Tree Risk

This chapter explores one of the five Strategic Objectives for tree management: tree risk. It sets out current priorities within this topic area and explores where action should be taken.

What is it?

Preston City Council owns and manages a range of publicly accessible open spaces across the city, including parks, cemeteries, and nature reserves, which contain the majority of Council-owned trees. Tree risk refers to potential harm from the failure of one of these trees.

The Council has a duty of care to take reasonable actions to reduce foreseeable risk of injury to people or property.

The approach to tree risk management set out in this Strategy applies to all Council-owned trees. Where responsibility for the management of Council trees has been transferred to other parties, the Council will require them to adhere to the principles set out in this Strategy, such as through lease agreements.

Where the Council provides arboricultural advice to any third party, it will do so in accordance with this Strategy.

How does this support the vision?

Tree risk management can provide the architecture for successful management of the existing tree population and therefore contribute to a healthy, thriving treescape.

Risk management is not the only objective, but the activity it requires provide familiarity with the treescape, data, systems, and opportunities for intervention that can support other objectives.

Effective management regimes will ensure that existing canopy cover is retained or enhanced wherever possible and will help to increase resilience to future challenges such as climate change.

The data collected as part of this process will help to establish an accurate baseline for the treescape, allowing the development of key targets and the measurement of future progress.

Efficient tree risk management saves money and avoids unplanned and unexpected costs. This supports a broader enhancement strategy by allowing resources to be diverted away from fixing problems and towards proactive improvements.

What are the Council's strategic objectives?

These are the key deliverables for the Council in this topic area:

Table 13 - Strategic Objectives for tree risk management

Proactive Philosophy

The Council aims to take a proactive approach to tree risk management, identifying and managing risks before failure occurs. For the purposes of risk management, a 'tree failure' is any harm that arises as a result of a change in the form or condition of a tree.

It might include a mechanical or structural breakage, or growth leading to a hazardous obstruction. Tree risk assessment by the Council will seek to identify all such risks, assess them, and act where the risk is unacceptable.

Tree risk management starts the moment a tree is planted. Species selection, planting techniques, tree pit design and adequate volumes of uncompacted soil all help with the successful establishment of healthy and problem-free trees, such as by increasing resilience to external factors such as pests and diseases.

Formative pruning and regular maintenance will remove potential weaknesses and improve the structure of the crown, reducing the chance of future limb failure. Establishing healthy tree stock from the outset will reduce the need for future tree risk management.

Small interventions by skilled operatives can avoid larger later remedial costs.

Preston City Council is fortunate to have many mature trees in its streets, parks and open spaces. As these mature trees start to decline, they may pose a higher risk of failure, which due to their urban setting, could result in harm to people or property if it is not managed.

The Council will develop a proactive approach to sensitively managing the decline of mature trees which is appropriate to their settings. For example, in some larger open spaces it may be possible to allow the natural decline of trees by moving targets away from potential harm or to reduce trees to standing monoliths and maintain them for biodiversity.

Above all, the proactive philosophy means looking for and anticipating risks rather than waiting for problems to materialise, and using resources efficiently to mitigate risks. In this, it is essential that decisions are taken systematically and objectively.

Action will be taken where it will be most effective in reducing risk, not where it will satisfy the most persistent complaint. Tree works will not normally be undertaken in response to a perception of risk, or a preference; doing so draws resources away from more effective interventions.

Tree Risk Inspections

In order to manage tree risk, the Council will undertake regular inspections of all trees that it owns. These will be planned in advance in a rolling programme. Trees in more sensitive locations will be inspected more often than those in less sensitive locations.

Although tree risk management is the core aim of this activity, it will also be integrated with other objectives to add value. Tree inspection can help to generate data, provide familiarity with the tree population, and inform decision making in other areas. It also requires good systems, all of which can deliver benefits more broadly.

The main output of tree risk inspections is the mapping of all the trees that the Council has responsibility for, along with sufficient information about each tree to understand and monitor its condition and make a risk assessment.

As a minimum, the following information will be recorded during tree risk inspections.

The format is not prescribed by this Strategy, but the core contents of each assessment should always include:

• Type of feature (tree, group, woodland)
• Species
• Age class
• Dimensions (e.g. height, stem diameter, canopy spread)
• Condition
• Risk assessment
• Works recommendations

It may be advantageous to expand the range of data that is collected during tree inspections in support of other objectives.

The data generated through tree inspections will provide the Council with a snapshot of its current tree population.

