IIRC, at least on the original Stinger, the heat signature of the trees and other objects on the horizon confuses the seeker head so you need a clear shot. You

IIRC, at least on the original Stinger, the heat signature of the trees and other objects on the horizon confuses the seeker head so you need a clear shot. You also need the target to be a certain distance away, otherwise the rocket can't maneuver toward it. Something close and literally at treetop level might not be an ideal target.