Natural and experimentally altered hydraulic architecture of branch junctions in Acer saccharum Marsh. and Quercus velutina Lam. trees

The functional xylem anatomy and the hydraulic conductivity of intact and treated branch junctions of the diffuse-porous sugar maple (Acer saccharum Marsh.) were compared to those of the ring-porous black oak (Quercus velutina Lam.). Maple shoots possessed greater growth intensity than those of oak. The extensive growth of the maple trees resulted in about a two-fold increase in xylem production in the maple branches. Branches were altered by removing a patch of bark from the base of a branch (near a junction) leaving a bridge of bark on the upper or lower side of the branch. The experimentally treated branch junctions revealed that, in oak, most (up to 92%) of the water flows in the lower side of a branch, where most of the large vessels occurred. In maple, most of the conductive tissue was observed to form in the upper side of the branches, which was equally or more conductive than the lower side. A treatment of longitudinal, parallel scratches in the bark-bridge, which reduced earlywood vessel width, substantially decreased conductivity (to only 15%) in oak, but had no effect on conductivity in maple. In maple, such wounding stimulated more wood formation and increased conductivity. In both trees, a narrow bridge at the junction induced more wood formation and higher conductivity in the branch. The mechanisms controlling wood formation and water flow in branch junctions of ring- and diffuse-porous trees are discussed.