A fictitious plant/fungis that is extremely tough and rooty, giving it a stump-like quality. It grows below (and possibly above) ground in the shape of a large fat cyprus knee or resembling a dome like stalagmite.
It is distinguished from its counterparts in that it only requires 1 z-level of space to grow though older larger versions can grow up to 2 z levels. It produces less wood than its counterparts, however (and possibly has a longer growing cycle).
The reason for suggesting this: I felt there was a need for a more simple underground tree farm, and stumpwood allowed for it. There might be other ways of simplifying tree farming, but I thought this would be the best way without having to add any new mechanics (hopefully).
Side notes:
Flavor Fluff: Dwarves dislike this wood because the younger versions are like a tough weed. Farmer dwarves can't stand it as it will grow in their crops making it difficult to turn soil. The young stumpwood can cause dwarves to trip causing extreme discomfort and sometimes death. Some dwarves like stumpwood for how tough and rugged it is for such a small plant.
It could look something like this.
Another possibility would be to make stumpwood a growable crop, but I shy away from that idea. Can you imagine growing a field of stumps? It would be every farmers nightmare.
This is just a suggestion. Name and aspects of the wood can be changed to better suit the desires of others, namely Toady and his brother, this is their game after all. Or, if no one is interested in this idea, left to be forgotten in the eternal abyss that is the suggestion forum.