XF 2.0 Subforums showing in forum list

If I have three "levels" of forum nodes, XF2 helpfully displays the "third level" inline in the main forum list underneath their second level forum, rather than as a whole forum themselves. I'd like it to do this for second-level forums too. So, imagine I have these forums:
  1. Motorcycle chat
  2. Car chat
    1. Ford
      1. Fiesta
      2. Mondeo
      3. Mustang
    2. Porsche
      1. 911
      2. Carrera
  3. Truck chat
then XF 2.0 will display these in the main forum list something like this:

Motorcycle chat
This is for chatting about motorcycles
Car chat
This is for chatting about cars
Ford
This is for stuff about Ford cars
Subforums: Fiesta, Mondeo, Mustang
Porsche

This is for stuff about Porsche cars
Subforums: 911, Carrera
Truck chat

This is for stuff about trucks

but what I'd like is for the "second level" forums to also be seen as sub forums, so that my main forum list actually looks like this:

Motorcycle chat
This is for chatting about motorcycles
Car chat
This is for chatting about cars
Subforums: Ford, Porsche
Truck chat

This is for stuff about trucks

How do I indicate to XF2 whether a forum should be treated as a "sub forum"?
 
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