MANAGING THE RISK FROM TREES GROWING ON LANDS OWNED OR CONTROLLED BY PUBLIC AUTHORITES
Tree risk in Northern Ireland. Tree risk assessments are usually designed to deal withthe danger ofa tree or part of a tree failing. In Northern Ireland falling trees generally cause death or injury to occupants of cars and this represents a risk of about 1 in two million of the population in any year. Risks posed to pedestrians or occupants of houses are much lower, although the risk of damage by trees to property (particularly to low load bearing structures like sheds, fences and glasshouses) can be significant.
Table 1: The chances of an individual being killed or seriously injured in any one year by a tree in Northern Ireland (compiled by the Author)*
Behaviour / Approximate Risk / CommentsBecoming a Tree Surgeon or climbing a tree / 1 in 900 / Four deaths in the last fifteen years
Crashing a car off a road / 1 in 60,000 / Until recently, about thirty deaths per year
Becoming depressed / 1 in 180,000 / About ten suicides associated with trees per year
Having a tree fall on you when driving / 1 in 1,940,000 / Seven deaths in the last fifteen years
Having a tree fall on you when walking / 1 in 40,000,000 / Less than one death per year in the British Isles
Being killed or injured by a tree while in a house / 1 in 150,000,000 / About one death per year in Western Europe
(* A very large afternoon storm may significantly alter these apparent risks)
As can be seen from the information compiled in Table 1, the three most significant risks posed by trees are usually attributed to the actions of the individual, rather than negligence on behalf of the tree owner. However, Public Authoritiesshould be aware of these risks and should adopt strategies to reduce the risk of these incidents on their property.
In particular, Public Authorities should ensure that all Tree Surgeons appointed by them are capable, through training and organisation, of carrying out their tree works in a safe and competent manner. An example of the basic minimum standards of training and organisation that should be required is attached in Appendix 1.
Most road traffic accidents involving trees are caused by drivers losing control of their cars, sliding off the carriageway and crashing, sideways, into a roadside tree. Such accidents carry a very high risk of death or serious injury. About 30% of these accidents will involve either trees already implicated in previous serious accidents, or those growing close to corners in what may be considered accident 'black spots'. Public Authorities should monitor any such incident that may occur on their property and consider, in collaboration with other informed parties, appropriate remedial action. (Such action may include the provision of crash barriers or, in extremis, the removal of the offending tree and its stump.)
Although the Author has no professional expertise on suicide in Northern Ireland, he is aware that patterns of behaviour can develop where certain locations, including Urban Parks, may be favoured. If incidents occur where trees growing within their property are used for suicide or attempted suicide, it is recommended that Public Authorities seek expert advice on appropriate staff training. They should also consider other responses, including management or removal of the tree and the management of surrounding vegetation.
Zoning. The variation in historically validated risks between various types of occupancy (illustrated in Table 1) supports the concept of zoning the risk from trees according to their potential target. The number of target zones normally identified in tree management systems varies. In the simplest case, only two zones are used, those that are occupied by people or property and those that are not (e.g. National Tree Safety Group Common sense risk management of trees, 2003). However, while this simple approach may work well for private landowners (who can control access within their lands and need only be concerned with those trees that could fall onto public, neighbouring or occupied lands) It is probably too simply for Local Authorities. While it is not usually desirable to include too much complexity into risk management systems, a third, intermediate, tree risk zone is probably needed in this instance. Such a system would identify high risk zones (trees within falling distance of car parks, roads, low load bearing structures, play areas and areas where non 'compos mentis' activity may occur), medium risk zones (trees within falling distance of busy or habitual pedestrian routes, adjoining gardens, high load bearing structures etc) and low risk zones (trees growing in areas with low or controlled pedestrian access, remote areas etc).
The classification of zones within lands owned or controlled by Public Authorities can only properly be carried out by permanent members of their staff, who know the sites well and are aware of seasonal and daily changes in occupation levels.
The risk of tree failure. Most large trees growing in Northern Ireland have a life span of between one and three hundred years. In the absence of intervention, most trees break up and fall close to or at the end of their lives, creating a crude 'risk' of an individual tree failing in any one year of between 1 in 100 and 1 in 300. However, the risk that a tree will actually fail in any given year increases with its relative age and condition. Thus, a young, healthy, tree population will have an exceedingly low risk of tree failure, while an aging tree population, containing diseased individuals, will have a relatively high risk of tree failure.
The conditions in which a tree actually fails are often weather related. Thus strong winds, a heavy fall of wet snow or exceptional growing conditions are usually required before a tree failure actually occurs. This further concentrates the risk posed by trees into definable, if somewhat random, periods of time. It also means that, for most of the time, all trees, even old diseased ones, create a very low risk.
