Just some game notes: I play small embarks (2x2) because I like to dig everything I can out and just rebuild everything with walls of my most common stone (great way to ditch that stockpile!). It was heavily forested with only one aquifer, and a lot of sandy soil underground.
So, I was trying to pierce my first aquifer (previously I had gotten luck and had aquifers that didn't take up an entire z-level), but the aquifer was on z-level 3, meaning I had to dig out z-level 2, and then channel the surface, to drop a z-level 1 block down. Well I did that, dropped a large (9x15) block of earth, and dug into the richest chalk i've seen yet. Loaded with gemstones, coal, and other goodies. I dropped it into a EXACT hole. Nothing scattered onto the cavern floor, except a lot of water. I dug some quick channels to drain it off back into the aquifer (not the big hole of water you see in the image, thats just my fishing hole.)
Afterwards, I floored up the top of the hole, on the surface, with chalk flooring, but left z-level 1 hole blank. Leaving it. I also put floors over all of the above ground ponds because I hate having stuff, like goblin bits, pollute them, and I might later drain them for a defense weapon.
Anyway, it wasn't even a season before the first sapling grew, and then 2, then 3, then 4, then 5... unfortunately I didn't notice it until one of the saplings got dwarf'd to death before I restricted traffic on the area. One even grew into a full grown tree.
Anyway Here is a screen shot (sorry its so fuzzy.)
In this is highlighted a Muddy High wood, but there is also two Muddy Chestnut Saplings, another Muddy High wood, and a Muddy Ash. As well as a dead shrub (It got dwarf'd), and a lot of grass. All are growing within the confines of the dropped block. Lumber was cut, and removed, along with all plants, before dropping.
Edit: also it says above ground, however if you check my indoor trade depot you can see the ramps leading up. Its really on z-level 2.