@TF
Go to the quick mission builder, get into an aircraft and fly through the "non invisible" AKA "not a forest" trees. You will see they don't do any damage.
If you got destroyed while flying low you will have hit the ground or some other object.
Except for you flew into those that are that can be placed around airfields as static objects. Those have a collision model of course.
I was talking about the generic ones on the maps as being harmless.