Ash Dieback in the National Trust

It won’t be news to anyone involved with Observatree that ash dieback will have a significant impact on Britain’s tree cover. Ash is one of our most important tree species both ecologically and culturally, and is generally considered to be the third most common tree in British woods, comprising around 12% of woodland tree cover. Ash dieback was first discovered in Kent and East Anglia in 2012, but it was probably here before then. The fungal disease is predicted to affect 70-95% of British ash over the next couple of decades, with many of these trees dying.

Ash Dieback and NT

The National Trust looks after 250,000ha of land, around 1.5% of the land area of England, Wales and Northern Ireland. This includes some 25,000ha of woodland with approximately 3,000ha where ash is the principal tree species and thousands of individual ash trees outside woods.

National Trust sites, like many others, experienced the gradual advance of the disease after 2012. Around 2017-18, the impact in chalk and limestone landscapes – which are typically dominated by ash but also provide poor growing conditions – began to become very evident, with ashwoods in places like the South Downs and White Peak starting to look very sickly and site teams having to devote significant time and effort in response. The summer of 2020 saw a dramatic spike in both the scope and severity of the disease, with decline noted across most National Trust sites in England and Wales (Northern Ireland is considered to be a couple of years behind, although the disease is well established).

We’ve had to focus our response on the tree safety management implications, particularly given the additional pressures of Covid-19 during 2020, but without forgetting the necessity to mitigate the effects on biodiversity wherever possible.

Tree safety management

National Trust teams survey their ash in summer in locations where they could affect people when they fail. Where trees demonstrate more than 50% crown loss, or other significant symptoms, we consider the necessity of carrying out safety work. There are nuances to this approach: older ash, particularly those outside woods, are more likely to respond positively in future years despite crown loss, so where possible we try to avoid felling them. Trees in woods are less likely to bounce back.

Ash trees affected by dieback can become more hazardous to work on quite quickly, so we need to find a balance between being confident any one tree is doomed, and not leaving it so long that it’s too dangerous to fell or climb. Even with this balanced approach in mind, National Trust staff had to specify essential safety work to around 40,000 ash trees during 2020/21, at huge cost to the organisation.

Biodiversity 

The immediate impact of ash dieback on species that rely on ash is well-documented, but in many woodland situations it’s difficult to fully understand the likely effects on ecosystems and species. Some of our ashwoods will change hugely, and there may be negative impacts on their special ground flora which relies on the light shade cast by ash and the quick decomposition of leaf litter. The ranger team in the White Peak have an exciting project underway to mitigate the impact in this respect, felling small coupes of ash to allow natural regeneration of both ash and other site-native canopy species, where parent trees exist. Where there are no other tree species already present, they’re planting the full range of site-native species, many of which are now missing from these special ‘ravine woods’ due to the history of human intervention and their particular ecology, like the two species of native lime.

Unmanaged mixed woods might, however, see a subtle improvement in habitat as dying ash let more light in. Some ranger teams are using the impact of essential safety work to amplify this effect, creating new glades and rides as they make paths and other access routes safe.

My biggest concern is the loss of ash trees outside woods. Unlike woodland, where we’re likely to see natural regeneration, trees typically don’t establish outside woods without care and protection from livestock, which means each new tree must be well-planned. We must ensure that we replace lost ash in the landscape – preferably at least two new trees for every lost ash – for their ecological benefit as well as to ensure the continuing beauty of much of Britain’s countryside.

Even this response won’t make up for the loss of our oldest ash. Ancient ash trees, like the pollards of the Lake District and many others, are particularly important for their wood decay habitat and for their centuries-long history of human use. They may respond better than younger, close-grown trees, but it’s inevitable that we’ll lose some. In these situations, we must ensure that we document special, place-making ash trees before they go, to ensure we have a record of these trees and landscapes – and to help us come to terms with the emotional impact of the change and loss.