We have a great view from our back porch - we can see our yard, our landlady/neighbors yard, and the houses beyond. In my landlady's yard, a huge beautiful Live Oak tree was set right at the back half of her yard. Then one night I heard a crack, and half the tree came down into her yard. Luckily, it was the half that was reaching out over her yard.
She tried to just take off the part that was on the ground. If this had happened out in a field you could probably get away with it. Unfortunately - because of the way the tree became "unbalanced" the other half of the tree had to come down as well.
The landlady actually had to have someone from the city to come out to inspect the tree to make sure it had to come down. Oak trees are protected in our city, so she had to have a permit. She told me that the City arborist said her tree had included bark at the split point and it did indeed have to come down. I wish I had photos of the tree removal process, but my camera had run out of space. It was quite the interesting endeavor, with the workers swinging around on a rope with chainsaws. Whoa.
So the arborist said that the tree had included bark. What happened, is that the tree broke at its weakest point. When branches grow at a very narrow angle, bark becomes an increasingly large part of the connection between the branch and the trunk. Bark isn't an attachment structure of a tree. So as the weight increases on the branch, the small amount of actual connective tissue comes under increasing stress and inevitably gives way.
Ideally in city trees, when they are young, you prune them to try to mitigate the threat of potential property damage.
To the right is a preferable branch arrangement with the lovely 45-degree angle. On the left is an example of a narrow attachment that will most likely form with included bark. Photos from Urban Tree Foundation
So once the first part came off, the second part of the tree was more likely to come down as well. Who knows when it would have happened, but it would definitely have been way more of an issue than the first part, and including other property owners. Not pretty!
So - I know all this because I just finished a Citizen Arborist class conducted by Friends of the Urban Forest in San Francisco. It was amazing! We touched on everything from planting, to pruning, to identifying trees. I just wanted to share this perfect example that happened in the middle of the class, so I knew exactly what was going on, and why they had to take down the entire tree.
Hopefully in the next few weeks I can talk more about taking care of urban trees as I'm actually out doing it in the city! First assignment: Assessing the condition of newly planted trees.
EDIT: The after picture.