This will be helpful in understanding the spatial distribution of tree cover across the area, and the current condition, age and species composition of the treescape. As re-inspections are completed, this information will be kept up-to-date and therefore remain useful as a baseline for decision making.

Systems

Tree risk management undertaken by the Council will use a GIS based approach comprising a map of tree locations, and a database containing the information produced by inspections.

To record information consistently during inspections, the Council will use a tree risk management software system. Observations and recommendations will be recorded electronically on site and saved onto a shared central system where all relevant staff members can access them.

Any employee expected to use tree risk management software will be given adequate training and will undergo refresher training as required. External contractors will also be expected to use the Council's system for undertaking inspections, or to produce data in a compatible format that can be uploaded directly into the database.

Risk assessment process

Tree inspections will be undertaken by an appropriately qualified arboriculturist and will involve a systematic risk assessment of each tree.

This will include an assessment of the likelihood that a failure could occur, the sensitivity of the surrounding context to harm, and the type and significance of harm that could occur in the event of a failure.

Risk assessment will be undertaken at defined intervals to ensure that it remains reliable as a description of a dynamic and organic asset. The frequency of ongoing inspections for trees will vary according to one or more elements of the risk assessment.

Inspections will be more frequent where deterioration in the reliability of the assessment over time could have greater consequences or is more likely.

The results of the risk assessment will be used to categorise trees in terms of their relative level of risk. A threshold will be established within the assessment process to define the level of risk that is tolerable.

Tolerable risk is an important concept in this approach. It represents risks that are broadly acceptable because they are outweighed by the many benefits that trees provide. They include both minor inconveniences that do not warrant intervention, and also more significant harms where a competent assessment has identified that the possibility of occurrence is sufficiently remote.

Risk management

Trees which, based on the outcome of the risk assessment, are hazardous (i.e. those with a level of risk that exceeds the threshold of tolerable risk) will be identified and appropriate remedial measures will be recommended, with input from other specialists or managers as required. Interventions will be designed to mitigate the particular combination of attributes that have been found to exceed the level of tolerable risk.

Interventions may also incorporate other objectives such as habitat creation or amenity where these do not run contrary to the core objective of risk management.

Interventions may include tree pruning and/or removal as well as other operations such as redirecting or relocating targets, providing structural support or bracing, or making changes to the context to reduce the likelihood of failure.

Interventions will be proportionate to the actual risk posed by an individual tree or tree group.

The Council can never guarantee that a retained tree is completely safe but disproportionate responses to actual risk can result in unnecessary intervention or widescale tree removal which will be avoided.

A timescale for completion of each intervention will be defined. This will be set in proportion to the urgency and significance of the hazard, and sometimes to reflect the optimum season for completion.

Proactive or routine interventions may be completed with a lower priority, whereas immediately hazardous or emergency situations will be given a higher priority.

Remedial tree works will be carried out as specified by competent individuals and verified by an appropriately qualified member of the arboricultural team. Completion of works will be recorded, typically with photographic evidence.

Surveys and surveyors

All tree risk inspections will be carried out via a rolling programme of surveys. These will be undertaken by competent arboriculturists with appropriate qualifications, training and experience.

Detailed inspections requiring specialist equipment (e.g. aerial, invasive, stress loading or scanning inspections) will be carried out by an expert and will usually require an external consultant. These are expected to be a relatively uncommon occurrence.

Tree Inspectors will be qualified to a minimum RQF Level 3 in Arboriculture. They will also have either:

(i) Lantra Professional Tree Inspectors course; or

(ii) Equivalent relevant qualifications and/or experience, and the support of Tree Inspectors that have passed the Professional Tree Inspector course.

Surveyors will undertake regular CPD and ensure that they are up to date with relevant policy and guidance.

Responsibility for quality assurance of all survey work will rest with a nominated Tree Officer or other appropriately qualified senior member staff.

Where external consultants are used, the same minimum standards will be applied as to Council staff, according to the role they are performing.

Resourcing

Initially, the Council will need to forecast survey requirements for all Council owned trees and allocate resources accordingly.

After an initial assessment has been undertaken, future surveying resources will be based on recorded inventory data.

Reactive Inspections

The Council will carry out reactive inspections in response to specific events such as tree failures or storm damage. The Council will develop a plan defining when reactive inspections will be carried out and how they will be undertaken.

Where an enquiry relating to tree safety is submitted by a member of the public, a tree inspection will be undertaken before any works are specified on the grounds of risk management.