queue trees have no download and upload, they have only upload. you only manage the speed at which packets go OUT the interface.
when you want to manage traffic for a single interface, you attach the queue tree to the outgoing interface, e.g. wlan1 or ether2.
that will manage the traffic towards those interfaces.
of course you can also have another queue tree towards the internet (WAN) interface, which manages the traffic in the other direction.
you could have different priorities for the traffic from different internal interfaces when you set the proper packet marks as Steveocee writes.
Using a "global" queue tree is only necessary when you want to manage the total traffic outbound to more than one interface.
E.g. you have 100 Mbps download from internet and you want to share that between wlan1 and ether2 without limiting either of them to a part of that total.
Then you can have a global tree that operates via marks set on traffic received from internet, and with a total limit at a bit less than 100 Mbit.