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First published online July 10, 2009; doi:10.3732/ajb.0800424 American Journal of Botany 96: 1445-1453 (2009) © 2009 Botanical Society of America, Inc.

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Water stress tolerance of shrubs in Mediterranean-type climate regions: Convergence of fynbos and succulent karoo communities with California shrub communities¹

² Department of Biology, California State University, Bakersfield, California 93311 USA ³ Department of Conservation Ecology and Entomology and Centre for Invasion Biology, Stellenbosch University, Matieland, 7602, South Africa ⁴ Biological Sciences Department, California State Polytechnic University, Pomona, California 91768 USA

ABSTRACT

Mediterranean-type climate regions are highly biodiverse and predicted to be particularly sensitive to climate change. Shrubs of the mediterranean-type climate region of South Africa are highly threatened, and their response to water stress has been comparatively little studied. Resistance to water stress induced xylem cavitation (P₅₀) and xylem specific hydraulic conductivity (K_s) were measured in 15 shrub species from fynbos and succulent karoo communities of South Africa. Species displayed a fivefold variation in cavitation resistance (P₅₀ of –1.9 to –10.3 MPa) with succulent karoo species displaying greater interspecific variability in P₅₀ than fynbos species. Principal components analysis (including P₅₀, minimum seasonal water potential, K_s, and xylem density) showed the response to water stress in fynbos species to be similar to chaparral species from the mediterranean-type climate region of California. The data suggest convergence of community and species-specific water stress "strategies" between these mediterranean-type climate regions with respect to their xylem traits. On the basis of the current study and reported plant death and dieback in these regions, woody species within the fynbos may be more susceptible to climate warming and drying than those within the succulent karoo that appear to be utilizing more diverse xylem strategies in response to water stress.

Key Words: cavitation resistance • chaparral • dioecious • fynbos • mediterranean-type climate • Mojave Desert • succulent karoo • water relations • xylem density

Received for publication 16 December 2008. Accepted for publication 18 March 2009.

FOOTNOTES

¹ This work was supported by an NSF Graduate Research Fellowship and Doctoral Dissertation Improvement Grant to A.L.J. and Michigan State University Department of Plant Biology. The authors thank L. Alan Prather, D. W. Schemske, F. W. Telewski, S. D. Davis, and L. M. Moe for valuable advice and discussions. A.L.J. and R.B.P. thank the Andrew Mellon Foundation and NSF Grant IOS-0845125 to R.B.P. for support.

⁵ Author for correspondence (e-mail: ajacobsen{at}csub.edu)

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