Does land turned to ash recover?

Solution 1:

This reddit thread describes a couple of other folks who encountered similar situations: https://www.reddit.com/r/dwarffortress/comments/21mjsp/cleaning_up_a_forest_fire/

Judging from the experiences in the thread, a small area containing ashes will recover quickly, 5 years or so. The grass and other growth around it seems to help. A large area of ashes will take longer, on the order of 20 - 30 years.

This process can be sped up by massive dwarven labor investment, as so many things can. Building over ashes tiles with a floor and then destroying the floor should remove the ashes.

Solution 2:

The ashes are simply smeared over regular ground. Grass, bushes, trees and other flora will regrow normally after a fire, and the ashes will disappear as the plants grow back. Since ash is a contaminant, you can also remove it by constructing buildings over it, such as floors, or by having your dwarves clean it.