First confirmed in Britain in 2012, ash dieback is a highly infectious disease of ash trees caused by a fungus called Hymenoscyphus fraxineus.
A high proportion of ash trees in Northern Europe have been infected and killed and the disease is now widespread in England and Wales. Southern and eastern counties have been the worst affected so far but the disease is already badly affecting ash trees in the Peak District and many northern counties.
The disease is spread when spores are dispersed by wind. Currently there is no known cure. Tests are ongoing to try to find and propagate from resistant specimens.
Based on data from European infections, up to 85% of UK ash trees outside woodlands are expected to die from it. In Europe the mortality rate for trees within woodlands has been around 70%.
Affected trees become unpredictably brittle as the disease takes hold. They can drop limbs or fail entirely in the later stages of the disease and can become dangerous to climb, making felling more difficult and dangerous.
How we are managing ash dieback
There are an estimated 250,000 ash trees in Sheffield, the majority of which are on public land (data from iTree survey carried out in 2017). The only way to remove the risk posed by badly affected trees in public places is to fell them.
In addition to felling badly affected trees, some trees in locations that are impossible to access with large machinery may need to be taken down in the early stages of the disease to ensure the safety of tree operatives – that is, before they become too dangerous to climb.
All sites owned by us have safety tree inspections on a regular basis. We are also surveying for Ash Dieback across the city to monitor the spread of the disease and we are increasing inspections to cover sites identified as high priority – those with high numbers of ash, or large ash, near roads, schools or other well-used areas.
Whilst we do need to fell infected trees before they become a public danger, we also need to retain the 10% of trees that may be resistant to the disease, so that a new generation of more resilient ash can establish. We will assess trees on a case-by-case basis in order to identify any with resistance.
We estimate that Sheffield will lose between 127,000 and 215,000 Ash trees in total, including between 76,000 to 130,000 trees on Council managed land over the next 10-15 years.
Since 2000 we have planted 290,000 trees across Sheffield, including those in parks, woodlands and on the highway. Because of this we have a head start on restoration and protection of the treescape, but it is vital that we continue to replace trees that are lost. This will be part of the council's environmental and greening strategy across the city.
You can download our plan to find out more about how are managing dieback.
Identifying ash dieback
The Forestry Commission and Tree Council has a wealth of information on Ash Dieback.
What landowners can do to help
If you have ash trees that could potentially fall on neighbouring land, roads or property, you should check your trees for obvious signs of ill health or dieback. If you are concerned, you should have the trees assessed by a suitably qualified arborist to establish their condition and the level of risk they pose.
Tree work on ash dieback affected trees is particularly hazardous and should not be carried out by unqualified people or contractors who are unaware of the particular dangers.
Private landowners have a duty of care under common law to ensure they do all that is reasonably practicable to prevent injury or damage to neighbours and anyone visiting their land. The Highways Act also requires them to ensure their trees do not endanger or impede people on roads and footpaths.
Businesses have additional requirements under the Health and Safety at Work Act to ensure their work places are safe.
Larger landowners and businesses should take the following steps, and it is essential that a qualified professional is used to carry out any tree work:
- identify how many ash trees you have and where they are
- assess their current condition - use percentage of canopy cover remaining
- identify where affected trees pose a risk
- appoint qualified tree specialists to remove hazardous trees
Tree contractors are at the greatest risk of harm from dying ash trees, so it is vital that they are fully qualified and insured and aware of the particular dangers of dealing with Ash Dieback affected trees. Trees can become unpredictably brittle and unsafe to climb even before they are half dead.
The Arboricultural Association provides advice for both tree contractors and those employing them.
Planning and Tree Preservation Orders (TPOs)
The full implications of the disease are still unknown, so it is not currently necessary to fell healthy trees. Applications for consent to fell unaffected trees (under tree preservation orders and conservation areas) will be judged on their own merits, and the potential for infection by Ash Dieback will not be a significant consideration. Trees already infected by Ash Dieback will again be judged individually on their merits, bearing in mind the location and the need to retain trees which may prove to be resilient. Felling infected trees under Statutory Plant Health Orders will be an exception.
Until further notice, the potential risk of infection by Ash Dieback will not be considered a significant justification for not making a TPO, although a confirmed case of Ash Dieback is likely to be a significant factor in the decision.
Dead and dangerous trees exemptions
The council must be given five days written notice of works on protected trees that are dead or dangerous and pose an immediate risk. Infection alone does not constitute an exemption, only cases of advanced decline or death. Felling required under a Plant Health Order is an exception.