To answer what seems to be the question. Difficult terrain?
It's not unlike a medium size creature that has to force its way thru thick brush ... assuming it's dense enough to be worth worrying about.
Keep in mind, a Grizzly Bear is large and an Elephant is huge per their Bestiary entries. Neither has any particular difficulty moving thru a typical forest or does much damage to the tree sized vegetation while moving on earth so you are talking about an unusually thick stand of trees to really do much. Certainly I have gotten fairly cinematic with scenes involving the surrounding forest but usually it has been restricted to the approach of some creature (building tension etc) rather than in combat but you could certainly have an Giant rip a small tree out if they found themselves short of weaponry in an encounter. Generally I'd think they'd be breaking off branches and such long before knocking whole trees down if that's what you are envisioning.
