There are a multitude of factors to weigh in this fundamental process.
Mitigation is the process of reducing risk. Measures to mitigate risk can be arboricultural, to reduce the likelihood of failure or the likelihood of impact; or they can be target-based, to reduce the consequences of failure and impact.
Tree risk assessors should resist the ultimate security of risk elimination based on tree removal and consider possibilities for retaining trees when practicable. Trees offer many benefits, so removal should be considered as a last option to reduce or eliminate risk. In many cases, there are other options available to reduce risk to an acceptable level.
These options include the following:
Target management: Movable targets within the target zone may be temporarily or permanently relocated. Mobile targets, such as pedestrian or vehicular traffic, can be rerouted or restricted from using the space within the target zone. These are often the solutions that will have the lowest impact on the tree and are therefore preferred if tree preservation is a primary management goal.
Pruning: Dead, dying, and weakly attached branches can be pruned in accordance with the applicable national pruning standards or the International Society of Arboriculture Best Management Practices for tree pruning. Wind resistance can be reduced with reduction pruning or pollarding, and to some extent, thinning. Topping is not recommended due to the long-term problems with weak sprouts and the entry of wood decay. Crown raising can eliminate lower branches that could be interfering with structures, pedestrian or vehicle traffic, signs, or safe views. Excessive raising, however, can reduce taper development, change sway patterns and limit the tree’s ability to damp the effect of dynamic wind loading.
Installing structural support systems: Structural support systems can be installed to limit movement of certain tree parts. Various types of hardware are used, depending upon the goals.
- cables (flexible braces) installed in the upper crown to limit the movement of weak junctions or codominant stems;
- brace rods (rigid braces) installed close to or through weak junctions, or through split sections;
- guys installed to improve anchorage and stabilize lean;
- props installed to support some leaning trees and low branches from below.
Tree growth regulators: Using tree growth regulators in conjunction with pruning may extend the reduction in wind resistance or clearance from utilities and structures for several years.
In some low-use locations, dead and decaying trees may be retained for wildlife habitat or other uses. Selection of suitable wildlife habitat trees must consider the risk, as well as its value for wildlife. One management strategy is to ensure that wildlife habitat trees are maintained at a height shorter than the distance to the nearest target.
Over-mature trees in natural settings may reconfigure as they age and deteriorate, a process sometimes called “natural retrenchment.” They may continue to grow trunk diameter while branches die and fail – reducing overall height of the tree and increasing stability. Where tree risk is a concern, tree risk assessors can imitate this process by recommending crown reduction. Eventually, however, even tall stumps without branches will fail when the roots decay. For this reason, the integrity of roots and the trunks of trees with targets that are retained for wildlife habitat should be monitored and removed if the risk exceeds allowable thresholds.
Mitigation may involve more than one action. The tree risk rating based on the highest risk factor assigns an overall tree risk, but there are typically other factors that also should be considered, which may require additional actions if the tree is to be retained. Once the highest risk factor has been mitigated, the tree risk rating goes to the next highest risk factor.
With some extreme-risk trees, mitigation may involve the immediate restriction of use within the target zone before removing the tree. Safe tree removal may require using a crane or an aerial lift because additional loads on the tree – from the weight of a climber or from lowering branches – may cause the tree to fail. However, on some occasions, tree removal is not necessary, even for high-risk trees. Removing the target, pruning, or installing a support system may significantly reduce the risk posed by the tree to a level that is acceptable to the client.
With a single, privately owned tree, there is little need for the prioritization of work. The owner will make a decision on the remedial treatment and schedule the work. The exception is with extreme-risk trees, where the tree risk inspector should recommend that the work take place as soon as possible and the target zone may need to be restricted immediately.
With populations of trees, such as in municipal or utility applications, work may need to be scheduled well in advance. The risk assessment can be used to establish the work priority to mitigate the highest risk situations first. If a tree with an extreme risk is discovered, the owner/manager should be notified promptly, and action should be taken as needed.
Residual risk is the risk remaining after mitigation. Following any mitigation action, there is a residual risk posed by that tree. With tree removal, that residual risk is brought to near zero; however, even stumps can pose some residual risk. The level of residual risk needs to be acceptable to the risk manager/owner. If the residual risk exceeds the acceptable risk level, the specified treatment might not be the best course of action. As previously noted, tree removal should not be recommended without due consideration of the benefits that would be lost.
Inspection frequency and timing
The inspection interval is the time between assessments. Because site and tree conditions change over time, risk assessments should occur on a regular, recurring basis when justified by the level of risk or target value.
