In much of the world, vast forests are a relic of a bygone age. A few forests are left as preserves for rare pokemon or maintained for timber, berries, or other resources, but far more are clear-cut to make room for the march of civilization. In Kalos, the forests, led by the Trevenant, fought back.
Trevenant are powerful pokemon in their own right, capable of numerous powerful and mysterious attacks; those few willing to accept human companionship served their trainers extremely well in battle. But far more dangerous for loggers and the settlements which send them is their ability to command even the trees not possessed by spirits; it is all but impossible for armies to tell which portion of the marching, rock-hurling forest is the one or more Trevenant commanding them, and no less difficult to slay an army of trees and woodland pokemon without first eliminating their commander. Any advance by the forces of humanity over the Trevenant was soon found to be illusory, for in emergencies Trevenant are more than willing to recruit both sides' dead to replenish their own number and fertilize the next generation of trees.
Through war after war, humanity has learned harsh lessons; today, those who would anger the forests, and the Trevenant who lead them, do so at their own peril. Trevenant, for their part, have understood this and refrained from attacking human settlements, but woe to any poor fool who seeks to trim even a sapling in Kalos without the Trevenant's approval. Depending on the Trevanants' opinions of one's intentions, the Wayward Forest can be either a gentle pathway or an unending labyrinth, and the other forests in Kalos are not much kinder.