Patterns in hydraulic architecture and their implications for transport efficiency

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Abstract
We evaluated whether patterns in hydraulic architecture increase transport efficiency. Five patterns are identified: area-preserving branching; variable trunk versus twig sap velocity; distally decreasing leaf specific conductivity (KL) and conduit diameter; and a decline in leaf specific conductance (kL) of the entire plant with maturation. These patterns coexist in innumerable combinations depending on the ratio of distal/proximal conduit number (F). The model of West and colleagues does not account for this diversity, in part by specifying F = 1 and requiring a specific conduit taper derived from the incorrect premise that kL is constant with plant size. We used Murray's law to identify the conduit taper that maximizes kLfor a given vascular investment. Optimal taper requires the ratio of distal/proximal conduit diameter to equal the ratio of distal/proximal KL. The smaller these ratios, the greater the kL. Smaller ratios are achieved by an increase in F. Conductivity and diameter ratios < 1 and F ≥ 1 in plants are therefore consistent with maximizing conducting efficiency. However, the benefit of increasing F requires area-increasing conduit branching, potentially leading to mechanical instability of trees. This trade-off may explain why tree stems were relatively inefficient with F near 1 and limited conduit taper compared with vine stems or compound leaves with F > 1 and greater taper. Within trees, the anatomies of a coniferous and a diffuse-porous species were less efficient than that of a ring-porous species, presumably because the latter allows conduit area to increase distally without also increasing total xylem area. This is consistent with decelerating sap velocities from trunk to twigs in ring-porous trees versus accelerating velocities in other types. In general, the observed architectural patterns are consistent with the maximization of transport efficiency operating within mechanical constraints.

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