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Ecology |
2 Department of Biology, Duke University, Durham, North Carolina 27708 USA 3 Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina 27708 USA
ABSTRACT
In the conifer genus Juniperus (Cupressaceae), many species are increasing rapidly in distribution, abundance, and dominance in arid and semiarid regions. To help understand the success of junipers in drier habitats, we studied hydraulic traits associated with their water stress resistance, including vulnerability to xylem cavitation, specific conductivity (KS), tracheid diameter, conduit reinforcement, and wood density in stems and roots, as well as specific leaf area (SLA) of 14 species from the United States and the Caribbean. A new phylogeny based on DNA sequences tested the relationships between vulnerability to cavitation and other traits using both traditional cross-species correlations and independent contrast correlations. All species were moderately to highly resistant to water-stress-induced cavitation in both roots and shoots. We found strong phylogenetic support for two clades previously based on leaf margin serration (serrate and smooth). Species in the serrate clade were 34–39% more resistant to xylem cavitation in stems and roots than were species in the smooth clade and had 
35% lower KS and 39% lower SLA. Root and stem resistance to cavitation and SLA were all highly conserved traits. A high degree of conservation within clades suggests that hydraulic traits of Juniperus species strongly reflect phylogenetic history. The high resistance to cavitation observed may help explain the survival of junipers during recent extreme droughts in the southwestern United States and their expansion into arid habitats across the western and central United States.
Key Words: juniper • Juniperus • phylogeny • vulnerability to cavitation • xylem cavitation • xylem embolism
Received for publication 10 September 2007. Accepted for publication 11 January 2008.
FOOTNOTES
1 This research was supported by an NSF Graduate Research Fellowship, an NSF Doctoral Dissertation Improvement Grant (no. 0308937), a Duke University Latin American and Caribbean Studies Travel Grant, and Duke University Biology Department grants to C.J.W. and an NSF CAREER grant (no. 97-3333) and an Andrew W. Mellon Foundation grant to R.B.J. The authors thank W. Pockman for lending his expertise and laboratory at the University of New Mexico; R. Addington, R. Banks, W. Cook, T. Crocker, S.-H. Oh, C. Moura, P. Selmants, and D. Willson for help in the laboratory or field; K. Ogle for help with fieldwork and statistical analyses; H. Maherali for helpful discussion; and the National Park Service, Jamaica Conservation and Development Trust, and St. Lucia Department of Forestry for access to field sites. They also thank members of the Jackson and Manos laboratories, A. Zanne, and two anonymous reviewers for helpful comments on the manuscript.
4 Author for correspondence (e-mail: jackson{at}duke.edu)
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