An underground tree farm can also double as a strip mine and animal pasture. Its easy to dig out multiple Z-levels on a mark. The entire Z-level. Just channel it out.
Make sure to construct up/down stairs for your main stairway! Constructed tiles cannot be mined. This helps prevent accidentally channeling out your access stairs and stranding your dwarves.
Then go from one edge of the map to the other. I designate it all as up/down stairs for the entire size of the cavern, and once the up/down stairs are all designated and mined out you can either channel it out one Z-level as a time, starting from the top, or you can construct a support pillar linked to a lever, mine out all connecting/supporting tiles, and pull the lever. Either way works.
All you have to do from there is flood it and drain it. Aquifers make this super easy. Make sure you have a drain built in advance.
This strip mining excavation will get you an enormous amount of stone, gems, ore, and open space. You can make a pasture zone and animals will graze on moss while still having a huge area growing trees. You don't need to farm at all. Just cover the entire thing in activity zones with plant gathering and your dwarves will continually harvest all of the naturally growing cave plants, easily enough to feed even a huge fortress.
These strip mines also make for interesting fortresses. Instead of flooding it, try building your fortress in this huge open underground area. Build it like the great hall of Moria. You'll have to construct all of your rooms/buildings within this artificial cavern, but you can get a lovely city with freestanding buildings while still being underground.