One thing that DOES happen in vanilla is that a sapling can mature in between the time a wall or floor is designated and the time it's built, in particular if building is held off by e.g. an invasion that keeps you away from the building site. In those cases a tree seemingly is present concurrently with the designated wall, but as soon as wall constructions resumes, the wall segment designation disappears and only the tree remains (and you'll have to cut down the tree to build on that tile).
I've never seen that happen with constructed roads, so I suspect designating the construction of a road immediately suppresses sapling maturation (like stockpiles do, for instance).
The way to ensure saplings do not mature in between the designation of the building of a floor/wall and finishing the work, you can prepare the ground by first "building" a dirt road, as that removes all vegetation, and then finish the building within 3 years (as saplings take 3 years to mature).
It can also be noted that sapling maturation is suppressed by stuff on top of it, such as logs (which also obscures the presence of a sapling), but as soon as that suppression is removed (probably within 10 ticks) the saplings whose maturation is overdue will either wither or mature. Thus, if you designate the building of a wooden floor from logs within that area, saplings may mature as logs are moved to build a floor segment at another location within the designated area.
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Topic: How to remove sapplings (Read 4366 times)