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In order to cope and survive the cold winter temperatures, trees close off the veins that carry water and nutrients to and from the leaves. They do this by producing a new, dense layer of cells at the base of the leaf stem called the abscission layer. This layer causes the leaf to die and the stem to weaken, so the leaf can fall from the tree. The tree will grow a protective layer where the leaf broke off, making the tree ready for its winter hibernation.