Question
I have a birch tree and the base of the tree by the roots appears to be rotted out or maybe a disease.  It is quite a large area but the tree itself appears healthy.  I think I saw a mouse burrowing into the open area as well.  What do you think this could be and can my tree be saved? It is at least 20 years old.

Answer
As long as the foliage of the tree is healthy looking and green and full the tree is OK. A tree can survive with the trunk being hollow. A decay fungi will get into the trunk through a wound and decay the woody non living inner part of a tree. I would fertilize the tree this fall with 10-10-10 fertilizer scattered around the tree and watered in good to help the overall health of the tree. Do not worry about the hollow in the trunk.