The topology of the xylem network is likely to affect the transport of water, propagation of embolism and plant survival and growth. Few studies have been conducted on the hydraulics of the entire water pathway in trees. We evaluated the hydraulic connections from roots to branches in a mature Quercus suber L. tree, through sap flow responses upon branch severing. Sap flow was recorded in branches, stem and roots by the heat field deformation (HFD) method. Results showed that roots, except for the taproot, were hydraulically connected to all branches, but the rest of the tree (stem, branches and taproot) was highly sectored. In the large roots that showed an integrated response to branch severing, the outer xylem was preferentially connected to the same side branch and the inner xylem to the opposite branch. The hydraulic sectoriality in branches, stem and taproot may be regarded as an adaptive trait to water stress. The integrated hydraulic structure of roots is advantageous under patchy soil conditions, but may allow the spread of root diseases. The HFD sap flow method proved extremely useful to calculate xylem flux connectivity between different organs of a large tree, providing a comprehensive picture of its hydraulic architecture.