If you peel birch bark in the winter, we call it "the winter bark", another layer, a tougher inner layer of the tree adheres to the bark, producing a stronger material.

如果你在冬天剥桦树皮，我们称它为“冬树皮”，另外一层更加坚硬的紧贴树皮的树层就可以生产出更强更硬的材料。