As far as I know, every MC is Fearless. The Fearless rule prohibits going to ground in any way.
Forests tend to be the trickiest of the terrain features, especially if you have loose trees in a defined area like where I play. We rule that since the trees can move to accomodate models that anything in them gets cover(unless it was a lot taller than the trees like Flyers or Titans). If the trees cannot be moved than LoS comes into play for determining cover for vehicles.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."