The challenge of tree height in Eucalyptus regnans : when xylem tapering overcomes hydraulic resistance

Recent research suggests that increasing conduit tapering progressively reduces hydraulic constraints caused by tree height. Here, we tested this hypothesis using the tallest hardwood species, Eucalyptus regnans.Vertical profiles of conduit dimensions and vessel density were measured for three mature trees of height 47, 51 and 63 m.Mean hydraulic diameter (Dh) increased rapidly from the tree apex to the point of crown insertion, with the greatest degree of tapering yet reported (b > 0.33). Conduit tapering was such that most of the total resistance was found close to the apex (82-93% within the first 1 m of stem) and the path length effect was reduced by a factor of 2000. Vessel density (VD) declined from the apex to the base of each tree, with scaling parameters being similar for all trees (a = 4.6; b = -0.5).Eucalyptus regnans has evolved a novel xylem design that ensures a high hydraulic efficiency. This feature enables the species to grow quickly to heights of 50-60 m, beyond the maximum height of most other hardwood trees.