The center of the trunk is xylem and the outside is phloem. Xylem transports water and inorganic salts from bottom to top, while phloem transports organic matter from top to bottom. Generally speaking, phloem is very thin. If peeled from the outside, organic matter will accumulate near the wound, and the roots will soon die without nutrition. But if it is only hollow, there will still be some xylem left, which will not have much impact on the tree.