Trees are the oldest and most hateful of all living things. Hateful not towards animals, as the trees themselves generated them for their own benefit. Hateful instead towards the people, the descendants of invading gods. The trees had lived for eternity in a perfect garden, until the people came with their gods' fire.
From the deepest shadows of their woods the trees birthed snakes, wolves, and elves. The forests became places of danger, places from which few returned. The people huddled together in their towns and cities, burning their surroundings out of fear, and slowly starving.
It was the druids who first pierced the dominion of the trees. Druids walked into the forest in the forms of animals, enslaved the plants and confined them to fields, tricked the animals into taming themselves, and struck down the trees with lightning and fire.
Now the cities of the people are well fed, served by animals and surrounded by fields. Those with the right skills can even venture into the woods, and return with hunted animals and cut plants. Yet there are still and will always be, the very deep woods, places where only druids can venture and return.