The problem is that the spell can be cast on over trees, and not on ground.
I think this is what he's asking for.
Otherwise you can use different style by creating a dummy spell for the AOE cast (Blizzard is preferable) and then use an summon spell with changed buff (ex: Summon Hawk, Summon Water Elemental) as well as changing the units for the summon spell, and use the dummy to use that summon spell.
PS: Forgot to mention that if you use the this way, you need to check the area for any tree and then destroy them (kill) in the trigger so it won't make units stuck between trees.