Last week I talked about a large white oak I came across on a client’s development site. This tree had some signs that could cause concerns about failure. One of the things we look at in such situations, in addition to the tree itself, is the presence or absence of potential targets.
What is a target? A target is something that could be hurt or damaged if the tree fails: a person, a building, etc.
How do we identify targets? Look at the tree. Make a circle around the tree with the radius of the circle being equal to the height of the tree.
It does not have to be exact; in very high winds, the tree may fail beyond the target line. If it is only part of the tree at risk of failure (I.e., a specific branch), the target zone may be smaller than the height of the tree. Hopefully the exercise is illustrative and gets you in the habit of visualizing target zones and targets.
With GIS tools, we can do this by buffering the points in a tree inventory. Take a look at the inventory below. The tree with the yellow arrow is a huge tree. However, if it fails, it should not land on any improvements (not quite the proverbial tree that falls in the forest, but close).
Now look at the tree at the red arrow. It is less than half the diameter of the other tree, but look at what could be impacted if it failed: a parking area and the cars in it to the south, and sidewalks to the south, east, and north. GIS tools let us make these buffered target zones partially transparent so we can see what lies beneath them – very useful in tree risk management plans and inventories.
So we’ve looked at two of the main considerations in deciding whether or not to save a tree on a construction project – whether or not the tree is likely to fail and whether or not it could hit anything if it does. To put this in a risk management framework, risk is defined as probability x consequences. In the next part of the series we will look at tree care industry standards and best practices for tree risk management.
