Hydraulic efficiency and safety of branch xylem increases with height in Sequoia sempervirens (D. Don) crowns

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Abstract
The hydraulic limitation hypothesis of Ryan & Yoder (1997 , Bioscience 47, 235–242) suggests that water supply to leaves becomes increasingly difficult with increasing tree height. Within the bounds of this hypothesis, we conjectured that the vertical hydrostatic gradient which gravity generates on the water column in tall trees would cause a progressive increase in xylem ‘safety’ (increased resistance to embolism and implosion) and a concomitant decrease in xylem ‘efficiency’ (decreased hydraulic conductivity). We based this idea on the historically recognized concept of a safety–efficiency trade‐off in xylem function, and tested it by measuring xylem conductivity and vulnerability to embolism of Sequoia sempervirens branches collected at a range of heights. Measurements of resistance of branch xylem to embolism did indeed show an increase in ‘safety’ with height. However, the expected decrease in xylem ‘efficiency’ was not observed. Instead, sapwood‐specific hydraulic conductivities (Ks) of branches increased slightly, while leaf‐specific hydraulic conductivities increased dramatically, with height. The latter could be largely explained by strong vertical gradients in specific leaf area. The increase in Ks with height corresponded to a decrease in xylem wall fraction (a measure of wall thickness), an increase in percentage of earlywood and slight increases in conduit diameter. These changes are probably adaptive responses to the increased transport requirements of leaves growing in the upper canopy where evaporative demand is greater. The lack of a safety–efficiency tradeoff may be explained by opposing height trends in the pit aperture and conduit diameter of tracheids and the major and semi‐independent roles these play in determining xylem safety and efficiency, respectively.