I just cut trees and build a floor above, so that pest, the trees, is not growing there. I wall off pastures (else animals will leave pasture), but above ground fields are an issue for how many seeds are allowed in game. The only cave-ins I do, is when leveling whole map to some lower level (mostly to get rid off the trees) and want to preserve some area for farming.
Trees grow way too fast imho. In 5 years time map can be so over overgrown with trees, that even caravans' wagons can not pass from edge to middle of the map, where usually I place my depot. This is most concerning imho right now, then getting farm land lowered with cave-ins.
Though getting farm land above ground, high in air, with forming obsidian (water+lava) and caving it (over built walls or other obsidian) is a game of chance. Even getting anything to grow higher into air (like on the roof of constructed building) with mud trick, can be a game of chance, as air's different levels have their own different biomes. People using DFHack are saying there is a way to spy on those biomes, as some can be even rising dead and can be used for some reanimation?
Yeah, the rocks spawned from thin air into fresh clay/sand, which can not be removed then by any means (even through additional cave-ins), are a painful part of DF. For now.