The hydraulic architecture of eastern hemlock (Tsuga canadensis)

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Abstract
Leaf specific conductivities (LSCs, hydraulic conductivity per gram dry weight of supplied leaves), Huber values (transverse xylem area per gram dry weight of supplied leaves), specific conductivities (hydraulic conductivity per unit transverse xylem area), and tracheid diameters were measured throughout the trunk and erown of 9- to 96-year-old trees of Tsuga canadensis (L.) Carr. By definition, LSC = Huber value × specific conductivity. Specific conductivity is controlled by wood anatomical features, especially tracheid diameter. LSCs, which indicate the relative water supply to different plant parts, decrease acropetally but are higher in the trunk than in branches and are particularly low in second-order branches and at branch insertions. The differential water supply is due to larger Huber values in the leader and to particularly narrow tracheids at branch junctions. In addition, as trunks enlarge they produce wider tracheids, resulting in greater specific conductivity than in supported branches. Based on comparison with results for Abies balsamea, the degree of "hydraulic dominance" in the younger parts of conifers is controlled by the Huber value and may be related to the degree of apical control.