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First published online July 10, 2009; doi:10.3732/ajb.0800237 American Journal of Botany 96: 1388-1398 (2009) © 2009 Botanical Society of America, Inc.

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Wood anatomy and wood density in shrubs: Responses to varying aridity along transcontinental transects¹

² Department of Ecology and Evolutionary Biology, Unit-3043, 75 N. Eagleville Road, University of Connecticut, Storrs, Connecticut 06269-3043 USA ³ Department of Biological Science, California State University Fullerton, P.O. Box 6850, Fullerton, California 92834-6850 USA

ABSTRACT

Wood density plays a key role in ecological strategies and life history variation in woody plants, but little is known about its anatomical basis in shrubs. We quantified the relationships between wood density, anatomy, and climate in 61 shrub species from eight field sites along latitudinal belts between 31° and 35° in North and South America. Measurements included cell dimensions, transverse areas of each xylem cell type and percentage contact between different cell types and vessels. Wood density was more significantly correlated with precipitation and aridity than with temperature. High wood density was achieved through reductions in cell size and increases in the proportion of wall relative to lumen. Wood density was independent of vessel traits, suggesting that this trait does not impose conduction limitations in shrubs. The proportion of fibers in direct contact with vessels decreased with and was independent of wood density, indicating that the number of fiber-vessel contacts does not explain the previously observed correlation between wood density and implosion resistance. Axial and radial parenchyma each had a significant but opposite association with wood density. Fiber size and wall thickness link wood density, life history, and ecological strategies by controlling the proportion of carbon invested per unit stem volume.

Key Words: aridity • cell size • cell wall thickness • growth form • life history • shrubs • wood anatomy • wood density

Received for publication 11 July 2008. Accepted for publication 6 April 2009.

FOOTNOTES

¹ The authors thank C. Goedhart, M. Nordenstahl, G. Pongetti, D. Ortega, F. Biganzoli, A. Rolhauser, S. Lambert, M. Mazon, and P. Grierson for help with fieldwork; E. Jobbágy, L. Donovan, B. D. Kloeppel, and J. Ansley for hosting site visits; and N. "Cano" Agüero for permission to conduct research on his land. C. Henry helped with the figures. They also thank three anonymous reviewers and the associate editor for thoughtful comments. Funding for this research was provided by a grant from the Andrew W. Mellon Foundation to H.J.S. and from the National Science Foundation to H.J.S. (award IOS-0641765) and C.S.J. (award IOS-0641569). The scholarship provided by CONACyT to H.I.M.-C. also is appreciated. This research was supported by the National Science Foundation under Cooperative Agreements DEB-9632854 and DEB-0218001, the Coweeta LTER Program. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

⁴ Author for correspondence (e-mail: hugo.martinez_cabrera{at}uconn.edu)

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