They cannot, whatsoever.
But, you can create a dummy unit which has a flying movement above the unit you want to be able to see above trees.
Then do whatever with the dummy unit, kill it? make it follow the ground unit for a period of time, or always?
If you just want to see above trees "in front of the unit", create multiple dummies with sight range of say 150 in a line in front of the unit, based on its "viewing angle"...
PS: The dummy unit would of course be invisible, invulnerable, with the locust ability, so on so forth.