Modifying CC0 Art/Work?

Modifying CC0 Art/Work?

If someone modifies a CC0 art/work, do the modifications made become CC0 as well or are they separate meaning the modifications could be under a different license while anything of the original work is still CC0?

For example I make a pine tree and license it as CC0. Someone likes the pine tree and adds decorations to it to make it look like a Christmas tree. What would the licensing be on the modified tree?