Hydraulic architecture of some diffuse-porous trees

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Abstract
The rate of flow of a dilute KCl solution through sections of stem, branches and twigs [in Acer pensylvanicum, A. rubrum, A. saccharum, Betula papyrifera and Populus grandidentata] was measured and expressed in .mu.e/h under conditions for gravity flow, per gram fresh weight of leaves supplied by that section of xylem. This is called leaf-specific conductivity (LSC). It is not uniform throughout the tree, LSC of the stem being higher than that of branches. Furthermore, vascular junctions, such as the path from stem to branch, represent hydraulic constrictions. Distribution of LSC in the tree is primarily based on varying vessel diameters. Vessel diameters increase from top to bottom in the tree stem. They are smaller in branches than in the main stem, and there is a distinct constriction of diameters at the base of each branch. Functionally this means that when transpiration begins the pressure has to drop more rapidly in the xylem of lower lateral leaves than in those at the top of the tree. It also means that under conditions of water stress peripheral parts of the tree are more vulnerable than the trunk.

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