Karl J. Niklas2 and Edward D. CobbDepartment 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 producedby 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 constructivecriticisms. Support from the College of Agriculture and Life Sciences, Cornell University, is gratefully acknowledged.
2 Author for correspondence (e-mail: kjn2@cornell.edu)
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NOTA DESTE BLOGGER:
O ScienceDaily trouxe esta pérola de Lógica Darwiniana 101:
1. Mathematical models for the distribution of light within the canopy predict that the photosynthetic rate of the entire canopy is maximized when the specific leaf area is lowest for leaves at the top of the canopy. This research provides new insight into the mechanism by which trees have evolved to obtain light and photosynthesize at the greatest rate.
2. "Our research shows that plants are highly integrated organisms that respond to their environments in ways that are every bit as complex as even the most sophisticated animals," Niklas said. "This research also shows that we still have plenty to learn about phenomena that we thought we understood very well."
Fui, sem entender bulhufas, pois o artigo não disse COMO que ao longo do tempo os processos evolutivos foram capazes de produzir um organismo altamente integrado e tão complexo quanto os animais...
