Well, I'm really not sure...
I may be wrong, but I think that the tiles (dense forest, light forest) have an impact on movement but not on LOS. I think that only foliage (1 tree, 2 trees, 3 trees, orchard-like aligned trees, brushes, etc.) has impact on LOS (but not on movement: you can move on dense, 3-trees forest tiles but not on treeless, dense forest tiles... ).