Girdling can be used as a valuable research tool to improve our understanding of the tight coupling between water (xylem) and sugar (phloem) transport. Therefore, double girdling was applied on young oak trees (Quercus robur L.) to manipulate the sugar flow by mechanically removing a complete band of bark at two different heights. The double girdling effects on both the water and sugar transport were investigated by analysing stem diameter variations, photosynthesis, xylem sap flow and content of carbohydrates. The double-girdled oak trees were divided in three stem zones: (1) the upper stem zone (U) still receiving new assimilates from the leaves, (2) the lowest stem zone (L) receiving only stored sugars from the roots, and (3) the middle stem zone (M) completely isolated from crown and roots. As downward carbon transport was interrupted by girdling, the stem expansion and carbohydrate content increased in U, indicating that U became the major sink instead of the roots. In contrast to U, stem expansion and carbohydrate content decreased in the two lower stem zones (M and L). Furthermore, a decrease in photosynthesis and sap flow rate was observed, which could be attributed to an indirect effect of girdling.
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