I really don't know how spread out they are .. The problem with cutting roots is that the main trunk probably is producing anti-gibberellens .. gibberellic acid, GA3 is a plant hormone that encourages plant growth. I'm sure you've notice that when you prune a plant that lateral growth, side shoots start developing. Plant tips produce an anti-gibberellen which suppresses lateral growth. By removing the tip you are encouraging growth by removing the source of the anti-gibberellens. I'm thinking that the trunk is probably doing the same thing, suppressing plant growth with anti-gibberellens so roots won't grow into new plants close to the main trunk that will compete for nutrients and water. The longer the roots, the less effect the anti-gibberellens have on the roots - probably why you see new plants popping up away from the main trunk.
This is all speculation on my part but I think the logic is sound.
