Doomtime — Doris Piserchia

It all began when someone tried to push Creed into the flesh pool to be ingested. The assassination failed, but Creed was never the same again. Because it launched the new cliff-dwellers of Creed's colony onto a new course of life - which could lead to humanity's re-emergence as Earth's masters.

In those far future days, Earth's masters were two trees. Not trees as we know them, but two Everest-high growths, whose sentient roots and fast-growing branches dominated every living thing on the world. Men lived between their arboreal combat.