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First published online December 18, 2009; doi:10.3732/ajb.0900249 American Journal of Botany 97: 27-37 (2010) © 2010 Botanical Society of America, Inc.

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Ontogenetic changes in the numbers of short- vs. long-shoots account for decreasing specific leaf area in Acer rubrum (Aceraceae) as trees increase in size¹

Department of Plant Biology, Cornell University, Ithaca, New York 14853 USA

ABSTRACT

Specific leaf area (SLA) is reported to decrease with increasing plant size among dicot tree species despite a strong positive correlation between SLA and relative growth rate. This diminishing returns in SLA may result from changes in the relative numbers of different shoot types bearing leaves with different SLAs as trees increase in overall size. This ontogenetic shift hypothesis was examined for 15 Acer rubrum trees differing in basal stem diameter (0.01 m D 0.62 m). Detailed analyses of the largest tree showed that short-shoots produced leaves with significantly smaller SLA than the leaves produced by long-shoots regardless of the location of shoots within the canopy. A combination of random effect and split-plot (main-effect) ANOVA models showed that >94% of the variance observed for SLA was attributable to shoot type rather than to the location of leaves in the canopy. Further, with increasing trunk diameter, the number of short-shoots increased rapidly relative to the number of long-shoots. Although the leaves of short-shoots gain disproportionately more surface area per unit mass investment compared to the leaves produced by long-shoots, our data show that ontogenetic shifts occurring at the shoot and whole plant level account for size-dependent decreases in total canopy SLA.

Key Words: Acer rubrum • Aceraceae • allometry • diminishing returns • functional foliar traits • leaf area • leaf economics • long-shoots • scaling relationships • shade leaves • short-shoots • sun leaves

Received for publication 20 August 2009. Accepted for publication 16 October 2009.

FOOTNOTES

¹ The authors thank Dr. Michael L. Christianson (AJB Associate Editor) for drawing attention to a critical reference (i.e., Wilson, 1966), Ms. Simona Despa (Department of Biological Statistics and Computational Statistics, Cornell University) for statistical consultations, and two anonymous reviewers for constructive criticisms. Support from the College of Agriculture and Life Sciences, Cornell University, is gratefully acknowledged.

² Author for correspondence (e-mail: kjn2{at}cornell.edu)

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