In a ring girdled plant:

In a ring girdled plant the root dies first. Like all vascular plants, trees use two vascular tissues for transportation of water and nutrients: the xylem (also known as the wood), and the phloem (the innermost layer of the bark). Girdling results in the removal of the phloem, and death occurs from the inability of the leaves to transport sugars (primarily sucrose) to the roots. This is an experiment to prove the translocation of food through the phloem.