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

¹ Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459. droyer{at}wesleyan.edu
² Department of Botany, University of Hawai'i at Mnoa, Honolulu, Hawai'i 96822
³ Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
⁴ Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia
⁵ School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Australia
⁶ Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Tartu 51014, Estonia
⁷ Department of Biology, University of Utah, Salt Lake City, Utah 84112
⁸ Department of Paleobiology, Smithsonian Institution, Washington, D.C. 20013
⁹ Department of Earth Sciences, Denver Museum of Nature and Science, Denver, Colorado 80205
¹⁰ Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia
¹¹ Centro de Ciencias Medioambientales, CSIC, E-28006 Madrid, Spain
¹² Present address: Department of Ecology and Evolutionary Biology, University of California, Los Angeles
¹³ Present address: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario M5S 3G5, Canada
¹⁴ Present address: School of Biological, Earth, and Environmental Sciences, University of New South Wales, New South Wales 2052, Australia

Leaf mass per area (M_A) 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 leaf mass, normalized for leaf area; this enables M_A 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 that M_A 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-M_A 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 M_A 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 M_A 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.

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