Green glass is nice, but the proper dwarven way is to cover the surface in obsidian...
My current understanding is that the only thing blocking tree growth on soil (or mud underground) is physical abuse of saplings, stockpiles, and tree canopies.
- Paving, flooring, and trampling are variants of the physical abuse method.
- Saplings under tree canopies appear to die off. Since young trees have smaller canopies than older ones clear cutting can actually result in more trees over time. This is especially bad with cavern trees, where many species have no canopy at all while young. As far as I understand, there is no other mechanism in place to control tree density. Toady plans to take nutrients, soil quality, ... into consideration at some point in the future. While that is good news for the future, it's bad news for the time up until then, as he's rather unlikely to do anything about the issue until he's prepared to do it fully.
- Stockpiles have the odd effect of suspending growth underneath. When stockpiles are removed, it seems all the pent up maturing takes place in a few ticks, so lots of the vegetation shrivel, while some saplings immediately mature into trees (but probably young trees, as I'd expect the tree growth counter to start at that maturation). I've got a suspicion that stockpiles do little or nothing to combat the FPS drain of vegetation, though.
- It can also be noted that sometimes cutting down cavern trees results in exposed rock where the trunk was, thus gradually depleting the area's ability to support trees as they are logged.
My main method of battling the tree FPS attacks is to embark in areas where only one embark tile has trees, the rest being badlands or ocean. In theory mountain would work, but I want perfectly flat embarks. Glaciers would work as well, but I avoid freezing biomes since there's no way to keep the dorfs away from getting killed by water/ice transitions, since traffic designations usually fail at that task.