Trees do not heal - they just shut off

It's true, actually. Not just a joke. In contrast to animals, who in general heal or renew damaged tissue, trees just shut off, or compartmentalise an area of damage or decay. The detailed mechanisms of this process are only now being studied. The credit for this discovery, or at least for making it explicit, largely goes to Dr. Alex Shigo, who worked for the United States Forest Service. More on him on http://www.shigoandtrees.com.

The main idea is that when damage occurs to a tree, it will react to the damage, creating a number of chemical barriers around the damaged area. These barriers seal off the affected area, which will then rot out. If the barriers do their job effectively, fungal and other agents which cause the rotting will not spread into healthy wood.

I'm not being too specific about the barriers as it's a while since I studied it and I'm by no means an expert. Another thing that I have observed trees to do is to shut branches down. These tend to be the unproductive ones - those that are not photosynthesising a lot. This may be be because they are overshadowed by other branches, or because they are sick or damaged. In any case, their leaves will go brown and the branch will start to rot. The clever bit is that the conical section of branch inside the trunk will be sealed off, so that when the branch actually falls off the tree, it leaves a corresponding conical hole in the tree.

Today I climbed a large plane tree and kicked off a dead branch, and it did exactly that. I also saw a nuthatch scurrying around on another tree. Perhaps the hole I left will become a home for a nuthatch.