This scene has only 4 sets of mesh data including the floor. The tree trunks have way too many verts.
I used alt D to make new objects with the same mesh. Never used that shortcut before. I’m not sure where it is in the menus but when I get into trouble with shared mesh data I just press U. It seems to work.
Under menu “Object” (Blender 2.8). It is called Linked duplicate, because it shares (linked) the same mesh data.
Shift D is just duplicating.