Trees and the Risk of Harm

Trees are large woody perennial shedding organisms. It is normal for trees to shed dead branches as a part of growth and it is not uncommon for trees to occasionally lose large branches particularly during inclement weather. Occasionally even a whole tree can fail particularly when there are significant structural defects present near the base of the tree.

The probability of the shedding of woody components collectively can be referred to as “the probability of failure”. Probability of failure by itself is of no consequence. Failure needs to occur whilst a target is present and the impact must be significant enough to cause harm.

When considering the Risk of Harm consideration needs to be given to the size of the part that is likely to fail. The smaller the part the lower the level of damage that is likely to arise in the event of such failure. In almost all residential situations branches and trunks up to 300mm in diameter are unlikely to pose an unacceptable risk (see below).

When considering targets we need to break them into two groups, living and inanimate (non- living) objects.

Inanimate objects include things like cars, fences and buildings. Inanimate objects spend most or all of their time stationary and they are at a greater risk of being hit by a falling branch or tree. Most damage caused by trees is fairly minor and the amount of damage is usually far less than what most people anticipate.

In spite of the fact that inanimate objects are “easy targets” insurance companies do not rate the risk of harm of trees as particularly high. They do not ask about the condition of trees on a property, nor do they require trees to be regularly inspected. The reason for this in real terms is two fold. Trees in fact cause very little harm and many trees that are close to catastrophic failure are easily identified by most people and are removed.

In contrast with inanimate objects people are moving targets and as a result there is a lot less likelihood of being hit by a falling tree part. Sensory perceptions also allow for us to take evasive action and as a result the risk of mortality from all trees in Australia is less than 1 in 8 million. This compares favourably with a mortality rate from cars at 1 in 12,000, drowning at 1 in 125,000, stairs 1 in 1 million, or lightening strike at 1 in 2 million.

The often-accepted rate for Risk of Harm is a risk of 1 in 10,000. Clearly many trees pose a Risk of Harm that is much smaller than this with the average being less than 1 in 1,000,000 and the risk of being killed by a residential tree in Australia being less than 1 in 17,000,000.

Shouldn’t we get rid of any tree that poses any risk at all? No! The reason is obvious… that would involve many trees being removed and the secondary affects of that would be even more devastating. We must weigh up the benefits against the risk.

Whilst trees generally have a low risk of harm, sometimes conditions exist that increase the risk of harm to unacceptable levels. Should we remove them at that stage? Perhaps! There are a number of management options and it may be more appropriate to manage the risk. Of course this is not to say that removing such a tree is wrong. There are many times when removal is the most appropriate option, particularly when replanting occurs.

Is there anything else that you should do to reduce the risk of being harmed by a tree? Yes there are a number of simple things that can be done, but remember the overall odds of being harmed by a tree are already very small. Here are some things that greatly reduce the Risk of Harm associated with trees. Because trees are less likely to fail during the normal range of wind conditions

Do not camp under large trees particularly during inclement weather. In a thunderstorm or during lightning strike never seek shelter under a tree.
As far as possible stay in doors during inclement weather. Houses are strong structures and the risk of being hurt by a tree whilst inside a house is less than 1 in 200,000,000. This adds real meaning to the idiom “Safe as houses”.
Avoid or minimize the riding of bicycles and motorbikes during inclement weather.
If you must drive during inclement weather, reduce your speed when traveling in a vehicle under or adjacent to large trees.

These 4 steps alone will reduce your risk of being seriously hurt by a tree by more than 50% and it is a lot easier than having a tree removed.