How does a gymnosperm branch ( Pseudotsuga menziesii ) assume the hydraulic status of a main stem when it takes over as leader?

In most gymnosperms, resistance to the flow of water per unit path length through the main stem is less than that of lateral branches. Using branches, leaders, and branches that have replaced missing leaders (‘branch‐leaders’), we tested the hypothesis that branch‐leaders are at a hydraulic disadvantage. Reduced xylem transport efficiency in branch‐leaders relative to leaders could be expected both because of an initial disparity in hydraulic capacity, and because of the relatively impermeable compression wood formed in branch‐leaders during shoot reorientation. By subsampling branch‐leaders, we also tested the hypothesis that opposite wood (formed directly opposite compression wood) is more permeable than normal wood, and could, therefore, compensate for the presence of compression wood at the whole shoot level. Fifteen months after leader removal, branch‐leaders were intermediate between branches and leaders in their ability to supply foliage with water, suggesting a transition towards leader status that was not yet complete. Increased hydraulic capacity in branch‐leaders was the result of increased xylem cross‐sectional area per unit foliage, rather than an increase in permeability. Among subsampled wood types from basal branch‐leader segments, opposite wood was significantly less permeable than normal wood, suggesting that it does not compensate for the presence of compression wood.