Here's a real simple real world example:
People have phones
Phones have phone numbers
Except, sometimes the same phone number goes to a fax as well as a desk phone, or a single phone number is shared between multiple people, or people have more than one phone number. So what sits at the top of the tree? Phone number or Person? If your answer is that it depends on the problem at hand then you haven't done data modelling, you've modelled a single instance within our problem domain. Or, in-other-words, a tree can model a document that gives a single way to handle phones numbers, devices and people, but that single tree model does not reflect the management of phones numbers,
devices and people within the enterprise.
More generally, you cannot represent any many to many relationship as a single tree.
So, the answer to your question is that these models are not ever best represented as trees.
