The scaling of safety versus efficiency in hydraulic architecture

Background/Question/Methods Tree hydraulic architecture exhibits patterns that propagate from cellular to tree scales. A challenge is to make sense of these patterns in terms of trade-offs and adaptations. We collected anatomical and physiological data from a wide range of trees from Panama and the United States to examine the consequences of a previously observed and seemingly universal trend for xylem conduits per area to decrease with increasing conduit diameter.

Results/Conclusions All species fell below the theoretical packing limit, which may reflect the compromise between maximizing area for conduction vs. mechanical support and storage. The angiosperm species were further from the packing limit than the conifers, likely because of the mechanical need for area devoted to fibers in angiosperms. When comparing a given mean conduit diameter, this difference in packing results in the theoretical hydraulic conductivity per wood area of conifers exceeding that of angiosperms. In many environments, though, angiosperms exhibit greater conduit diameters than conifers, and achieve higher conductivity. Variation in conduit diameter may have two complementary influences: one being compromises between efficiency and safety and the other being that conduit tapering within a tree maximizes conductance per growth investment. Area-preserving branching may be a mechanical constraint, preventing otherwise more efficient top-heavy trees. In combination, these trends beget another: trees have more, narrower conduits moving from trunks to terminal branches.  This pattern: a) increases the efficiency of tree water conduction, b) minimizes (but does not eliminate) any hydraulic limitation on productivity or tissue growth with tree height, and c) is consistent with the scaling of tree conductance and sap flow with size. These patterns reflect tradeoffs between hydraulics, mechanics, storage and development at scales from cells to trees.