Nature of the mechanical constraint on hydraulic efficiency

Evidence for a support vs. transport conflict was most obvious in the conifer wood — where the two functions are performed by the tracheids that make up most of the wood volume. What prevents conifer wood from matching the efficiency of leaf xylem and vine wood? The answer is that if it did, conifer trees would likely fall over. The dashed line in Figure 4.4 represents "area-preserving" transport networks, i.e., vascular systems that have the same total cross-sectional area of functional xylem from base to tips. A conifer tree above the dashed line in company with the vines and leaves would be top heavy with more cross-sectional area and bulk in their upper branches than in its trunk.

This result exposes a basic support vs. transport trade-off. Achieving higher conductance for a given network volume requires "area-increasing" conduit networks (Figure 4.4, above dashed line). Achieving the tallest self-supporting structure for a given volume requires a tapered column with its area-decreasing pattern [35] (Figure 4.4, below dashed line). The two functions cannot be simultaneously optimized if the supporting cells are also transporting conduits. The conflict is not relevant for vine wood, which is less self-supporting, or for leaf and Psilotum shoot xylem, which is not involved in mechanical support. We interpret that these mechanically unconstrained networks are free to be area-increasing and more efficient. One of the vine species fell near the area-preserving line and also deviated more from Murray's law than the other (Figure 4.4, C. radicans). Vine stems can be transitional between self-supporting vs. tension structures, depending on species, stem position, and development [36,37].

The specialized wood of angiosperms can at least partially resolve the support vs. transport conflict by having separate support vs. transport cells. They can have an area-increasing conduit network while maintaining area-preserving or even area-diminishing branching of the wood as a whole for mechanical stability (Figure 4.4, Fraxinus datum). Ring-porous trees represent the extreme of this angiosperm strategy by localizing transport to a few large vessels embedded in a much greater area of fibers.

Sap velocity measurements reinforce our anatomical results. Relative sap velocity in trunk vs. twig is a proxy for relative cross-sectional area of functional conduits — assuming steady state flow conditions and conservation of mass flow. Conifers and diffuse-porous trees show accelerating sap velocity from trunk to twig [38-40], consistent with a reduction in conduit area and less efficient transport networks in the conductance per volume sense. Ring-porous trees, in contrast, tend to have decelerating velocity from trunk to twig [39], consistent with more efficient area-increasing conduit networks. Unfortunately, we know of no comparable data for vines or leaf xylem, but we would expect decelerating velocities based on the anatomy (Figure 4.4).

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