Thinking a bit about it I don't think it would be a good idea to add the generated karma to the top poster or even a fixed fraction of it. The fraction should decrease with distance from the root because all intermediate commentes also deserve a share of the pot and the pot shouldn't increase by itself. The function of distance should be between harmonic and exponential I think. Or it could be tuned by sampling actual comment trees (only that your whole idea is to influence the shape of the tree).