This is in reference to Dwarf Fortress 40d (v0.28.181.40d). Later versions of this game have significantly changed the related mechanics.
I just embarked on a map with 6 trees on it. There is a small (about 100 tiles total) bit of greenery, and the rest is a mix of red sand and felsite. If I pump water up and turn the ground muddy, will the forest spread? Would the muddy section have to be next to the existing green area?