Sci-fi plot: Fungi are actually alien artificial life. They were designed as a huge family of microorganisms designed to recreate an alien biosphere. However, when they landed on earth, they encountered something which changed these plans.
specifically the ontology described during a "pink storm" in A Day for Damnation. A fungal-like substance (the pink fuzz, which looks suspiciously like cottonwood storms south of Seattle) is the catalyst for the rest of the ecology.
Also, be prepared that the series was never completed (IIRC he had a son with autism that took time away from writing).