FOSSIL LEAF ECONOMICS FROM SCALING PRINCIPLES

Leaf dry mass per unit leaf area (LMA) is a key functional trait representing the dry-mass cost of deploying photosynthetic surface. Species investing in a high LMA tend to have lower mass-based photosynthetic rates but achieve longer leaf lifetimes (LL), such that their lower revenue per unit investment per time is compensated by a longer-lasting revenue stream. This LMA-LL trade-off is also coordinated with variation across species in dark respiration rates, leaf nitrogen and phosphorus concentrations, and insect herbivory rates, forming a "leaf economics spectrum". Thus far, investigation of the leaf economics spectrum has been limited to present-day vegetation because of the inability to determine LMA for fossils. Here we show for woody angiosperms a new and easily-measured scaling relationship between petiole and lamina dimensions that enables the estimation of LMA for compressed fossil leaves, and thus the positioning of past species along the leaf economics spectrum. Application to two well-understood Eocene lake floras (Republic, Washington, USA; 49 Ma and Bonanza, Utah, USA; 47 Ma) quantifies the contrasting ranges of leaf functional diversity between sites and demonstrates for the first time a significant negative correlation between LMA and insect herbivory, consistent with extant floras.

Additional contributing authors: Barbara Cariglino³, Phyllis D. Coley⁷, Asher D. Cutter⁸, Kirk R. Johnson⁹, Conrad C. Labandeira⁸, Angela T. Moles^4,10, Matthew B. Palmer⁸, and Fernando Valladares¹¹, (7) Dept. of Biology, Univ. of Utah, Salt Lake City, Utah 84112, (8) Dept. of Paleobiology, Smithsonian Institution, Washington, DC 20013, (9) Dept. of Earth Sciences, Denver Museum of Nature and Science, Denver, Colorado 80205, (10) School of Biological Sciences, Victoria Univ. of Wellington, Wellington, New Zealand, (11) Centro de Ciencias Medioambientales, CSIC, E-28006 Madrid, Spain