select_tree: Only save last term descendent

Hi there,

Is there a way with the taxonomy field in the select_tree format to only save the last term descendent?
E.g. with the following term structure:
- Parent 1
- - Child 1
- - Child 2

If I select Parent 1 in the 1st select box and Child 1 in the 2nd select box, I am only looking to save the term Child 1 and not also Parent 1.
If however Parent 1 is selected in the 1st select box and nothing is selected in the 2nd select box, it would only save Child 1.

Thank you!