The tree structure is pretty useless and incomplete on a traditional forum. The only information you have is when the user explicitly opts in to a quote-reply.

The tree structure is pretty useless and incomplete on a traditional forum. The only information you have is when the user explicitly opts in to a quote-reply. Most replies are written in the quick-reply box (no parent) because they are responding to what is being said in the last X posts. Assuming orphan posts are top-level replies to the topic is nonsensical, and you can verify this by toggling on threaded-view on forums that support it. The next best thing is to just link the upstream post from a post that quoted/replied to another, which is what Discourse and traditional forums already do.