Toz began by scraping away a thin layer of bark from the tree. Since the tree didn't just look like it was made from stone and the surface also had the characteristics of stone, he used his sword. While creating a spot on the tree of thin bark, he was extremely careful not to penetrate the bark all the way. He needed to thin the bark down so he could force the pipe through and into the tree and drain the sap, but if he scratched away too much and the bare trunk came into contact with light, there would be no way for him to get sap from that tree since it would all petrify.
The insides of the trees were extremely sensitive to all light since their primary weakness was being struck by lightning. And so, every time a part of the tree not already covered by the protective layer of stone came in contact with light, that part and then some would petrify almost instantaneously.
