Tree rings are said to show how old a tree is. So showing tree rings that indicates an older tree, it could either depict you as an old soul or that you are strong enough to grow. But it could definitely depend on your own story.
Cal constantly struggles with his belief that of the two of them, Aron has the good and he holds the bad, balancing the good and evil out. This can be seen in yin and yang, in which opposite forced are interrelated.