title: The challenge of tree height in Eucalyptus regnans: when xylem tapering overcomes hydraulic resistance.
abstract: *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.
authors.affiliation: Treeline Ecology Research Unit, Department of TeSAF, University of Padova, Viale dell'Università 16, I-35020 Legnaro (PD), Italy.
authors.forename: Giai
authors.initials: G
authors.lastname: Petit
authors.affiliation: Faculty of Agriculture, Food and Natural Resources, University of Sydney, Sydney, NSW 2006, Australia.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Pfautsch
authors.affiliation: Treeline Ecology Research Unit, Department of TeSAF, University of Padova, Viale dell'Università 16, I-35020 Legnaro (PD), Italy.
authors.forename: Tommaso
authors.initials: T
authors.lastname: Anfodillo
authors.affiliation: Faculty of Agriculture, Food and Natural Resources, University of Sydney, Sydney, NSW 2006, Australia.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Adams
mesh_terms: D005052:Eucalyptus / Q000033:anatomy & histology* / Q000502:physiology; D057446:Hydrodynamics; D018547:Plant Stems / Q000033:anatomy & histology / Q000502:physiology; D014197:Trees / Q000033:anatomy & histology* / Q000502:physiology; D014867:Water; D052584:Xylem / Q000033:anatomy & histology* / Q000502:physiology
chemical_list: D014867:Water