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JXB Advance Access published online on September 25, 2003
Journal of Experimental Botany, doi:10.1093/jxb/erg272
© 2003 by Oxford University Press
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Received April 30, 2003; accepted July 15, 2003
© 2003 Society for Experimental Biology

RESEARCH PAPER

Hydraulic efficiency and safety of leader shoots and twigs in Norway spruce growing at the alpine timberline

Stefan Mayr 1*, Barbara Rothart 1, and Birgit Dämon 1

1 Institut für Botanik, Universität Innsbruck, Sternwartestr. 15, A-6020 Innsbruck, Austria

* To whom correspondence should be addressed. E-mail: stefan.mayr{at}uibk.ac.at.


  Abstract

Xylem within trees varies in its hydraulic efficiency and safety. Trees at the alpine timberline were expected to exhibit a hydraulic architecture protecting the leader shoot from winter embolism. Hydraulic and related anatomical parameters were compared as well as seasonal courses of winter embolism in leader shoots and twigs of Norway spruce trees growing at 2000 m. Leader shoots had a 1.4-fold higher specific hydraulic conductivity (ks) as well as a 4.9-fold higher leaf specific conductivity (kl) than side twigs. Vulnerability to drought-induced embolism was lower in leader shoots with a 50% loss of conductivity occurring at a water potential ({Psi}50) 0.7 MPa lower than in twigs. Higher ks and kl were related to 1.2-fold wider tracheid diameters in leader shoots. Lower vulnerability corresponded to smaller pit dimensions but not to wood density. High ks and kl reflect the hydraulic dominance of the leader shoot, which is important for its water supply during summer. Low vulnerability protects the leader shoot from embolism during the winter season. In field measurements at the timberline during the winter of 2001/2002, conductivity losses of up to 56% were observed only in twigs while leader shoots showed little or no embolism. Results demonstrate that leader shoot xylem is both hydraulically efficient and safe.

Key words: Conductivity, conifer, hydraulic architecture, hydraulic efficiency, hydraulic safety, leader shoot, timberline, twig, vulnerability, winter embolism.


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