The Radiant Grove (Pathfinder)

From publisher blurb:

In the lead up to the Great War, a cave system in a once-unspoiled park in western Wisconsin was turned into a dumping ground for radioactive and biological waste. Somehow in that toxic ground, a miraculous tree bloomed. In the darkness it ate the radiation rather than depending on light from the sun.

Fifteen years ago, a band of humanoid apes discovered this "Tree of Life" and built their home in it's branch. Their leader, Longmane, has long believed that a time will come when humans die off and apes inherit the Earth. Now the signs Longmane reads tell him that time is now.