Safety and efficiency conflicts in hydraulic architecture: scaling from tissues to trees

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Abstract
Tree hydraulic architecture exhibits patterns that propagate from tissue to tree scales. A challenge is to make sense of these patterns in terms of trade‐offs and adaptations. The universal trend for conduits per area to decrease with increasing conduit diameter below the theoretical packing limit may reflect the compromise between maximizing the area for conduction versus mechanical support and storage. 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: (1) increases the efficiency of tree water conduction; (2) minimizes (but does not eliminate) any hydraulic limitation on the productivity or tissue growth with tree height; and (3) is consistent with the scaling of tree conductance and sap flow with size. We find no hydraulic reason why tree height should scale with a basal diameter to the two‐thirds power as recently claimed; it is probably another mechanical constraint as originally proposed. The buffering effect of capacitance on the magnitude of transpiration‐induced xylem tension appears to be coupled to cavitation resistance, possibly alleviating safety versus efficiency trade‐offs.