Tree object changes affect its prefab

Hi,

I have a set of tree object prefabs. I'd like to have several variants of the same tree in the scene, like with leafs and without leafs. Moving one tree to the scene and make a duplicate. Now I have 2 identical trees. I want to remove leafs from one of them: I select "Unpack prefab" (or "Unpack prefab completely" - it doesn't matter) to unlink the object from its prefab. The selected tree's name is now isn't blue in the hierarchy window, all right. But when I'm trying to remove leaf groups from the unlinked tree, the changes affect the prefab like they were still linked.

What's the problem? How can I get a cloned tree that is not bound to the prefab?