may also limit natural regeneration. 

Scotland

• Risk: Serious
• Mitigation: Not effective

Great Britain/UK

• Risk: Serious
• Mitigation: Not effective

Overall Genetic Health status explanation

Despite widespread distribution and abundance, population losses are predicted to be catastrophic. Intensive management strategies, such as resistance breeding, require huge investment and benefits to wild populations will only be realised with persistent and long-term intervention. Ex situ collections are very strong, with good representation of contemporary diversity, but do not secure a long-term future for ash genetic diversity and presence of viable and evolving ash populations. Managing populations to promote natural regeneration is the most effective way of promoting long term adaptation to ash dieback through natural selection. 

In situ genetic threat level

• In situ Risk for Scotland: Serious
• In situ Risk for UK: Serious

In the face of emerging pathogen threats, major population losses and limitations to regeneration present a high risk of genetic variation loss. 

Confidence in in situ threat level

• Confidence score for Scotland: High
• Confidence score for UK: High

Assessment based on good demographic data from Europe, where the longer-term effects of ash dieback can be more easily assessed; direct data on genetic variation, population differentiation and biology.

Current actions are: 

1. projects to understand and map stresses on ash stands, understand the genetic basis for tolerance (including resistance, avoidance, tolerance),
2. breeding programs to promote tolerance,
3. control movement of nursery stocks and timber to prevent arrival of emerald ash borer into the UK,
4. maintain any tree that appears healthy and promote recruitment to maximise the opportunity for continued natural selection,
5. designation of Gene Conservation Units. 

Population

Demographic

N pops assessed/monitored in Scotland = 2/2
N pops assessed/monitored in UK = 5/5

Genetic

N pops assessed/monitored in Scotland = 2/2
N pops assessed/monitored in UK = 5/5

Deliberate interventions to support change in the genetic composition of the UK ash population seem essential. Work to establish the rate at which natural selection might deliver adaptive escape for the species would be valuable, along with assessment of the efficacy of different interventions to promote regeneration. Tree improvement and breeding may deliver varieties with elevated tolerance to the disease and efforts to pursue this are recommended.