It's interesting to me that you were growing the BBB in New Jersey. Magnolias are generally considered a southern tree, but that particular one, the BBB has been certified to survive temps. of -20 degrees, and has been sold as far north as PA, so that puts you right in that geographic area. As for the root ball under these trees, it is pretty much a true ball. They are usually dug in the field with a machine, which takes out a ball of soil with the roots, so you can figure the tree has a root ball that is about as deep as the distance it extends out from the trunk. I do not believe there is a true tap root, and the fact that the tree has been dead a few years, means a lot of the smaller roots are dead and rotted away by this time. I do not believe it would be a difficult stump to dig, and another tree probably would not have difficulty growing near the stump. Point of interest--my wife's mother was a Bracken. She and her father, along with her three brothers were all in the nursery business. The youngest brother, Ray Bracken found that tree growing on his nursery property and identified it as having special characteristics. He grew it to maturity, documented it's special features, had it patented, and after a good deal of trial and error, learned to mass propagate it, and that's where your tree came from.
