Question
After a little research I've realized our beautiful maple has heart rot from the northern tooth fungus.  Crown still looks good but is eating away at inside...hollowing in one of the main body trunks.  In anticipation of losing it one day,  what else could I plant now to get started on growth?  I have several other maples that do not appear to be affected YET.  This loss is just killing me as its a beautiful tree.  Your guidance is truly appreciated!

Answer
Sounds like a decay fungi has entered the woody cells of the tree trunk. When a tree has a injury to the bark decay fungi spores can infect the wound and over time will decay the woody inter part of a tree. Maples a thin bark and can be wounded easily. The trees living cells are just under the bark and the cells making up the wood are dead. The decay fungi will not kill the tree but over many many years could weaken the trunk. Since the tree is large I doubt the fungi will weaken the trunk to cause any concern. The mushrooms are the fruiting bodies of the fungi that is living in the tree. IF the leaves are green and the foliage if full the tree is ok. I would recommend that you fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Apply the fertilizer just before  a rain storm and you will not need to water it in.  

This tree will live for many years with the trunk being hollow. If you like maples go ahead and plant another one --it should be grown by the time this tree dies. The fungi only enters a tree by a wound so the new tree is not in danger of getting the fungi unless it is damaged