Tree Stem Butt Rot

Here are 2 healthy trees that I will watch to see if the exposed parts of the stems of these trees will cause them to be in distress. Currently there seems to be no problems at all.

This Bitternut Hickory seems to have some butt rot in the base of the tree. 

Here the E. Cottonwood tree seems to have grown together at the base and further up the stem of the tree. Why was there a gap in the middle? Why did the tree wait to grow together further up the stem?

This tree does not seem to have the problem the Bitternut Hickory has. There is not as much decay happening with this tree.