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Fossil leaf economics quantified: calibration, Eocene case study, and implications

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Royer, DL and Coley, PD and Cutter, AD and Johnson, KR and Labandeira, CC and Moles, AT and Palmer, MB and Valladares, F and Sack, L and Wilf, P and Lusk, CH and Jordan, GJ and Niinemets, U and Wright, IJ and Westoby, M and Cariglino, B (2007) Fossil leaf economics quantified: calibration, Eocene case study, and implications. Paleobiology, 33 (4). pp. 574-589. ISSN 0094-8373

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Abstract

Leaf mass per area (MA) is a central ecological trait that is intercorrelated with leaf life
span, photosynthetic rate, nutrient concentration, and palatability to herbivores. These coordinated
variables form a globally convergent leaf economics spectrum, which represents a general continuum
running from rapid resource acquisition to maximized resource retention. Leaf economics are little
studied in ancient ecosystems because they cannot be directly measured from leaf fossils. Here we
use a large extant data set (65 sites; 667 species-site pairs) to develop a new, easily measured scaling
relationship between petiole width and leafmass, normalized for leaf area; this enablesMA estimation
for fossil leaves from petiole width and leaf area, two variables that are commonly measurable in leaf
compression floras. The calibration data are restricted to woody angiosperms exclusive of monocots,
but a preliminary data set (25 species) suggests that broad-leaved gymnosperms exhibit a similar
scaling. Application to two well-studied, classic Eocene floras demonstrates thatMA can be quantified
in fossil assemblages. First, our results are consistent with predictions from paleobotanical and paleoclimatic
studies of these floras. We found exclusively low-MA species from Republic (Washington,
U.S.A., 49 Ma), a humid, warm-temperate flora with a strong deciduous component among the angiosperms,
and a wide MA range in a seasonally dry, warm-temperate flora from the Green River
Formation at Bonanza (Utah, U.S.A, 47 Ma), presumed to comprise a mix of short and long leaf life
spans. Second, reconstructed MA in the fossil species is negatively correlated with levels of insect
herbivory, whether measured as the proportion of leaves with insect damage, the proportion of leaf
area removed by herbivores, or the diversity of insect-damage morphotypes. These correlations are
consistent with herbivory observations in extant floras and they reflect fundamental trade-offs in
plant-herbivore associations. Our results indicate that several key aspects of plant and plant-animal
ecology can now be quantified in the fossil record and demonstrate that herbivory has helped shape
the evolution of leaf structure for millions of years.

Item Type: Article
Journal or Publication Title: Paleobiology
Publisher: Paleontological Society
Page Range: pp. 574-589
ISSN: 0094-8373
Identification Number - DOI: 10.1666/07001.1
Date Deposited: 07 Apr 2008 14:31
Last Modified: 18 Nov 2014 03:35
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