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First published online September 3, 2009; doi:10.3732/ajb.0800410 American Journal of Botany 96: 1779-1786 (2009) © 2009 Botanical Society of America, Inc.

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Long-term relationships among atmospheric CO₂, stomata, and intrinsic water use efficiency in individual trees¹

Department of Biology, Boston University, 5 Cummington Street, Boston, Massachusetts 02215 USA

ABSTRACT

Leaf-level responses to increases in atmospheric carbon dioxide (CO₂) concentrations could have large implications for water and carbon cycles. We investigated whether stomatal density, guard cell length, and intrinsic water use efficiency (iWUE) of 27 individual trees growing at the Arnold Arboretum in Boston, Massachusetts have responded to changing environmental conditions over the last 100 years. We examined leaves from 74 herbarium specimens collected from three genera—Acer (maples), Quercus (oaks), and Carpinus (hornbeams)—from 1893 to 2006. During this period, global average atmospheric CO₂ concentrations increased by approximately 29% (86 ppm), and temperatures in Boston increased by 1.8°C. Stomatal density and guard cell length were negatively correlated in oaks and hornbeams. Although stomatal density declined and guard cell length increased over time, the changes were not dependent on the magnitude of changes in CO₂ concentrations. Intrinsic WUE did not change significantly over time. Our findings suggest that iWUE may not respond to changes in CO₂ concentrations over the lifetimes of individual trees, possibly because of compensating changes in stomatal density and guard cell size. We provide an example of a method that can enable researchers to differentiate between genetic and plastic responses to global change in long-lived trees.

Key Words: Acer • Arnold Arboretum • carbon dioxide • Carpinus • herbarium specimens • intrinsic water use efficiency • Quercus • stable isotopes • stomata • trees

Received for publication 7 December 2008. Accepted for publication 17 June 2009.

FOOTNOTES

¹ The authors thank the National Science Foundation, the Arnold Arboretum of Harvard University, and Boston University for providing funds for this study, and M. Dosmann, K. Tu, I. Woodward, and two anonymous reviewers for helpful comments on previous drafts of this manuscript. The Arnold Arboretum gave permission for collecting samples and assistance preparing the specimens. They also acknowledge the assistance of T. Blute with microscopy. The authors thank A. Socci, L. Scott, S. Wheatley, and S. Wood for helping to prepare samples for isotope analysis, and R. Michener for analyzing the samples on an IRMS.

² Author for correspondence (e-mail: abe{at}wildlife.org); present address: USA National Phenology Network, 1955 East 6^th Street, Tucson, AZ 85719, USA; and The Wildlife Society, 5410 Grosvenor Lane, Bethesda, MD 20814, USA

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