Forests

A forest has a Line of Sight value of 5 -- models in or behind it can't see or be seen through more than 5" of forest between them and the enemy, but can see and be seen normally through less than that (as long as a Line of Sight can otherwise be drawn). A forest's own "base" doesn't otherwise block line of sight.

Cavalry, monstrous cavalry, or chariot models that march, charge, overrun, flee, or pursue through a forest must take a Dangerous Terrain test, as must Flying models that begin or end a flying move inside one.

A unit counts as being in soft cover if the majority of its models are within a forest. The same applies to a target beyond a forest if the shooter's own line of fire passes through it (even if the forest's trees have since been rearranged or removed in-game -- treat them as still being in their original position). This penalty doesn't apply if the shooting model is itself firing out of the forest it's standing in.

A unit with the majority of its models fighting in a forest can never gain rank bonus -- it's too difficult to hold tight formation among the trees.

A few examples of forests follow, using the rules above unless stated otherwise.

Any unit at least partially within an Ancient Forest suffers a -1 penalty to its Movement value.

A unit with the majority of its models within an Abyssal Wood suffers -1 to its Leadership -- this has no effect on models with Immunity (Psychology).

Whenever a spell is successfully cast by (or at) a model in a Blood Forest, every unit wholly or partially within it suffers D6 Strength 4 hits. Once those hits are resolved, the Blood Forest itself moves 2D6" in a random direction, moving around any intervening terrain by the shortest route -- this doesn't move the models that were inside it; the forest simply walks off and leaves them behind.

Any unit at least partially within a Fungus Forest is subject to the rules for Stupidity. A Forest Goblin unit at least partially within one is also Stubborn.

Any model in a Venom Thicket has the Poisoned Attacks special rule (close combat attacks only). Any model moving through one must also take a Dangerous Terrain test, representing its attempt to fend off the venomous critters within.

Roll for any unit at least partially within a Wildwood at the end of the Movement phase -- on a 4+, it suffers D6 Strength 4 hits.