What is a Tree Risk Assessment? And What is an “At Risk Tree”?

A downed tree can cause all kinds of problems. Trees fall on and damage structures, take out power lines, and block roadways.

Because of this, it’s important to assess whether a tree has become hazardous, particularly after a weather event, but also in general. This process is called a tree risk assessment.

What is a Tree Risk Assessment?

A tree risk assessment is typically performed by a qualified arborist. The arborist will assess a variety of factors that include soil conditions, what structures are endangered if the tree falls, the overall health of the tree, wind exposure, etc.

Typically, arborists who perform this kind of work have the ISA Tree Risk Assessment Qualification, which includes specialist training in assessing trees. A tree risk assessment includes both a visual assessment and various other tests including static load tests, using sensors to map strain, digital surveys, etc. Work is being done to make tree risk assessments more objective.

The arborist will then assess the risk according to a matrix that takes into account the danger of the tree falling, the likelihood of it hitting something important if it does, and the amount of damage it might do. For example, a small branch coming down will do much less damage than if the entire tree falls.

At this point, the tree may be classified as an “at risk tree” which may need to be mitigated or removed.

What is an “At Risk” Tree?

An “at risk” tree is a tree that is particularly likely to fall. Arborists class trees in four levels by likelihood of failure.

• Improbable. The tree is unlikely to fail even during severe weather.
• Possible. The tree could fail, but it’s unlikely to happen under normal conditions.
• Probable. The tree might be expected to fail even under normal conditions.
• Imminent. The tree is on the edge of failure even if nothing happens.

A tree that is at imminent risk of failure should in most cases be removed. So, what kind of things in the tree risk assessment could indicate to an arborist that a tree is about to fall? Here are some examples.

• Significant cracks in the trunk
• Missing or abnormal bark, which can indicate an infection or structural issue.
• Sapwood damage or decay
• Heartwood decay
• Large cavities or hollows in the tree. Some tree species can stand for a long time completely hollowed out, while others become vulnerable.
• Dead structural support roots
• Significant weakness of the soil under the roots

Arborists look at all of these while conducting a tree risk assessment. For example, one thing which can cause failure is soil saturation after heavy rain. This can weaken a tree’s structural support but is generally temporary. An arborist is likely to recommend temporarily supporting the tree until the soil dries out. They also consider the species of the tree. The structural qualities of the wood, for example, can help establish whether a large cavity in the trunk is a problem or something that particular tree can simply live with. Species also determine what pests the tree might be vulnerable to, which is important as damage can sometimes allow an entry point for systemic pests. A tree risk assessment may not always be necessary, though. 

At risk or hazard trees need to be quickly identified after a disaster, storm, fire, or other incidents. A quick visual assessment can sometimes identify them, such as if a tree is leaning or has cracks in the trunk.

What Mitigation Options Might an Arborist Recommend?

Mitigation options might vary. For example, if a tree is in the middle of a grassy area where few people go and there are no structures, an arborist might recommend minimal intervention after completing the tree risk assessment in hopes that the tree will survive if left alone.

Meanwhile, they might recommend the immediate removal of a tree that is endangering an occupied structure such as a home.

Between these two extremes are a variety of mitigation effects, which include:

• Pruning and removal of dead or diseased branches, which might also involve reshaping the tree so that it is at less risk in the future.
• Relocation of the “target,” that is to say the thing the tree or branch might fall on. For example, rather than cut down the tree, you could remove a picnic table to a different location.
• Maintenance or treatment to improve the health of the tree and help it to recover from the damage.
• Bracing and supports to keep a tree upright or support branches while the tree recovers.
• Monitoring the tree for decay or disease which might have been triggered by the damage or the pruning done to correct it.

Needless to say, the arborist might recommend more than one of these based off the results of the tree risk assessment…or all of them…when dealing with a particular at risk tree. Certified arborists prefer not to remove trees if they can be avoided, and will make every effort to save a tree if possible. So, they might suggest that you prune dead branches and also remove furniture from under the tree, at least temporarily. If a tree does have to be removed, you should talk to the arborist about the best replacement, whether it’s the same species or a different one.

Should You Have Your Trees Assessed Regularly?

Typically, tree risk assessments are done after a storm or disaster to ensure that damaged trees are removed or mitigated. However, there is some value in having trees assessed regularly.

Doing so can help you predict risk and may reduce your liability. An arborist can also give you advice on how to better maintain the health of your trees in general and how to reduce risk before an event. Such mitigation might include pruning to reduce wind resistance, applying fungicides to treat infections before they become serious, using fertilizer, installing lightning protection, and even choosing which trees to plant in the first place.

Giving your trees a regular checkup can help with risk management and also with protecting their health and beauty.

If you have had a weather incident and have damaged trees, Essel’s arborists can help by performing a high-tech tree risk assessment and tagging trees for removal or other mitigation. You can then work with your regular arborist to mitigate the damage and protect as many of your trees as possible.