The risk that a tree will fail increases with certain definable attributes. For example, certain species are prone to producing narrow forks that become unstable as the tree grows, while other species can have similar narrow forks that remain stable. Obviously, trees that have died or contain significant amounts of deadwood in their crowns become dangerous, however, the rate they become dangerous differs with species. There are also a number of infections that predispose trees to failure. In many cases an infection with a particular disease will have a different risk associated with it, depending on the tree species infected. Finally, damage to a tree, through vandalism, storms or inappropriate construction operations, can increase the risk of failure. Sorting out the specific level of risk associated with these defects requires a detailed knowledge of trees and it on is this knowledge, however incomplete, that the Arboricultural Consultancy industry is based.
Monitoring and reporting tree risk. Legal Case Law has established the principal that tree owners must be prudent in their ownership. This is usually interpreted as meaning that they should make themselves aware of the apparent condition of their trees and should take appropriate action if it is clear to them that a tree is not safe. Case Law has also established that the owner should use those resources normally available to him to exercise this care. In the case of Public Authorities these resources will include their own staff, many of whom (Groundsmen, Gardeners and Safety Officers) will have some professional knowledge, as well as the services of the specialist knowledge of a Tree Officer or Arboricultural Consultant. Finally, Public Authorities may also be expected to have produced a risk assessment that provides a framework in which the risk from trees is addressed.
The monitoring of trees should be proportional to the risk they pose to people or property. Common sense risk management of trees usefully defines three levels of monitoring of a tree stock. These are;
Informal observations. These are observations made by anyone with a working knowledge of the trees in a particular location. They are not made as part of a formal assessment of the trees and help to ensure that obvious defects and hazards are identified. Public Authorities should encourage all of their employees, particularly those employed in grounds maintenance, to report defects or decline in Local Authorities owned or controlled trees, regardless of their surrounding targets. Local Authorities should also welcome reports from neighbours or members of the public. These reports should be recorded and investigated by staff capable, through training, of deciding if and what action is required to maintain site safety.
Formal Inspections. These are regular, walk over, inspections of trees owned or controlled by the Council that may cause harm if they failed. They should be carried out by dedicated members of staff with a knowledge of trees and are designed to identify obvious hazards within the trees. It is recommended here that these formal inspections are carried out every August and again following storms. The member of staff should log the date of formal inspection, as well as any defects within the inspected trees, and they should have sufficient training to identify obvious hazards.
Detailed inspections. Individual, detailed, inspections should be carried out on all mature trees in high risk target areas. Detailed inspections may also be required in medium risk target areas if they contain mature trees that are likely to fail. They should also be used where a second opinion is considered appropriate on trees growing in low risk target areas, and where they are required for planning applications. A record of the trees form and condition, and recommendations for tree works designed to ensure site safety, should be produced for each tree surveyed. It is usual that trees subject to a detailed tree inspection are marked on site by a numbered tag, so that they can be identified at a later date. Detailed tree inspections may also contain additional information designed to ensure that the report conforms to the recommendations contained in BS5837:2012. Recommendations for tree works contained within them should conform to those detailed in BS3998:2010.
Detailed tree inspections are normally repeated every five years and should be completed by an appropriately competent person, experienced in tree inspections. They should be completed by the most appropriately qualified person to have reviewed the inspected tree. An appropriate tree inspector in this instance will hold a qualification in Forestry or Arboriculture to an NQF level of 5 or greater, will have a few years experience in providing detailed surveys of trees, will be adequately insured and will be capable of defending their opinions at Court.
Recommendations. It is recommended that Public Authorities produce a tree risk assessment strategy, designed to ensure, as far as is reasonable practicable, that their trees are maintained in a safe manner. In particular, the Public Authority should;
- Ensure that all tree surgeons appointed by them are competently trained and organised to safely carry out tree surgery operations
- Review all incidents of death or serious injury associated with trees growing on their property (including road traffic accidents and suicides) and react in a way that minimises future incidents.
- Establish, using a competent member of their own staff, a three tier priority system for zoning their trees, with high priority zones (within falling distance of car parks, roads, low load bearing structures, play areas and areas where non 'compos mentis' activity may occur), medium priority zones (within falling distance of busy or habitual pedestrian routes, adjoining gardens, high load bearing structures etc) and low priority zones (low or controlled pedestrian access, remote areas etc).
- Implement a three tier priority system for monitoring their trees, with informal observations of potential tree risk encouraged from members of their own staff, neighbours and the public, formal walk over surveys of all potentially dangerous trees carried out, at regular intervals, by dedicated members of their Grounds Maintenance staff and detailed inspections of high risk trees, carried out, at regular intervals, by a trained, competent professional.
- Provide training to their staff. In particular, a short (circ. half day) course on the identification of basic tree hazards should be made available to all permanent members of their Grounds Maintenance staff, with a more detailed (two to five day) course on tree inspections made available to those supervisors and managers tasked with completing the formal, walk over surveys.