This month (June 2019), the Government published Conserving our ash trees and mitigating the impacts of pests and diseases of ash: A vision and high-level strategy for ash research, setting out a "roadmap" to guide research funding and recommending a range of actions within the framework of its wider "Tree health resilience strategy", launched in May 2018.
This set out what it called a "resilience circle" of three self-reinforcing segments, namely resistance (minimising a threat or absorbing its impact); response and recovery (dealing with an outbreak to enable existing trees to recover); and adaptation (making longer-term changes to aid survival of trees, woods and landscapes).
While for ash this largely means dealing with the consequences of ash dieback (ADB), known to be present in the UK since at least 2012, the strategy also emphasises the importance of having measures in place to deal with the emerald ash borer (EAB), which it says "presents a looming threat and will be a key focus for future studies" (see box).
Defra is already supporting the development of a new nationwide map of ash using remote sensing technologies and is establishing the UK’s first "archive" of ADB-tolerant trees while continuing to screen for more such individuals. "A conventional breeding programme for trees providing tolerance to ADB could be implemented relatively easily," it adds, though this could be accelerated through advanced breeding techniques.
Already the genetic basis for tolerance has been extensively studied. Identifying markers for this can help identify tolerant trees in the field "to inform better management of existing ash populations", it notes.
But it points out: "Whilst tolerance exists, the climate and site appear to play a large role in how trees succumb to the disease, including soil type and moisture, air humidity, temperature, stand age and stocking density."
It adds: "The disease has been observed to progress quickly in young trees, trees growing in stressed conditions and in ash-dominated woodlands with higher levels of leaf litter and consequently spore loads. Fewer symptoms have been observed so far in ash trees growing on well-managed open sites, such as parklands."
A perhaps less well-known aspect of the disease is that in larger trees, especially on wet sites, the pathogen can also cause basal lesions at the root collar, often associated with secondary pathogens such as Armillaria, making trees structurally unstable. The incidence of such lesions "also has a genetic component", the strategy states.
For all these reasons, "the impact and required management of ADB is expected to vary considerably by site and context", it notes. Among practical measures to consider, "removal of leaf litter may be an effective way to reduce the level of inoculum in urban environments, and fungicides may have a small role to play in treatment and prevention, for example in nurseries", it suggests.
As for adaptation, "a measure which can be implemented early is replacing ash with a diverse mix of other species, including other Fraxinus species where applicable", while natural regeneration "will encourage the process of natural selection for tolerance, so healthy trees should be maintained for as long as possible to ensure regeneration from tolerant mother trees".
The Woodland Trust has welcomed the publication of the strategy, but its senior conservation adviser Dr Nick Atkinson says: "We would urge the Government and land managers to ensure that replacement trees are UK-sourced and grown to reduce the risk of importing other diseases from abroad. We would also advocate using native species, which are best able to adapt to climate change and co-evolved with the rest of our wildlife."
The "looming threat"
The devastating impact of the East Asian pest emerald ash borer (EAB) on North America’s ash trees means the UK must be vigilant against its arrival here, according to the strategy. However, "the cryptic life cycle of the beetle and the delay in observable symptoms mean visual surveillance is likely to be ineffective for early detection", it warns.
Yet eradication "is only likely to be possible for localised outbreaks where the beetle has not had time to spread". It points out: "Tree injections have been used to protect high-value trees in Canada and the USA but are not currently approved for use in the UK."
Four non-native parasitoids released in the USA that prey on EAB eggs and larvae "are having a positive impact on ash recovery", while another identified in Russia "may also be of value". Movement restrictions of ash trees, branches, logs and firewood, accompanied by a public awareness campaign in the USA, "have shown value in reducing spread".
Now that the pest has established in western Russia, "particular effort should be made to facilitate and incentivise collaboration between scientists and stakeholders in countries across the likely pathways of EAB", it urges.
"It is particularly important to understand the interaction between [EAB and ADB], through studies in places such as Russia where they are co-existing on novel hosts, and to predict how this interaction is likely to play out in the UK environment."