Limited post multi level tree experience would explain it. While the scheme would work, it's a huge overkill. my understanding is that the single tile trees yielded fairly little wood, while the current trees provide quite a lot (usually: saguaros don't provide much). Thus, a single 31*31 room in the soil level would provide a fair bit of wood once it starts to produce trees. And, keeping traffic away from the tree farm is good advice.
However, it's usually faster to secure a cavern and drain a lake than to set up a tree farm, but that's provided you want to secure the cavern rather than keeping it open to keep the danger level up (but still somehow have a safe wood supply).
Depends on what you're doing.
Single-tile trees are 1 wood, yes, and the largest trees now can produce 50+ wood (I have woodcutters with lots of time on their hands, now...) although last I checked, underground trees like tower caps were bugged to only produce 1 wood, as well.
Still, thanks to heavy objects being slow to move, and stone being, you know, HEAVY, I found it worthwhile to actually, for the first time in Dwarf Fortress, treat wood as a readily expendable resource, and actually construct above-ground structures with the stuff, (bee houses and some exterior pallisades, not actual housing - I'm not SO far gone I'd make a dwarf sleep in a WOOD house...) liberally consume charcoal and make clear glass, maybe even use
potash as fertilizer!
Maybe it's still gross overkill from old habits dying hard, but Dwarf Fortress is a game of better being safe than sorry. Plus, you can always use drowning chambers for other uses.