Why Trees Fall

Understanding and Mitigating the Risk of Tree Failure

It’s impossible to predict precisely when a tree, or a portion of one, will fall. Couple that with the damage a failure can result in and you get RISK! By determining how likely a tree is to fail, you can make informed decisions and effectively mitigate that risk.

A tree will fall if the force or load applied to it is too strong. When you consider the force of gravity applied to a tree is increased 25% by rain and up to 800% by ice, you can imagine that more failures will occur during severe weather. But trees fail during typical conditions too and it’s important to understand why.

Species and age of the tree also play a role in tree failures. All species have a failure profile that details issues commonly observed in that species. For example, maples, ash, elm, birch, and conifers are prone to branch failures and should be routinely inspected for signs of branch weakness.

Explore photos of structural defects in trees, which may predispose them to failure - even under normal weather conditions.

Photo Gallery: Seven Warning Signs of Tree Failure
Explore structural defects which may predispose trees to failure – even under normal weather conditions.

Beyond the tree itself, soil and site characteristics are important. Some soils become easily saturated with water and root plate failures are more likely to occur. A change in grade or compaction of the soil (due to construction or even foot traffic) will also have an impact.

Determining the Risk of a Failure

If a structural defect or other issue exists on your property, don’t rush to assume that the tree should be removed. The key is determining the severity of the defect and degree of risk. On a commercial property, that risk is not just about the likelihood of the failure itself. The surrounding site, damage that could occur and the safety of people on the property must also be accounted for.

A severe or extreme defect increases the likelihood of a failure – particularly during inclement weather. This might be a dying tree or perhaps one with severe wood decay.

But consider a tree with a more moderate defect. On a woody property border, the tree owner/manager may choose to make no investment at all and let nature take its course. Take that same tree and position it next to a structure and the scenario changes. Given the potential outcome, the tree may now pose an unacceptable level of risk. However, based on the situation and tree, options other than removal exist that can decrease the likelihood of failure.

Lowering the Risk

Sometimes simple, easy-to-implement solutions can reduce the risks associated with tree defects. Removal of dead, weakly-attached and hanging branches can immediately improve safety on a property. This type of pruning and other routine care, like regular fertilization, keeps plants healthier – and better able to withstand the forces of gravity and wind that are responsible for toppling trees.

Construction Protection

Cordon off areas around trees when construction work is being done nearby to avoid root damage and soil compaction.

When an entire tree falls, an issue with the roots or soil is often to blame. Alleviating soil compaction can improve the conditions in which roots grow, enabling development of a strong root system. Given the damage that can be caused to roots during construction, remember to cordon off large areas of soil around trees when work is being done nearby.

The architecture and form of the tree can also be strengthened. Guiding good growth form is best accomplished when a tree is young, but maturing trees can also benefit from pruning when a defect like codominant stems exists. Brace rods can be installed through branch unions or between multiple stems to provide rigid support. In the upper crown, high-strength steel cables can help limit the movement of supported branches so they are less likely to fail.

The sooner potential hazards can be identified, the more likely the issue can be addressed, thereby saving the tree and minimizing the risk of potential failure. When the severity of the defect is extreme and the consequence of failure is likely damage to people or property, that risk should be mitigated with removal. This will always be dependent on the tree, site, and level of risk acceptable to the tree owner/manager.

The Bottom Line

Understanding why trees fail and when they should be removed is critical to managing a landscape budget. This knowledge enables a property manager to prioritize care of trees while maintaining a safe landscape.

Trees with severe defects can be removed to prevent unexpected failures that cost excessive time and money. Moderate defects can be inspected to help gauge the life of the tree and improve the ability to plan for replacement costs. For example, a tree with slight decay may pose no immediate threat. Routine monitoring can identify when the decay, and threat of failure, begins to worsen. With this understanding, a property manager can better plan for removal and replacement costs – and minimize unanticipated expenditures.

While nature is unpredictable, risk is manageable. Knowledge is the first step followed closely by the ability to use that knowledge in making both short and long-term decisions about tree care and removal.

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