On residential and commercial properties, timing of the initial risk assessment and frequency of future assessments is rarely at the discretion of the tree risk assessor. However, after a tree has undergone the initial assessment, an inspection frequency should be recommended based upon the level of risk and the goals of the client. The inspection interval typically ranges between one and five years, but it may be more or less often depending on the age of the tree, level of risk, specific conditions, client goals and resources, or regulations. For example, a moderate-risk tree may have an inspection interval of three to five years; a high-risk tree, one to two years. Generally, it is a good idea to inspect trees with known structural weaknesses and/or high-value targets after major storms or other exceptional events on the tree site (such as forest clearing, trenching, or other construction work) to identify damage or changes in condition that may have occurred.
The time between tree risk assessments conducted by or on behalf of municipalities, utilities, and other entities that manage large populations of trees is often defined by the risk managers acting on behalf of those agencies or their controlling authority.
Public agencies, utilities, and large property managers may identify zones of similar tree population, site usage, and facility type. The inspection interval for each zone is then specified. Zones of higher priority should be inspected more frequently than zones with lower priority.
Scheduling assessments in a specific season can aid the assessment. For instance, the crowns of deciduous trees are more easily assessed for failure likelihood in the winter than in the summer when in full leaf, because the branch structure and unions are more visible. If a particular decay fungus produces annual fruiting bodies (conks, brackets) at specific times of year, but which subsequently degrade or disappear, identification of that decay will be easier if the inspection is scheduled to correspond with the fruiting body emergence.
Staggered inspection intervals, such as every 8 or 16 months, will allow the tree risk assessor to see the trees in different seasons. For the utility arborist, assessments can be combined with periodic inspection for clearance.
After conducting a tree risk assessment, the information, conclusions, and recommendations need to be communicated to the owner/manager, client, designated person, or agency. The preferred method is in a detailed written report, since this protects both tree risk assessor and client from misunderstandings. In some instances, however, the report also may be verbal or videographic.
A work order with a recommendation for risk mitigation also may be considered a form of a risk assessment report. The tree risk assessor should maintain copies of all risk assessment reports.
The following items should be included in a detailed written report or other accompanying documentation:
- Name of the tree risk assessor and the date of assessment.
- Statement of the assignment scope of work.
- Location or identification of the tree(s) assessed.
- Level of inspection (limited visual, basic, or advanced) and details of the method (for example, “Basic assessment, using a mallet and a probe”).
- Targets, occupancy rates, likelihood of impacting the target, and potential consequences of failure.
- Site factors that were considered (history of failure, storm patterns).
- Documentation of the likelihood of failure, such as a list of tree conditions, structural defects, and response growth that were observed; measurements of defects (size and shape) may be included.
- Risk assessment and conclusion (for example, “The likelihood of a failure striking the house is ‘likely’ and consequences to the house could be significant; therefore, the level of risk is ‘high’ “).
- Options and/or recommendations for mitigation (for example, “Move target, prune, or remove the tree”).
- Residual risk information.
- Recommendations for reassessment.
- Limitations of the assessment
Limitations of tree risk assessment
Limitations of tree risk assessment arise from uncertainties related to trees and the loads to which they are subjected. The scientific study of tree failure is relatively young; there is still much to learn. As an example, the specific wind speed and direction that will cause a specific tree to fail is unknown. There are tools for determining wood condition, but the requirements of a specific tree to avoid failure are not known.
Tree risk assessors perform assessments with limited information about the structural condition of the tree itself and the environment that affects it. For instance, root decay may be present but not visible. Similarly, abnormally strong winds that create loads greater than the tree can bear are not always predictable. Experience, training, and education influence how the tree risk assessor sees the situation and analyzes it to form an opinion about what is likely to occur in the future.
Tree risk assessors should include the limitations of their assessments in their risk assessment report, including the limitations of the methodology used, and any limitations related to the ability to access or assess the tree, site, or potential targets.
Some of the limitations that are common to risk assessment reports include:
- Tree risk assessment considers known targets and visible or detectable tree conditions.
- Tree risk assessments represent the condition of the tree at the time of inspection.
- The time period for risk categorization should not be considered a “guarantee period” for the risk assessment.
Talking About Mitigation
Most tree risk assessors will recommend the following:
1. Extreme-risk trees should be mitigated as soon as possible. Immediate action may be required to restrict access to the target zone.
2. High-risk trees should be mitigated as soon as it is practical, when the work schedule or pruning cycle allows.
3. Moderate-risk trees may be mitigated and/ or retained and monitored. Mitigation may be conducted when budget, work schedule, or pruning cycle allows, preferably before seasonal storms develop.
4. Low-risk trees should be retained and monitored (if appropriate) and/or mitigated, if deemed necessary, when the budget, work schedule, or pruning cycle allows.