I have always found this attribute amazing and quite scary. We have this tree in our front yard that was was planted the same time as every other tree in our neighborhood and we have always wondered why i was growing so slow and why it is not as big as the others on our street. Yesterday Holly (my wife) may have found a reason.
What you are looking at is a piece of string sticking out of the tree. You can see the mark on the left where a string was tied all the way around the tree and the tree had to grow around it. Now this normally is not a big deal like in the following picture where a tree is growing through a fence.
The tree has enough living tissue elsewhere to compensate it nutritionally loss. But in the case of a simple string around the ENTIRE tree it cuts off almost all growth of a tree. If you cut 1/4in deep around a tree it would die. The only real living tissue on a tree is the viscous film underneath the bark. This material is what changes the trees nutrients into new growth and eventually into heart wood (the colored center of trees) if this film is broken then the tree will stop growing. Lucky for our tree since the string was absorbent it allowed for the films continued growth and transportation of nutrients. The scar tissue that remains continues to chart directional growth of the bark.
If you find this stuff interesting make sure to take my furniture class in the spring.
Now the reason this is scary is you never know sometime what is inside a tree. This is a huge safety concern for woodworkers. This is why Georgetown College owns a handheld metal detector for woodworking. Because 50 years ago someone could have nailed a sign up on a tree with a nail and over the years the nail is now part of the tree and can get hit by a tool and cause harm to the worker and the tool.
Now some species of trees like the mahogany family will choke out other species underground to steal their light and nutrients. It is crazy to think about these slow moving combatants waging a super slow war beneath our feet. These super slow creatures wage war on sidewalks, houses, fences among other occupants made be people. It is sad to see them get removed when they have worked so hard to get to where they are. It has always been a mixed emotion for to cut down trees. It is not a habit I like to take part in. But I love to work with them after they are removed from the ground. I (like many other woodworkers) still see the wood as living after it is harvested. It breaths and moves and has its own will that you cant stop. It is just awesome to be part of the conversation and collaboration with what a tree wants to become.