OOC: In the PHB, it describes "dense foliage" as one of the things providing 3/4 cover, and the cover sections says that cover applies to missile and melee attacks. It's wierd, I know, but I read it several times. Since the undergrowth is physically restraining our weapons, as opposed to concealement which simply obscures the target, everybody in the dense foliage benefits from 3/4 cover.