It was shocking to see the prevalence of the disease in our area when the trees were in full leaf this Summer, a large number of the trees which had been showing some sign of the disease in 2019, had deteriorated dramatically over the Winter months and come back into leaf with less than 50% of their canopy cover.
It was depressing to have to spend the Summer and early Autumn marking up Ash trees to be felled along roadsides, well-used footpaths and railway lines.
Ash dieback is caused by a fungus called Hymenoscyphus fraxineus; it is also commonly known as Chalara Fraxinea.
Thought to have originated in the Far East and probably arrived in mainland Europe and now the UK due to the movement of plants as part of global trade. The fungus spreads rapidly as its spores are windborne. It kills affected trees from the top down – initially killing the most recent growth on the outside of the crown, then progressively moving down to affect the whole tree. Individual trees vary in their tolerance, and the tree’s existing health and other factors (such as drought and secondary pathogens like honey fungus) can affect the rate of decline. We are seeing a rapid increase in trees in severe decline this year. Current thinking is that 70-95% of ash trees will be affected.
The threat to safety
Any dying or dead tree can pose a hazard as either its branches, trunk and/or roots can fail once in decline. Ash trees that are dying from dieback may pose an increased risk as they can shed branches, I have personally witnessed some, shedding their entire crown. There is an increased incidence of root-heave as the disease progresses.
Secondary infection by honey fungus can also make them more susceptible to root heave or stem failure. It is important to stress, however, that we should only take action where there is a genuine risk to safety – it is otherwise important to leave dying ash trees alone both to identify those that are tolerant to the fungus, and for the habitat value in decaying wood.
Most organisations are following the Forestry England’s guidance and reducing or removing trees, where they are either dead or have lost over 50% of their crown or display other symptoms of concern (e.g. basal lesions, honey fungus). It is vital for the safety of forestry workers and arborists that ash trees that pose a risk are worked on at this stage; further into their decline they can become extremely hazardous to work on, either by felling from the ground or climbing.
It is still necessary to apply for the appropriate permissions to fell trees which have Ash dieback disease.
Example (from Tree Council Toolkit)
Ash dieback obviously also poses a threat to our native biodiversity, specifically those species associated with ash woods and with open-grown ash trees like veteran pollards and those found on field boundaries. Many places have plans in place to maintain the conservation value of our woods and trees so far as possible.
Whilst there is no evidence of full resistance to the disease, research and experience in Europe indicates that up to 5% of the ash population may be genetically tolerant to ash dieback. This natural tolerance in some trees provides an opportunity to maintain ash in the UK because the tolerance may be inherited.
Best practice is to leave as many trees standing as possible so that we don’t inadvertently fell tolerant ones; we may not always be able to do this in terms of managing risk but any tree that doesn’t pose a risk should be left.
Woodland trees and open grown trees
The experience from the continent is that ash trees grown in woods don’t recover once they’ve got ash dieback, and decline year on year until they die. Open grown or field trees, in contrast, may recover or continue to live with the disease even after significant crown loss. Where reasonable, it may be worth just dealing with the immediate safety concern in open grown trees even with 75% crown loss (i.e. removing dead branches over a usage zone) to give them a chance to respond in future.
Replacing Ash Trees
Replacement tree planting should take account of site constraints. It is important to diversify the species and to think about provenance when selecting trees in order to maximize the landscapes resilience to pests, diseases and climate change. Natural regeneration should be considered, where it is appropriate to do so, in particular from ash showing tolerance to ash dieback.
There is no cure for ash dieback, but good biosecurity practice should always be followed, whether working in woodlands, in parks or open spaces, or in residential gardens. By doing so, you will help reduce the risk of introducing and spreading tree pests and diseases.
There are no restrictions on the movement of ash timber, branches or leaves, but a plant health order made in 2012 prohibits all imports of ash seeds, plants and trees into GB, and all inland movements within GB of the same material.
It’s not all doom and gloom, natural regeneration in mixed species broadleaf woodlands can be dominated by ash saplings, as they are fast growing, they can out compete many other slower growing species. With ADB affecting young ash in woodlands, other species will have more of a chance to get established.
As ash trees die, this will help create glades and clearings within the woodland. These open spaces favour woodland-edge plants and their associated insects, birds and animals. Not to mention the important habitat their standing and fallen deadwood provides.
Victoria Stanfield Cert. Arb & For, Guest Editor
Victoria has been involved in conservation management of trees and woodlands for the best part of 20 years. She has a particular interest and experience in the management of Ancient and Veteran trees and is currently working as Area Ranger for the National Trust in the Bristol area. She is a BTF Tree Champion.