Serves the lazy Dorf right, I mean how lazy/slow do up have to be in order to stay in a room long enough for a tree to grow and block you in???
With quantum trees, it's a little different, of course...if you ever find a sapling of a quantum tree, give it a wide berth, since you never know quite when it's going to grow into a tree proper: you might be impaled by a low-lying branch if you're not careful.
If I wanted, I could cast gigantic floors of obsidian in successive layers above the map, then dig 'em out, flood them, expose to sunlight, and then go above one floor, right? Thus giving me 15 or 16 Z-levels of outdoor tree farm.
Unless you're dropping a huge obsidian structure of several hollowed out levels, which by the way won't be exposed to sunlight if they're on top of each other. If you want to drop then down individually, unless you have some way to stop it from crushing the one below it, I don't think a tile exposed to sunlight can be "brought back to life" if you dig out the obsidian on top of the crushed out level.
I think the idea was to build a floor and walls above the ground, flood the floor with magma, cast it to obsidian, dig it out leaving an obsidian floor exposed to sunlight, muddy that floor to grow trees on, and then repeat on the next layer above. I don't think cave-ins were a part of the picture at all.
You might even be able to skip building the floor on the second, third, etc. layers if you cast two layers of obsidian and then dig out the lower one. The floor won't be exposed to sunlight in that case, but the game might already consider it 'sunlight-contaminated' if it marks squares with no floor in them at all as sunlight-contaminated, since it doesn't seem to ever un-mark them. Might require testing.
My thought on the new test results was, for anybody with above-ground-wood-loving nobles: the condoarborium is back on!