Just to explain more clearly:
I made an aspen tree with aspen "leaf" level 3 components grouped together inside a level 2 "branch" component grouped together inside a level 1 "tree" component.
Then I used the aspen tree component to make a birch tree in a new file. I replaced the Aspen leaf .tif texture with birch leaf .tif, and changed the branch shapes a bit.
Then I make a new landscape file and insert the birch tree. Later I insert the aspen tree. However the Aspen leaf.tif texture is now replaced with the birch leaf .tif texture. It seems SketchUp remembers that the leaf level 3 component was the same.
Changing the components and sub-component name before importing makes no difference. The only thing that helps is exploding the leaf component.
This would be a nice solution, except the tree is much more complex than explained above, with tons of different level 3 leaf components, which are not easy to find. Also I modeled a handful of trees before discovering this problem. So, either a solution to the issue of help figuring out the logic behind this problem would be greatly appreciated.