I thought that damage was based on animation actually hitting unit, at least thats how it worked in ETW. If the trees and arrows can collide with one another then that could explain the lack of damage to units. You have the arrows leaving the forest hitting trees as well as arrows entering the forest and hitting trees. Its not like a unit suddenly walks into the woods and gains +500 to range defense allowing a samurai to have 40 arrows sticking out of him claiming to be the master pin cushion.