Forest fires are started from lightning. When a fire is spawned, it will propagate across the terrain, spreading to nearby trees, shrubs, and bushes. The fire will continue burning until it has ceased to find suitable vegetation to propagate to. The larger the fire, the further its reach, so large flames can jump from tree to tree, whereas a small fire may only be able to ignite other plants immediately beside it.
Forest fires contain the power to potentially wipe out entire sections of the map from being habitable for herbivores, sometimes requiring migration to other parts of the map to find green vegetation.
Close proximity to forest fires will sharply reduce comfort. Standing in a forest fire will kill you.
Rain makes it more difficult for forest fires to spread. Due to this, fires are most likely to propagate aggressively during dry lightning.