Thank you Embothrium! This is not a forest region - just a single tree in the middle of my yard. I planted an elderberry on the other side of the stump at about the same distance this spring, and it seems to be doing fine (although I didn't come across any roots of this size when planting that one). The weeds seem to like the area as well. :) Is honey fungus a problem outside of thickly forested areas?

It sounds like the root may be connected to a different tree. Large Japanese maples are still fairly small trees, with fairly small roots.

Mad_gallica, in that case, it may not have been a Japanese maple. It was half dead when we bought the house and we had it removed right away; I can't remember what the leaves looked like. The stump is about 2.5' in diameter, so not a small tree by any means. There are no other trees within 25' of this one, so I doubt this root belongs to anyone else.

Hudson Valley is within a forest region. Likewise a single building lot or even a neighborhood is not a region. But it is part of a region.

Gotcha! I will do a little research on honey fungus, then!