Tree safety

If you have an ash tree or trees on land that you own, it is your responsibility to survey for signs of the disease, and if it is present, to act in a structured and proportionate manner taking into account any risks to public safety.

The Government’s response to managing Ash dieback comprises a series of high level, national objectives. These are set out in DEFRA: Chalara Management Plan, March 2013.

The pages here provide landowners with more practical local advice on what you should do if you suspect your trees have ash dieback, including how to identify trees in high risk locations, and when to take action. If diseased trees in your ownership require tree works or felling, we also highlight the key legal considerations, such as whether you need a felling licence and how to protect wildlife. Where tree works would impact on an adjacent highway, we also outline highway authority procedures for closures of roads and rights of way. We also provide advice on ongoing monitoring and survey techniques.

Key safety considerations

  • You will need to consider whether active intervention is required or not. Key things to be aware of are:
  • Trees on your land are your responsibility
  • Once ash die back has infected an ash tree the tree can be at considerable risk of structural failure. The tree can shed branches and limbs, or the whole tree may even collapse.
  • Large trees have the potential to kill or seriously injure people and damage property and vehicles – this should be the paramount consideration for tree owners when deciding how to manage ash trees affected by ash die back.
  • A key health & safety consideration for tree owners should be “what is in the target area around the tree should it fall or shed limbs?”. For example the risk posed by the tree to infrastructure (power lines, gas lines & telecoms), highways, areas around occupied buildings (especially schools, hospitals, shops, housing & offices etc) and other peopled areas, should be very carefully considered.
  • Preventing Access to Trees – if a tree owner has concerns about the safety of a tree, it may be prudent to cordon off the tree(s) to prevent access to the area where a tree may fall, as a short-term measure until a tree can be properly inspected.
  • Inspection of Trees should wherever possible be carried out by a competent tree inspector who has relevant experience necessary, to assess the tree, and the risks of inspecting & recommending work to be carried out on the tree.
  • Tree Surgery should wherever possible be carried out by a competent tree surgeon who has relevant experience necessary, to assess the tree, and the risks or inspecting & carrying out work on the tree.
  • Disclaimer – this is not intended to be an exhaustive Health & Safety Guide, only a list of the key considerations for tree owners.

Useful guidance on managing and mapping the risks from trees can be found at the National Tree Safety Group Guidance. Whilst ash dieback is not specifically mentioned it is considered the document is comprehensive enough in its guidance and scenarios that parallels can readily be taken and applied to management of tress infected by the disease

Trees in high risk locations

The risks posed by trees is determined by its target area and the potential for harm to occur should partial or whole tree failure happen, and its effect on people and property in these situations.  Should no target exist, then there is no risk to people or buildings and infrastructure. High target e.g. major road networks- Low target e.g. Low use public footpath.

Target:  person or object, whether mobile or fixed, within the potential zone of impact of a tree or its branches, which might be harmed as a result of the partial or total failure of the tree NOTE The term can also refer to a pedestrian or vehicular route where harm might thus occur

Risk control: If the purpose of the tree work is to control risks to people or property, the relative priority of the work should be determined by a tree failure risk assessment. Decisions about priority should take account of any relevant factors that vary with the time of year, such as site usage, windiness, occurrence of drought or waterlogging, the potential for snowfall and unseasonal frost, or hazards associated with certain species at particular times of year, that are likely to influence the degree of risk to people, including operators, and property.  Definitions of target and risk Taken from BS3998 tree work recommendations

When to take action

It is becoming widely accepted that once more than 50% of a tree’s canopy is observed to be affected by ash dieback (and not a separate disorder) it is unlikely that the tree will recover. At this point its levels of vigour are likely to be such that the tree will be unable to resist other diseases. It is possible that it will be the effects of these secondary pathogens that will lead to tree failure and not the actions of ash dieback in the first instance, however in risk management terms the site owner/ manager will see little difference.

The girdling effect of the disease occasionally observed upon branches and stems, particularly at branch junctions, may lead to fracture at these points due to increased local stresses on dysfunctional wood. Close observation should therefore be made to determine the extent of any lesion and its location if it is desired to retain the tree for any reason.

As deadwood sections increase in size they are likely to be shed and where vulnerable targets are in the fall path, works are expected to become necessary to control the risk, either by way of whole tree felling, pruning out dead/ dangerous parts or moving the target where possible.

Again, it is important to have a clear understanding of the relationship of the tree to targets so that proportionate and appropriate management interventions are undertaken, if any.

Monitoring the situation

If an initial survey of ash trees on your land shows no signs of the disease, please monitor the situation and plan to survey again – we recommend that tree surveys are no more than two years old.

Advice on surveying techniques

As normal observers with even basic knowledge of tree observation will be aware, a typical ash tree can be a somewhat unkempt tree even at its best. It may contain large amounts of deadwood, crowns of low vigour, cavities, cankers and occasional epicormic shoots at the base and along larger limbs. These are normally as a consequence of disease or human action such as plough damage or pesticide spraying affecting hedgerow trees.

This typical form and habit of the species can make disease observation difficult therefore a means of readily calculating the vigour (hence health) of a tree was required. In the absence of a readily available field testing kit for the disease a canopy vigour percentage technique was agreed.

This allows a rapid means of taking a ‘snapshot’ of the canopy heath of a single, group or population of ash trees up to a regional level if resources are available. Necessary skill levels are low and cost of survey/ equipment minimal.

As with all surveys, the greater the sampling base, the greater the level of accuracy that can be ascribed to the results.

As it is recognised that a number of similar symptoms arising from pests and diseases, climatic events and human activities may mirror some indicators of ash dieback on localised levels it is recognised that the survey technique will be of greater use when taken over a wider survey area. This reduces the risk of disproportionate results due to localised outbreaks of honey fungus, plough damage or insect activity, etc.

The canopy percentages are broken down into the following bands which specifically describe the percentage of live foliage observed (canopy remaining)

100-75%, 75-50%, 50-25%, 25-0% and dead trees.

For the purposes of the Devon County Highways canopy percentage survey a tally count was taken of all ash trees encountered along 1km lengths of both sides of 7 randomly selected roads in each borough.

This survey, now concluded, has provided a baseline data set and snapshot of the condition of those ash trees encountered. It is anticipated the survey will be repeated to improve the knowledge base of the geographical and temporal spread of the disease in Devon.

As the tree declines, sections of deadwood will coalesce and should be removed where a target may be affected by their fall, there is no requirement to seek consent under Tree Preservation Order legislation, Conservation area Regulations or from the Forestry Commission for the removal of deadwood.

If the amount of deadwood reaches levels where over 50% of the crown is now seen to be infected it is considered that there is little chance of the tree recovering and removal should be strongly considered.

It is recommended it is at first established what level of risk is posed by the trees and what the effects of ash dieback will have on this level of risk in terms of its rate of infection prior to any works commencing.

Intervention will generally only be considered necessary when more than 50% of the crown is dead (not intended to be prescriptive, only a guide).

Arisings should be disposed of locally in accordance with biosecurity measures as defined within the Plant Health Order legislation.

(The requirement for securing felling licenses or gaining consent under tree protection legislation is a matter being considered for variance if the effects of the disease spread as quickly as is projected).