Alternatively/horizontally, something like this would also work
Subforums: Subforum1 (number of posts), Subforum2 (number of posts), etc.
On some forums it is even possible to distinguish new/unread posts (but I'm ready to live with that not being an option here).
I'm not sure how many levels of nested subfora we might end up with but at each level of drilling down, it would be useful to show that something is happening/has happened. The point is to indicate to visitors that a forum or subforum is active - without
which (further) activity is less likely.