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I think (2) is not correct,
but I found such sentences on the net.
How do they sound to native speakers?
Well, the tree has already fallen. It's Ok if, say, the tree rolled off a truck and then hit you, then you could say with much dread, "I was hit by a fallen tree!"
falling tree: a tree falling to the ground hit me.
hit by fallen trees 
It's Ok if, say, the tree rolled off a truck and then hit you, then you could say with much dread, "I was hit by a fallen tree!" 
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