If a tree fails, is there a reason?
One thing I’ve often heard from people in the past when talking about tree failure is, “that tree or branch just up and fell.” I wouldn’t go as far as saying it is a pet peeve, but this thinking does bother me a bit as it shows a lack of understanding of trees and how they should be thought about in the landscape. Trees do have inherent risk. They are dynamic, sometimes very large living plants that have to deal with all of nature’s forces, but this doesn’t mean that they die and fall one day out of the blue. There is always more to it.
A combination of causes
It is useful to think about tree failures as a mathematical formula. Instead of the variables being numbers or symbols, they are things such as decay, root damage, weather, structural defects, or other natural factors. One of these alone most likely won’t equal tree failure, but adding all of these elements together just might.
A story of tree failure at Minnetrista
Let me present an illustration from a tree recent failure in Oakhurst Gardens at Minnetrista. The tree was a young catalpa. Perhaps thirty feet tall. One of the gardeners found the tree with its top broken and hanging during a campus search for storm damage. After using a rope to pull the top down safely, the gardeners and I used a chainsaw to clean up the mess. This isn’t where it stops for us however. We always want to try to understand why the tree failure happened in the first place.
It turns out that in this particular tree there were co-dominant leaders. Co-dominant leaders are stems that are usually close to or equal in size that compete with one another to be the tallest branch of the tree. Co-dominant leaders usually have very narrow attachment angles to each other. As the branches grow larger, the union is not strengthened like a horizontal branch is in relation to a trunk. This encourages what is called included bark, and makes for a weak branch union.
The Minnetrista gardens also happened to receive snow the previous night, and at this time the leaves were still on the tree. This can be very bad news for trees. The extra weight of snow sticking to leaves can very often overload a branch that would normally be fine without leaves.
A formula for failure
So the formula fills in. Co-dominant stems (weak union) + included bark + overloaded branches = tree failure. It is very possible that the addition of wind would have had a strong effect on the tree’s failure as well, but this cannot be proved.
The catalpa failure ended up being a pretty simple case, but figuring out the cause of tree failure can get more complicated. What is important is to learn from these experiences so you can attempt to prevent tree failure in the future. In the case of this catalpa, a little training and pruning for structure when it was younger would have prevented this failure from ever happening.
Have you experienced tree failure before? What were the causes? Let me know in the comment section below!