The Wiki also lists: natural=fallen_tree that seems more accurate since this one in particular is not an obstacle to anyone, but has very few uses (26). I don't know about the tree stump because I only saw the fallen tree/log. If we want to be very precise we could add both the fallen tree and the tree stump as two different nodes, but a survey would be needed in this case.

For the moment I change it with obstacle=fallen_tree since is the most used tag.