The distribution of resistances along the hydraulic pathway is controlled by the tapering of xylem conduits

Despite on abundance of information on the anatomical and hydraulic features of the xylem transport system, there is still a lack of evidence on how the former may affect the latter. We carried out a comparative analysis of hydraulic and anatomical data to test the actual role of conduit tapering in controlling the distribution of hydraulic resistance along the longitudinal axis of a branch. We measured the total hydraulic resistance (R) of the main axis of 9 sycamore branches by measuring the water flow at the top apex. We repeated the measurements for reduced branch lengths. The scaling of R with the distance from the apex (L) was then compared with that predicted by a theoretical model built upon the actual conduit tapering of the analyzed branches. R scaled against L with a scaling exponent (d) that was always <1.0 (from 0.29 to 0.64). The scaling exponents from the actual measurements were not significantly different from those estimated by the model. The results highlight the essential role played by the tapering of xylem conduits in controlling the distribution of hydraulic resistances along a branch or stem. Conduit tapering is likely to be the most effective strategy to minimize the effect of height on the whole-tree hydraulic resistance. Finally, the results implied that vessel furcation was unlikely to occur in the branches of sycamore.