Tree Risk Assessment

When we assess a tree for potential risks, we tend to pay attention to whether or not the tree or branch is likely to fail.  Yet likelihood of failure does not necessarily constitute a high risk tree.

Likelihood of failure is only a part of the equation. 

An old tree growing in a forest that is clearly in the process of dying does not mean that the tree should be labeled a high risk.  Take the same tree in the same condition and place it in the centre of Gore Park and suddenly the risk rating changes. 

It is the same tree, it is just as likely to fail in Gore Park as in a forest so why is the risk so much greater in the latter scenario? Likelihood of impact.  The likelihood of impacting persons or property in a forest is much lower than at a park.

This is why when we evaluate a tree we try our best not to isolate just the tree, but also pay special attention to the context.

Context is Key

Before assessing the health of a tree, we gather as much information about the context that the tree is in.  “I see you have a doghouse back here, does it get used?”  “Do you use your backyard regularly?”  “Do you store valuables in your shed?”  “This patio looks pretty recent, when was it built?” “Were roots axed during construction?” “Was there previously a tree beside this tree?”

I once looked at a very large mulberry tree and parked underneath it was an almost equally large playground.  I asked the customer, “How often does the playground get used?” “Almost every day” he responded, “we run a daycare out of here.”  “So potentially even on windy days?”  “Ahhhh, yes” was his answer. 

I also noticed several branches that had tears on them, indicating previous branch failure.  This weighed heavily in my decision to recommend tree removal.  The final recommendation would have been completely different if the customer had said, “My kids use to use this playground but they are getting older and haven’t played on it in years.” 

Below is a chart to help determine a tree’s risk that has been borrowed from the International Society of Arboriculture (ISA) in their Tree Risk Assessment Qualification program for which DeVos Tree Care is certified in. 

ISA Risk Rating Matrix

According to this chart, a very large tree in a person’s backyard does not necessarily justify a high risk rating.  Even if just one branch breaks off, the consequences could be severe (see The Neighbours Tree), yet according to this chart its risk rating is low.  This is because the likelihood of the tree or a branch falling AND hitting someone/something is considered unlikely.   

The areas where there is the most concern is when there is a high likelihood of a branch breaking AND hitting someone/something and in addition to this, the consequences are high.  For example, a corvette that is parked 24/7 underneath a large and mature tree that has already shows lots of signs of failure would be considered high/extreme on the risk matrix.

This may not mean removing the tree, perhaps just a conversation with the car owner about finding a new place to park.  Suddenly, with the car moved,the risk rating could potentially drop down to low.

Unsure where your tree might fit on this matrix?  Give us a call and we would gladly be willing to have a look. 

Lastly to understand a few signs on your tree that factor into high likelihood of failure; have a read through part 2 when we list the top 5 signs of tree or branch failure.

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