The reason trees don't care about what the density of an embark is supposed to be is that there are only seven things that keep them from growing (there might be something I've missed, though):
1. Physical abuse of saplings (trampling, dirt roads, etc.).
2. Unsuitable surface (surface clay [but not clay exposed via channeling], stone, pebbles, etc.
3. Lack of soil beneath for roots (surface trees only).
4. Lack of headroom (any cover at the level above the level they grow on, except bridges. They grow through bridges, but any tree part, including twigs blocks maturation). I suspect wind mills only block the area under center tile, but don't have any actual proof.
5. Biome not supporting trees.
6. Stockpiles (they'll block further maturation of what's underneath, but if you leave stockpiles for a number of years and then remove them suppressed saplings will immediately die or mature). It also seems stockpile suppression still causes you to suffer FPS degradation.
7. Deep fluids. I suspect a sapling that's managed to take root in shallow water can mature even if the water level is raised later, though. I'm not sure saplings can take root in shallow magma either.
The above means every area supporting trees will eventually reach the same end density regardless of initial density and totally disregarding the description of the biome (unless the ground restricts the locations supporting trees severely).