The small pot may it make look like I was an enthuastic root pruner. Actually it's been a vole who bit off almost all the roots during last Winter, when the tree was at its outside location. All that had been left of the rootstock fitted easily into the small pot. I really had not been very sure whether it would survive when I dug it out in Spring, but it did. It'll get a special protection this Winter. Maybe the whole tree will need a re-shaping in the future because of the lacking of branches at the right side.